capacimetre femto

Femto CapacimètreSchéma électronique

Plans électroniques gratuits sur Internet

Le premier capacimètre de précision pour condensateurs de faible valeur.

Grande précision.Mesure des femtofarads.

1% d’erreur sur 1 picofarad.6 gammes de mesure.

Faible bruit et faible dérive.Facile à construire.

Common op amp circuitry.Breadboard it in minutes.

Solves the speed problem for small capacitors.

CONTENTS

Introduction Triangle Wave Generator Table 1. Range Values Current-to-Voltage Converters Peak Detector Zero Adjust Calibrating Common Mode Error A/D Converters - page 2. Measuring Reference Capacitors Alternative Reference Circuit The Box Printed Circuit Boards - page 3. Power Supply Construction Hints Measuring Large Capacitors

FIGURES. Figure 1. Overall Schematic. Figure 2. ADD3701 - Analog Circuits. Figure 3. Digital Circuits for ADD3701 with LCD. Figure 4. Pin Connections for LCDO04. Figure 5. Pin Connections for LCDO02 and ICL7106. Figure 6. Analog Circuits for ICL7106. Figure 7. ADD3701 Digital with LED. Figure 8. Referencing Caps by Timer. Figure 9. Referencing Caps by Crystal Oscillator. Figure 10. Box. Figure 11. Pattern for Box. Figure 12. Markings for Box.

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Figure 13. Display Board for 3710 LCD. Figure 14. Main Board for 3701 LCD and LED. Figure 15. Small Board and Switches. Figure 16. Power Supply. Figure 17. Increasing Resolution. Figure 18. Test Circuits.

Introduction.

The Femto Capacitance Meter measures in the low femtofarad area having a resolution of 10 femtofarads (fF). The meter has six ranges allowing a maximum measurement of 4 microfarads.

This allows one picofarad to be measured with 1% error. The need for this is in the fact that the best meters by previous designs would have 20-50% error on 1pF.

This precision requires elimination of test leads. Instead insertion pins are used. Since capacitors are not measured in-circuit, there are advantages in removing test leads which create clutter.

The theory starts with the formula for analyzing capacitance. It is this:

C = I

V/s

It says: capacitance equals current over volts per second. It means that if one side of a capacitor is grounded and a constant current is applied to the other side, the voltage will increase at a constant rate. Measuring the rate of voltage increase is the usual method of determining the capacitance. But it doesn't work well for small capacitors, because timing between two reference voltages in a small amount of time is difficult. However, the procedure is quite precise at low speeds; and it can be used for measuring reference capacitors, as shown later.

What the femto capacitance meter does is hold the volts per second constant while measuring the current. Current can be measured at high speeds easier than volts per second can.

One of the advantages is that the circuits are totally analog; so the resolution is limited only by noise. The circuits are fortuitously highly immune to noise, because three capacitors function as noise filters - a reference capacitor, the capacitor being measured and a peak detector capacitor.

The constant volts per second are produced by a precision triangle wave generator.







Applying the triangle wave to one side of the capacitor being measured results in a constant current on the other side. The current is detected with a current-to-voltage converter which has a square wave at the output. A peak detector measures the amplitude of the square wave. The resulting voltage is put on a digital display through an analog-to-digital converter, which is a voltmeter on a chip. A digital voltmeter could be used for reading. (Gaining voltage by 2 may be needed, as explained under Current-to-voltage Converters.) Figure 1. shows the overall schematic.(You can right click the image, if you have images turned off.)


Triangle Wave Generator.

The triangle wave generator uses the standard procedure, except that a reference voltage is added for high precision.

Let's look at the details starting with the comparator (LM311). It produces the change of state when the amplitude of the triangle wave reaches the appropriate points. The amplitude of the T-wave is not critical; only the slope is. The amplitude is set at 10V P-P (±5V) for the first three ranges (small caps), and 1.8V P-P for the upper three ranges. The reduced amplitude has the only effect of increasing the frequency; so the lowest frequency in range 6 is about 32Hz.

The comparator detects the amplitude through a resistor divider between its output and the T-wave. The noninverting input (2-pin) is controlled by the resistor divider; so it crosses the threshold voltage (ground) when the T-wave amplitude is appropriate. The 3-pin sets the threshold voltage.

