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38

Analog Applications JournalAnalog and Mixed-Signal Products February 2001

Pressure transducer-to-ADC application

IntroductionA major application of operational amplifiers (op amps) isconverting and conditioning signals from transducers intosignals that other devices such as analog-to-digital con-verters (ADCs) can use. Conversion and conditioning areusually necessary because the transducer and ADC rangesand offsets are rarely the same. Op amp circuits are alsouseful in signal filtering for compatibility with ADC circuits.

This article shows how to use a bridge-type transducerfor measuring gas or liquid pressure, and for measuringstrain in mechanical elements. A basic understanding ofactive and passive analog devices and their use is helpful

to use this article to complete a design.Transducer informationThe sensor tested in this application is a pressure trans-ducer SX01 produced by SenSym Inc. It is one of a set ofsolid-state pressure sensors with available full-scale rangesof 1 to 150 psi (7 kPa to 1 MPa). Three types of pressuremeasurement are available: gauge, differential, and absolute.

TRW, another supplier of such sensors, has an on-lineproduct catalog with a link to data sheets at www.gesensing.com

The device evaluated here has a full-scale pressure of1 psig (7 kPa). Its price is low relative to other devices inthis range; however, the low price comes at the expense ofno temperature compensation, a drawback that can beovercome by adding inexpensive, external compensation

components. The SenSym data sheet defines three circuitsfor this purpose; the method chosen here uses an NPNbipolar transistor and two resistors.

Excitation source informationFor a bridge transducer to work, it must be excited by avoltage source. Because the stability of the excitation volt-age affects the accuracy of the measurement signal, a reg-ulated voltage source is necessary. This applicationassumes the availability of a regulated 5-V supply.

Choice of ADCFor this design, the TLV2544 ADC was selected because ithas an analog input range of 0 to 5 V (see Reference 1).Ideally, the span of the amplified sensor signal should fill

this range. The voltage needed to power this device is asingle 5-V supply.

Choice of op ampThe ADCs 0- to 5-V analog-input range and the use of asingle 5-V power supply required a rail-to-rail outputdevice. The op amp also needed to be able to handle thefull input range of the transducer. For these reasons, theTLV2474 was chosen (see Reference 2).

Defining the circuitThe amplified pressure transducer signal is connected toan ADC. Since the ADC connects to a microprocessor orDSP, final calibrations can be done in software. Therefore,the 0- to 1-psi range should span the center of the ADCsrange. For calculating gain, the output range of theamplifier is 1.25 to 3.75 V. Figure 1 is the schematic of theamplifier circuit for this application.

The output of the circuit is

(1)

When R7 = R6, R5 = R2, and R4 = R1, Equation 1 reduces to

(2)

Solving for R3yields

(3)

The sensitivity of the bridge is typically 4.0 mV/V/psi. Thepressure is 1 psi and the excitation voltage is 5 V.Therefore, the differential output of the sensor (VIN2VIN1)from 0 to 1 psi is 20 mV. Setting R1 = R4 = R6 = R7 =20.0 k (1% value) and R2 = R5 = 2.0 k results in a full-scale output range of 2.5 V when R3 = 3.478 k.

A unique feature of this precision instrumentationamplifier is the ability to control the total gain of theamplifier with one resistor.

Calibration devicesThe resistor that controls gain is R1. A potentiometer hasa larger temperature coefficient and is more likely to driftover time than a fixed resistor. Placing a fixed resistor inseries with a potentiometer minimizes this problem. The

values calculated in the following equations are based onabout 10% gain adjustment.

R3A = R1 R1(5%) = 143 k (1% resistor) (4)R3B = R3(10%) = 50 k (Cermet potentiometer) (5)

The potentiometer for adjusting offset, R13, is not critical;but a 10-k multiturn potentiometer uses 0.5 mA.

One of the goals of design is to reduce componentswithout compromising function. If the offset voltage of theop amps is low enough, replacing potentiometers withfixed resistors is possible. Offset and gain calibration

RR

V V R

V V R

OUT REF

IN IN

31

2

2 1 6

2

1

=

( )

( )

.

V V V VOUT IN IN REF= +

+( ) .2 1 12R

R

R

R

1

3

6

2

V VR R

R

R

R R

R R

R

VR

OUT IN

IN

=+

+

+

+

24 3

3

7

5 7

6 2

2

11

2

2 RR

R

R

RV

R

R R

R R

RREF

3

3

6

2

5

5 7

6 2

2

++

+

.

