Lecture 25 1

DC Measurements

Lecture 25 2

DC Measurements

• DC Measurements include current, voltage, resistance, and power.

• Section 2.8 should be titled “DC Measurements: a Historical Perspective.”

• A digital multimeter is one type of sensor system.

Lecture 25 3

Digital Meters and Oscilloscopes

• Most multimeters and oscilloscopes are now digital.

• A digital multimeter or a digital oscilloscope has an analog-to-digital (A/D) converter.

• Most digital meters and all digital oscilloscopes have one or more processors.

Lecture 25 4

Data Acquisition Systems

• In many applications, digital meters and scopes are being replaced by data acquisition cards that fit into a computer.

• The data acquisition cards have A/D converters.

• The computer provide processing and storage for the data.

Lecture 25 5

A Generic Digital Meter

Input Switchingand Ranging

Amplifier

A/D Converter

ProcessorDisplay

Lecture 25 6

Voltage Measurements

Hi Com

10V

1V

100V

Lecture 25 7

Voltage Measurements

• For voltage measurements, a more detailed model of the first three meter components looks like this:

+

-

Vin 10G +

-Vin

A/DConverter12VDC

Hi

Com

Lecture 25 8

Voltage Measurements

10V setting:

Input resistance of amplifier is > 10GGain of amplifier is 1

+

-

Vin 10G +

-Vin

A/DConverter12VDC

Hi

Com

Lecture 25 9

Voltage Measurements

1V setting:

Gain of amplifier is 10 (we learn how to make an amplifier with gain of 10 in Section 3.3)

+

-

Vin 10G +

-10Vin

A/DConverter12VDC

Hi

Com

Lecture 25 10

Voltage Measurements

100V setting:

Input resistance of meter is 10MGain of amplifier is 10

+

-

Vin

10G+

-10Vin

A/DConverter12VDC

Hi

Com

9.9M

100k

Lecture 25 11

Model for Meter

The ideal meter measures the voltage across its inputs. No current flows into it; it has infinite input resistance.

10M Ideal MeterHi

Com

Lecture 25 12

10M Ideal MeterHi

ComR

Meter Loading

The 10M meter resistance in parallel with R may change the voltage that you measure.

Lecture 25 13

Loading

• When measuring the voltage across R, we need to make sure that R is much less than 10G or 10M, depending on the meter setting

• If R is close to 10G10M, significant current flows through the meter, changing the voltage across R.

Lecture 25 14

Loading Example

• Without Meter: voltage is 100V

• With Meter: measured voltage is 83.3V

10M Ideal MeterHi

Com2M50A

Lecture 25 15

Current Measurements

AmpCom

10V

1V

100V

Lecture 25 16

Measuring Current

• Large Currents (> 100mA):

– The current to be measured is passed through a small resistor (called a shunt resistor) and the resulting voltage across the shunt resistor is measured.

– From the voltage, the current can be computed.

Lecture 25 17

Measuring Current

• Small Currents (< 100mA)

– An amplifier is configured to convert the current into a measurable voltage.

– This is called a virtual ground configuration.

Lecture 25 18

Shunt Resistor

The shunt resistor of known resistance Rs is placed into the current to be measured:

+

-

Vin10G +

-100Vin

A/DConverter12VDC

Amp

ComRs

Lecture 25 19

Shunt Resistance

• What value of Rs should you use to get a full range reading with 1A?

• What value of Rs should you use to get a full range reading with 100mA?

• What value of Rs should you use to get a full range reading with 10mA?

Lecture 25 20

Rs Ideal MeterAmp

ComR

Meter Loading

The Rs shunt resistance in series with R may change the current that you measure.

Lecture 25 21

Measuring Resistance

The known current Is creates a voltage across R which is measured.

Is Ideal MeterHi

Com

Unknown R

Lecture 25 22

Measuring Resistance

• What value of Is should you use to get a full range reading for 100?

• What value of Is should you use to get a full range reading 10k?

• What value of Is should you use to get a full range reading with 1M?