As the comparator changes state, it opens and closes the transistor, which functions as a saturation switch. With the transistor closed, op-amp A functions as a unity gain buffer for a reference voltage of +2.49V created by an LM336. When the transistor opens, it forces the output of op-amp A to approximately -2.5V. The negative voltage is noncritical.

From the buffered reference voltage, range setting resistors go to op-amp B. The 2.49K resistor produces exactly 1mA of current, because the other side (2-pin of op-amp B) is at ground level. That current flows into a reference capacitor in the feedback loop of op-amp B. When the reference capacitor is 1nF, the slew rate of op-amp B is 1V/µS (using the formula shown earlier). Table 1. shows the critical values for each range.

Clarifying Descriptions: Separate insertion pins for the test cap are used for the lower three ranges and the upper three, going to op amps C and D. Then two pins are used at the output of op amp B, so they can be positioned better for the test caps. Also, the upper three ranges, at op amp D, have an extra pin for wide capacitors, and it connects directly to the other pin on op amp D. Figure 12 shows this.

Notice in the range chart that the slew rate defines the measurement and is precisely controlled. The frequency is not precisely controlled, because the negative voltage on the triangle wave is crude. The slew rates would decrease by a factor of ten for each range; but in range 4, the reference capacitor is 100 times larger (100nF) while the resistor is 100 smaller (2.5K), so the slew rate is unchanged from range 3. This is ok, because the measuring resistor in op amp D is tailored for it by being ten times smaller (Op amp D is for ranges 4, 5, and 6.)

At switch 2, which changes to 18K for the upper three ranges (4, 5, and 6), the amplitude of the triangle wave is reduced to 1.8V P-P for the purpose of increasing the frequency by a factor of five. Thereby the frequency is 3.1kHz in range 4 instead of 560Hz, which it would be, since the slew rate is the same as range 3. Table 1. Range Values.

range max frequency C/V resolutionC = 1mV slew rate

1 40pF 45kHz 10pF/V 10fF 1V/µS

2 400pF 5kHz 100pF/V 100fF 0.1V/µS

3 4nF 560Hz 1nF/V 1pF 0.01V/µS

4 40nF 3.1kHz 10nF/V 10pF 0.01V/µS

5 400nF 320Hz 100nF/V 100pF 0.001V/µS

6 4µF 32Hz 1µF/V 1nF 100V/S

Current-to-voltage Converters.

Two current-to-voltage converters (CTVS) are used: op-amp C for the lower three ranges, and op-amp D for the upper three. Separate test pins are used for

simplification and to reduce noise pick up.

The feedback resistors are of the size required to correlate output voltage with capacitance. However, there is a gain of 2 in the A/D converter; so the voltage produced by the CTVs is half of what it otherwise would be. If a voltmeter is used for readout, an op-amp with a gain of 2 should follow the peak detector. Doubling the size of the feedback resistors would do, but wave shape will change, and it and must be corrected by reducing the feedback capacitors while scoping for results. Adding an op amp would be easier.

(Procedure for gain of 2: Use identical resistors [such as 1.00k & 1.00k] for 2 pin to ground and feedback, while 3 pin is input.)

The input resistors for the CTVs are invisible. They function as partial substitutes for the feedback capacitors in removing overshoot due to stray capacitance and input capacitance. They cannot be removed without changing the wave shape.

Stray capacitance should be minimized around the inverting inputs, as maintaining a good wave shape is important. Overshoot will cause the peak detector to read high, while too much rounding causes the read to be low. Also, the comparator is kept a couple inches away from the input of op-amp C, because transition noise picked up through the air produces a trailing spike on the wave. Peak Detector.

The peak detector does not require a polypropylene storage capacitor, when the values shown are used. If however, the storage capacitor is larger, soak time starts to become relevant for fast pull-down (called set); and then polypropylene may be needed. A 220nF metal film capacitor is adequate for a normal switching diode (1N4148 or 1N914B). A leakage current of 5 to 10nA for the diode is optimal. (A procedure for measuring it is described later.) At 32Hz, a reverse current of 10nA on the diode results in 0.71mV of ripple on the capacitor; and a correction of over-voltage occurs at 45mV per second. For pulling down large amounts of over-voltage, a resistor of 100k is used with a momentary switch. Zero Adjust.

The noninverting inputs of the CTVs are used for zero adjust to remove the effects of op-amp offsets and cross capacitance. (Cross capacitance is the capacitance between measuring pins.) There are two op-amps in series which influence the offset of the output: the CTV and the first op-amp of the peak detector (op-amp E). The second op-amp of the peak detector (op-amp F) does not influence the net offset, because its output voltage is determined by the first one.

If each op-amp is specified for a maximum offset of 10mV, the worst case offset would be ±20mV. A few mV are added for cross capacitance. With that many

millivolts, the adjustment becomes touchy; and therefore, 12.9mV are trimmed internally in parallel with the external potentiometer which adjusts for the same amount. This allows for ±10mV when switching between lower and upper ranges, plus 5.8mV for cross capacitance, which is equivalent to 116 femtos. There are normally 27 femtos of cross capacitance. Cross capacitance is determined by scoping the output of the CTV in range 1 without a test capacitor; but use of a scope should not be necessary for construction.

The peak detector will not measure less than 3mV, which is 60fF in range 1. Therefore, zeroing out the cross capacitance requires adding enough to be measured. It can be increased to 80fF or more by putting a wire on the board near the T-wave pin and bending it toward the other pin. If a scope is not being used, add a little extra with a partial wrap-around of about 80°. Another alternative is to eliminate cross capacitance with a ground shield between the pins. Then zeroing out offsets can be done with the internal trimmer only eliminating the need for an external zero adjust. To do that, there should be no significant difference between the offsets of the two CTVs (less than 0.5mV); or trim each separately. Calibrating.

If a 3½ digit meter is used, calibrating to 0.5% is possible. A 4½ digit meter can calibrate to 0.2% or better. Drift becomes limiting at around that point. Polypropylene reference capacitors produce most of the drift. They should be polypropylene for low dissipation and flat frequency response. Their temperature coefficient is not given; but it appears to be around 300ppm/°C. That means that a 30C temperature change produces about 0.1% change in measurement.

With a 3½ digit meter, resolution will probably be more limiting than accuracy, particularly with so many critical values having digits of 249. The resolution can be increased through various techniques. The most significant would be to reset the reference voltage to about 1.990V with a resistor divider. Circuit adjustments are shown later. The range setting resistors then have to be tailored accordingly; but the CTVs should not be changed.

If round numbers are used for calibration components, you will have to start with about 20 of each size to select from. The suggested procedure is to allow the reference capacitors to be a little bit small; and then make the range setting resistance a little bit large by using two resistors in series. Also, a small capacitor paralleling the reference capacitor can be used for rounding that number. The small capacitor can be measured reasonably well on a breadboard circuit of the femto capacitance meter. Procedures for measuring reference capacitors are described later.

Use metal film resistors wherever precision is required, because they are low in drift. If however, they are a small percent of the total, their drift becomes noncritical.

If there is a significant amount of offset in op-amp A, then measure the reference voltage at its output. To do so, stop the oscillation by shorting between the base and emitter of the transistor. A jumper wire attached to resistor leads suffices.

For high accuracy calibration, measure the offset of op-amp B and add or subtract it from the reference voltage. Common Mode Error.

Op-amp B produces an unusual error at the input, which I call common mode error. It shows up as a square wave on the inverting input. It only occurs when the output is slewing, as with a triangle wave, but not with a square wave; and it is slew rate dependent. It is probably caused by an internal capacitor. For LF351 or LF353 by National Semiconductor, it is ±37mV; for LF411 - ±35mV; for LF356 - ±31mV; for TL081 - ±58mV; each in range 1. In range 2, it is one tenth that amount, being slew rate dependent. That amount is subtracted from the reference voltage in determining the voltage across the range-setting resistor.

Page 2. Femto Capacitance Meter. A/D Converters.

(Nowdays, digital panel meters are available, which eliminates the need to interface a display to a voltmeter chip.)

The prototypes use ADD3701 for 4,000 counts. Any of the variety of display driving A/D converters could be used. The 3701 will drive either LCD or LED displays. LCDs are easier to see under varied lighting conditions; and the low current is advantageous. But if 4,000 counts are used, LCDs are more difficult to construct, requiring 6 additional chips. If 2,000 counts are used with an LCD, an ICL7106 can be used; and it drives the display directly. The prototype LCD meter uses 4,000 counts. The circuits for both 3701 and 7106 LCDs are shown.

Femto Capacitance Meter.

Figure 2. ADD3701 - Analog Circuits.

Figure 2. shows the analog circuits for the 3701. The chip measures to ground with a 5V supply and has a gain of 2; so 2V input produces 4,000 counts. The reference voltage, going into pin 18, is adjusted to about 2.010V. The adjustment range shown is the same as the specified range (+29mV, -13mV, relative to 2V); but it is too touchy; so two trimmers might be used. The 50K trimmer is for offset adjust.

Each ground connection around the A/D is supposed to go directly to the ground hub to prevent ground currents and noises from producing errors.


Figure 3. Digital Circuits for ADD3701 with LCD.


Figure 3. shows the digital circuits for the 3701 with an LCD. A square wave of about 44Hz is needed on the display. It is created by a 74C86 exclusive OR chip and is distributed by 4543 drivers. The A/D determines which driver functions through pins 21, 22, 23 and 24, which go to the latch disable pins.

Two phases are used with the square wave. One phase (pin 3) goes to the back plane of the LCD (pin 1); and the opposite phase goes to each segment which shows. The 4543s pick up the square wave in the same phase as the back plane (entering at pins 6).

The decimals are driven by the 10 pin of the 74C86, which is always out of phase with the back plane. However, there are 100K resistors which connect the decimals to the back


plane; and they hold the decimals off, unless the rotary switch overrides the resistors with the opposite phase.

All of the segments which are continuously blanked are connected to the back plane. Segments will not blank if left unconnected. They must be in phase with the back plane to blank.

Overflow blanks 4 digits and shows a 1 at the left. The decimal stays on during overflow. The overflow signal from pin 7 of the A/D goes to the blanking inputs (7 pins) of the 4543s and to the XOR for showing the 1 by changing the phase of pin 3 of the LCD.


Figure 4. Pin Connections for LCD004.

LCD

1

2

3

4

5

6

7

8

9

10

11

12

4543

- *

- *

- 11 of 74C86

- *

- 13 - (1)

- 12 - (1)

- 11 - (1)

- 3 & 6 on SW-1

- 13 - (2)

- 12 - (2)

- 11 - (2)

- 1 & 4 on SW-1

LCD

21

22

23

24

25

26

27

28

29

30

31

32

4543

- 9 - (4)

- 14 - (4)

- 15 - (4)

- 10 - (3)

- 9 - (3)

- 14 - (3)

- 15 - (3)

- *

- 10 - (2)

- 9 - (2)

- 14 - (2)

- 15 - (2)

13

14

15

16

17

18

19

20

*

- 13 - (3)

- 12 - (3)

- 11 - (3)

- 2 & 5 on SW-1

- 13 - (4)

- 12 - (4)

- 11 - (4)

- 10 - (4)

= pin 3 of 74C86

33

34

35

36

37

38

39

40

- *

- 10 - (1)

- 9 - (1)

- 14 - (1)

- 15 - (1)

- *

- *

- not connected

The 4,000 count LCD is an earlier Digi-Key part number LCD004, which is no longer extant but should be the same as more recent ones. The pinout is shown in Figure 4. along with the connections, which mostly go to the 4543s.


Figure 5. Pin Connections for LCD002 and ICL7106 Digital. LCD002

1

3

e 9

d 10

c 11

e 13

d 14

c 15

e 17

d 18

c 19

b 20

a 21

f 22

7106

- 21

- 19

- 18

- 15

- 24

- 14

- 9

- 10

- 8

- 2

- 3

- 4

- 5

- 6

74C86 (for decimals)

1 - 2, 5 (SW-1)

5 - 1, 4 (SW-1)

13 - 3, 6 (SW-1)

3 - 16 (LCD)

4 - 12 (LCD)

11 - 8 (LCD)

7, 8, 9 - 37 (7106)

2, 6, 12 - 21 (7106)

14 - +5V

10 - not connected

back plane - 1, 2, 28, 38, 39, (LCD); 2, 6, 12 (74C86)

not connected - 4, 5, 6, 7, 33, 34, 35, 36, 37, 40 (LCD)

10 (74C86); 20 (7106)

center pin of SW-1 - +5V


g 23

b 24

a 25

f 26

g 27

b 29

a 30

f 31

g 32

- 7

- 11

- 12

- 13

- 25

74C86 = CD4070BCN

The LCD for ICL7106 is an earlier Digi-Key part number LCD002. It is identical in pinout to the one above except without the MSD (most significant digit); and the corresponding pins are not connected. The pinout for LCD002 is shown in Figure 5.


Figure 6. Analog Circuits for ICL7106.


Figure 6. shows the analog circuits for the ICL7106 with an LCD. The chip needs a negative voltage of at least -2.5V; so it uses the available -8. 5V. It has no offset voltage; and there is no device variation on the reference voltage. So the reference voltage is set at exactly 0.500V. Twice that amount, or 1V, is the amount of input voltage that reads full scale. Since full scale is 2,000 counts, there is an effective voltage gain of 2, as required.

Either an external or internal reference voltage can be used with the 7106, as shown. The external reference would be easier to use, since it is already available. The internal


reference is based upon a zener diode which sets the 32 pin at 2.8V below V+. That reference is low in drift; but it has a wide tolerance; so it should be measured first and the resistor values tailored to the measurement.

The large capacitors can all be metal film except the one for integrating (pin 27), which should be polypropylene.


Figure 7. ADD3701 Digital with LED.

Figure 7. shows the digital circuits for 3701 with an LED display. The LED is 4 digit multiplexed. The digit select is buffered and inverted with an interface chip. The segment select pins have 18 ohm resistors to limit the current to 4OmA. Since each segment is on 25% of the time, the average current for each is 10mA. The cathode of each decimal point is switched to ground, while a 300 ohm resistor at the anode limits the current to 10mA.

Overflow is handled internally showing OFL; so the 7 pin of the 3701 is not connected. Measuring Reference Capacitors.

Reference capacitors are the two on op amp B (100nF and 1.00nF).



Figure 8. Referencing Caps by Timer.

Figure 8. shows a circuit for measuring reference capacitors by using a timer, which a frequency counter should have. One microsecond of resolution is desirable. When calibrated, the circuit is a complete capacitance meter. But there is no need to calibrate it for this purpose. The critical values are measured and calculated as odd numbers.

The first part of the circuit is the same precision triangle wave generator that the femto capacitance meter uses. What would be the reference capacitor is in this case the test capacitor. The resistor which determines the current flowing into the capacitor, Ra, is soldered on from the top; so it can be changed. If it is 1M, it produces about 4,000 µS for the 1nF capacitor. The 100nF cap produces 400mS; so Ra could be smaller.

Two comparators are used for upper and lower thresholds, with about 10V between them. That voltage needs to be precisely measured to determine the volts per second.

There is about 5mV of hysteresis on each comparator for stability at low frequencies. To


calculate it, there is 5V across the 270K feedback resistor before the change of state. The resulting current through the 270 ohm resistor produces 5mV. For the lower comparator, there is 10V across the feedback resistor prior to the change of state. The hysteresis for the upper comparator is subtracted from the reference voltage; and for the lower one it is added. So the effects cancel; and the quantities can be ignored.

The AND gates at the output have the purpose of preventing an occurrence during the unused half of the cycle. The voltage going in is limited to 5V with a diode network.

The circuit could be set up on a breadboard; but for high precision, it should be soldered into an etched board.

The circuit picks up 60 cycle hum easily at the high impedance level; so it may need to be shielded. To do that, use a piece of tin or aluminum with four sides bent up to form an open box. Put paper in the bottom to insulate; ground it; and put the board in. Run the wires over the top. The top does not have to be covered.

The capacitors could be soldered onto the board; but I use insertion pins and solder them onto the board. Shielding between them is needed, if small capacitors are measured.

Add 3pF to the value of the 1nF capacitor, because the cross capacitance of op-amp B will be about that much. Alternate Reference Circuit.


Figure 9. Referencing Caps by Crystal Oscillator.

If you don't have a frequency counter with a timer, you can use a crystal oscillator for measuring the reference capacitors, as shown in Figure 9. This procedure is not as precise as the previous one. A breadboard can be used.

The capacitor being measured is in the feedback loop of an op-amp. A reference voltage is pulsed into the noninverting input, which causes the output voltage to increase at a fixed rate based upon the current flowing through the 1.00M resistor. The resistance can be an odd value; but it needs to be precisely measured. The reference voltage is the voltage across that resistor. After the pulse, the output voltage stays high and is measured with a meter. The output voltage divided by the time of the pulse determines the volts per second.

The crystal oscillator will have enough accuracy without measurement. Its signal is reduced to the appropriate frequency and goes into the clock input (3 pin) of a flip flop which functions as a pulser. The output pulse will have a time duration of exactly one duty cycle of the input frequency. For the 100nF capacitor, the input frequency is 16Hz producing a pulse of 0.0625 seconds. For the 1nF capacitor, the frequency is 256Hz producing a pulse of 3.906mS.

The 6 pin of the flip flop produces an up pulse which opens the line to the reference voltage through a CMOS switch (4066). The 5 pin of the flip flop produces a down pulse which closes the line to ground.

The offset voltage must be thoroughly trimmed out to prevent a slide in output voltage due to current flowing through the input resistor. For the 100nF capacitor, that is not difficult to do; but for the 1nF capacitor, it becomes a little problematic; and more refined offset trimming might be needed.

A momentary switch removes the voltage across the capacitor for repeating the measurement. When the 1nF capacitor is being measure, a 1M resistor is used with the reset switch to remove contact bounce. It is omitted for the 100nF capacitor.

With the 100nF capacitor, the output voltage will be about 1.5V. That measurement will have a potential accuracy of about 0.2%. The 1nF capacitor will produce an output voltage of about 9.6V; and its measurement is consistently high by about 1% or 10pF. I don't know why. Theoretically, the reason would seem to be input capacitance, since there is a common mode voltage increase; but adding capacitance makes little difference indicating otherwise. At any rate, subtracting 1% from the measurement of the 1nF


capacitor should produce a useable quantity.

Momentary switches that work good on a breadboard are the flat switches or post switches. Twenty two gage wires are soldered onto the pins for breadboard use. The Box.


Figure 10. Box.

The box is designed for convenience of use. That means an upward sloping front panel with everything on it, as shown in Figure 10. Two boards are used, attached to the top panel, so the switches can run across the center, while insertion pins are at the bottom, and the display is at the top. Everything but the switches is attached to the boards. The power supply, however, is on the bottom of the box.

Of course you don't buy a box shaped like that; it is bent out of aluminum sheet metal, which is available from heating and cooling shops or metal supply shops. The preferred thickness is 0.040 inches (1.0 mm). A detailed box pattern is in Figure 11.


Figure 11. Pattern for Box.



Markings are shown in Figure 12.


Figure 12. Markings for Box.

Toggles are the preferred switches. Toward the upper left is the on-off switch for ac line voltage. There are 20cm wires going to it from the bottom of the box. They are allowed to coil up in the box. If they touch the reference capacitors, they can induce a few parts per thousand error, so an aluminum plate jutting out from the side is placed over them. If it is desirable not to have ac wires coiled in the box, the ac switch could go on the back panel,


which is attached to the bottom. If LCD is used, the on-off switch could be eliminated leaving the meter on most of the time. But if LED is use, the meter should be turned off except when used, because heavy currents in the display will heat the box producing about 3ppt drift in a half hour. Switch-2 is a three pole toggle located near the center between the rotary for stages and the potentiometer for zeroing.

The insertion pins (see description on page 3.) are soldered onto the copper side of the board; and they project through holes in the aluminum. For the upper three ranges, two pins are used on the T-wave side, because some of the large capacitors have wide spaced leads.

If capacitors have short leads which will not fit in the pins, alligator clips can be used by soldering 20 gage wires onto the ends of flexible wires for insertion into the pins.

If banana jacks are desired for test leads, they could be attached to the board near the pins. A suitable jack for that is Radio Shack's 274-661. The plastic top can be removed leaving a small diameter jack which can be attached to the board without soldering. An extra attachment screw for the board should be located near the jacks.

If wires are used for remotely located jacks, they will be antennas for picking up transition noise from the comparator, which will require the comparator and its output wiring to be covered with grounded tin for shielding.

The momentary switch which pulls down over-voltage (called the set switch) is located on the lower board towards the right. The Panasonic post switch is good for that. It solders to the copper side of the board, which is upward during use.

On each side of the box (inside) is a small aluminum bracket with a hole for gathering wires. One is 2cm below the on-off switch for the ac wires; and the other is directly across from it for four power supply wires (±8.5V, +5V, and ground). Each of those wires is about 20 to 30cm long, so the top of the box can be separated from the bottom, which has the power supply on it.

Page 3. Femto Capacitance Meter. Printed Circuit Boards.

There are two PC boards attached to the front panel - a large one above the switches, and a small one below them.

With a 4,000 count LCD (using 3701) there is an additional large board for the display and driving chips (4543s). The XOR oscillator switch is also on that board. The board is stacked on the same screws as the other large board, with 5/16" (8mm) spacers between them. In this case only, the display is on the component side of the board. It is socketed into molex pins.

http://nov55.com/cap/cap3.htm#Pins


Figure 13. Display Board for 3701 LCD.

The board is shown in Figure 13. The decimal point wires go through the board near the rotary switch. They need to be color coded 30 gage kynars.

Quite a few jumper wires will be needed, as usual with complex digital boards. If you use dry transfers, you might get all of the bus lines on; but if a marking pen is used, jumper wires will probably be needed for some of them. There are a few short jumper wires under the display; and those wires have to stay on the copper side of the board; so they are soldered on last.

Eleven wires link to the other board through a header; but there is no header on the display board. The wires are attached directly to the board near the corner. The wires then extend 17cm to a male header. I use machined pin sockets for male and female headers, because they save space and are readily available. When soldering wires into the top of the socket, a low temperature iron is used along with liquid rosin for fast soldering, so as to not melt the plastic. However, there should be enough space available for other types of headers.

The wires to the header include 4 chip selects (to 1 pins of 4543s), 4 bits of binary code (A, B, C, & D), The overflow signal to blanking pins (7 pins), +5V and Ground.



Figure 14. Main Board for 3701 LCD and LED.

(Not all components shown.)

Figure 14. shows the main board when using a 3701. The comparator should have a bypass capacitor, 1-10µF, between pins 4 and 8.

If two range setting resistors are used in series, the op-amp can be slid back further. The resistors stand vertical to save space. Both leads are attached to the board for stability. The long lead is in front for attaching the wire to the rotary switch.

The LED display is on the copper side of the board. Therefore, the pins are attached by wires only, as if a perf board were being used. A socket is not used.

Seven resistors are located above the pins. They stand vertical. Thirty gage wires go from resistor leads to pins. Being so short, they can cross without insulation.

Since the LED produces heavy digital currents, a capacitor of 1,000 µF must be located near the A/D, between pins 1 and 25, even if the voltage is regulated with an LM340. Otherwise, digital noise causes the A/D to produce errors.


On the LED board, the 3701 is moved close to the edge to save space; and therefore, most of the digital connections require jumper wires, which are about as easy to use as drawing lines anyway.


Figure 15. Small Board and Switches.

Figure 15. shows the small board and switches. The wires going over the boards are 30 gage kynar. They are soldered to wire loops which have both ends in the board, so soldering from the top does not cause them to unsolder. Tinned bus is preferable for loops, but not essential. Small quick-connect pins might be located. if experimenting with components is desired, use loops for soldering them on from the top. This procedure is suggested for the small capacitor with the reference capacitor, because you may decide later to improve the accuracy based upon measurements of reference capacitors.

Where supply wires jump onto the board, a couple of 10µF capacitors or larger are used as a matter of principle.

The rotary switch can have resistors coming off the pins, if space saving is needed. Power Supply.


Figure 16. Power Supply.


Figure 16. shows the power supply. For an LCD meter, it would not have to be that rugged.

For an LED meter, the 5V regulator needs to be heat sank to the bottom of the box. For that, an aluminum bracket is made - bent twice creating a U shape. The chip then sits above the edge of the board.

The box is of course grounded at some point. The ac power line has a 10M resistor going to ground for dissipating static charge.

A shielded transformer is preferred. Insertion Pins.

The insertion pins used on the prototypes were adapter pins designed for breadboards. They are no longer available, but a new type is produced by Mill-Max and sold by Digi-Key (near IC sockets in catalog). A suitable size is 0.025-0.037", which is Fig. 16, Digi-Key part number ED5009. The very large capacitors will not insert, but they can be held in place while measuring. Holding large caps does not alter their measurement significantly.


The next size up can be used. It is Fig. 24, ED5013. Small leads will be loose, but they can be bowed inward slightly, which locks them in place.

Construction Hints.


Figure 17. Increasing Resolution.

Figure 17. shows the use of a 1.985 reference voltage for increasing the resolution of calibration measurements.

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Figure 18. Test Circuits.


To measure capacitor leakage, as shown in Figure 18., apply a voltage, then allow it to sit 30-60 seconds and remeasure the voltage using a FET op-amp as a buffer. To measure the reverse current of a diode, put it in parallel with the capacitor.

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To measure the offset of a comparator, increase the voltage on one of the inputs through a resistor divider, while watching for the change of state. Note the voltage at which change of state occurs. If the polarity is incorrect, reverse inputs.

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Solid wire solder if far better than core type. Core type puts rosin everyplace except where it belongs, causes carbon to coat the iron, and can destroy sensitive parts before getting a connection. Liquid rosin can be put directly on the connection, which speeds soldering and keeps it off the iron. Then the solder is picked up on the tip of the iron, which frees one hand.

But better than rosin is citric acid disolved in distilled water. It won't create a burnt carbon coating, and it washes off easy with water. It will conduct a small amount of current if not removed.

The copper on the board should be coated with solder after etching, which speeds soldering as well as preserves the surface. When the copper surface is cleaned with acetone, citric acid will coat it more uniformly.

Apply liquid citric acid with a toothpick, except on a large surface, where cotton swabs


work good. Use citric acid saturated, which means there should be some undisolved cristals on the bottom. Citric acid is available at pharmacies.

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For an LED display, a lense is needed to show contrast. Plastic windshield tinting will stick on with water in place of a lens.

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When making the box, do not drill the holes in the tabs until the bottom is drilled. Marking through the bottom will align them properly for sheet metal screws.

Perspectives.

One reason why the Femto Capacitance Meter is needed is because small capacitors could not previously be measured adequately. For example, fractional picofarad capacitors might be manufactured with high precision, but they would all be off by about 20-50%, because the manufacturer could not measure them any closer than that. On top of that, markings are often nonexistent or impossible to read on small capacitors. So they were a real problem.

Test leads could be used for everything above 100 pF, but I didn't include them, because they are a real nuisance. They sweep things around on a workbench, and they tangle when stored.

Adapter pins (for breadboards) are no longer available for insertion of the test capacitors. But there are a variety of styles of pins available. Buy different types and test them. Another alternative would be to use tinned can steel to construct insertion pins. Roll it wide at the top for large leads and narrow at the bottom. Then solder it to the copper side of the board. It can be adjusted after attaching by pinching with side cutters or prying open. If small leads do not make good contact, they can be bowed inward slightly to create a suitable stress force.

I designed for a lot of external adjustments. Most of them could probably be eliminated with no more than 1% error. But they are desirable for high precision and certainty. If for example, a lot of ripple were placed on the peak detector storage capacitor, a pull down switch would not be needed. However, the pull down (called set) is so fast and convenient that I see no reason to exclude it.

Zero adjust does not measure on the negative side; all negative values read zero. Therefore, zero is adjusted to 1, which can be ignored or subtracted from the read.

Some persons would want a capacitance meter to look like the ones in the stores - battery powered, pocket size, rectangular, test leads, measuring inductance. Those characteristics are not conducive to the precision that is sometimes preferred, and they are not ideal characteristics. A meter needs a sloping front face for easy reading. Infact, I made an aluminum bracket for a voltmeter for sloping it. And why battery power for something that is not carried around. Capacitors cannot be measured in-circuit, so a capacitance meter is not carried around.

Creating a digital display is quite a bit of work. If you need the meter but don't have lot of time, the thing to do would be to use a voltmeter for readout. But the sloping box would still be desirable for controls.