Texas Instruments IncorporatedSignal Conditioning: Op Amps

By John Bishop

Application Specialist, Advanced Analog Products/Op Amp Applications

http://www.gesensing.com/http://www.gesensing.com/http://www.gesensing.com/http://www.gesensing.com/

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Texas Instruments Incorporated Signal Conditioning: Op Amps

39

Analog Applications Journal February 2001 Analog and Mixed-Signal Products

R3a

2 k

2 k

C1

100 nF

R4

VIN2

+5 V

C310 nF

R2

Q1

NPN

20 k

1430 VREF

TLV2474

6

57

3220

20 k

U2

BRIDGE

+5 V

R14

VOUT

10 k

R5

R15

10 k

U1a

TLV2474

2

31

4

11R13

R7

R8

3320

C610 nF

R9

C210 nF

500

+5 V

VIN1

U1b

+

+

+

R3b

R120 k

C5

10 nF

U1cTLV2474

9

108

+

+

R6

C710 nF

20 k

TLV2474

13

1214

U1d

+

10 k

Figure 8. Op amp circuit for the pressure transducer-to-ADC application

would then be done using software in a DSP or micro-processor. This is possible because the bottom and top25% of the input range of the ADC are not presently used.In this condition, VREF would be initially set by replacing

R13 with a voltage divider. The offset drift of the op ampscauses the output to move up or down into the unusedareas. Variations on the resistors cause small gain errors,but these should be of less concern than the offset volt-age. Instead of calibrating with potentiometers, using theoffset and gain variables in calculations can generate acalibrated output.

Signal filteringIf the transducer is installed on the amplifier board, theinput filter circuits and shielded wires are not needed.

Connecting a transducer to an input subjects the wiringto noise signals because of the surrounding electrical andmagnetic environment. To prevent this noise from inter-fering with the measurement signals, some shielding isnecessary. Using a twisted pair from the transducer to theconversion circuit and shielding this pair (grounding theshield at the instrument) reduces the noise.

Even when the transducer is connected through cor-rectly shielded cabling, some noise is brought into theamplifier along with the measurement signal. Without aninput filter, the op amp would act as an RF detector, con-verting high-frequency signals from other devices intosignals with low-frequency components. Placing a resistorand capacitor on the input forms a low-pass filter and

prevents radio-frequency signals from interfering with themeasurement signal. The frequency response of this filter is

(6)

Thus, if R14 and R15 are 10 k, and C2 and C6 are 10 nF,then fC is about 1600 Hz.

The next two stages have capacitors in parallel with thefeedback resistors. The frequency response of these filtersis also defined by Equation 6. Using 20 k for the feed-back resistor gives a cutoff frequency, fC, of about 800 Hz.

ReferencesFor TI information related to this article, you can downloadan Acrobat Reader file at www-s.ti.com/sc/techlit/litnumber and replace litnumber with the TI Lit. # forthe following document.

Document Title TI Lit. #

1. TLV2544, TLV2548 2.7-V to 5.5-V, 12-bit,200-kSPS, 4-/8-channel, Low-power SerialAnalog-to-Digital Converters withAutopower-down, Data Sheet . . . . . . . . . . . . . . .slas198

2. TLV2470, TLV2471, TLV2472, TLV2473,TLV2474, TLV2475, TLV247xA Family of600-A/Ch 2.8-MHz Rail-to-Rail Input/OutputHigh-Drive Operational Amplifiers withShutdown, Data Sheet . . . . . . . . . . . . . . . . . . . . .slos232

f1

2 RC

C =

.

Continued on next page

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Texas Instruments IncorporatedSignal Conditioning: Op Amps

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Analog Applications JournalAnalog and Mixed-Signal Products February 2001

Document Title TI Lit. #

3. Understanding Basic Analog-Ideal Op Amps,Application Report . . . . . . . . . . . . . . . . . . . . . . . .slaa068

4. Single Supply Op Amp Design Techniques,Application Report . . . . . . . . . . . . . . . . . . . . . . . .sloa030

5. Active Low-Pass Filter Design, ApplicationReport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .sloa049

6. Application of Rail-to-Rail OperationalAmplifiers, Application Report . . . . . . . . . . . . . .sloa039

7. Ron Mancini, Sensor to ADCAnalogInterface Design,Analog Applications

Journal (May 2000), p. 22 . . . . . . . . . . . . . . . . . .slyt1738. Signal Conditioning Wheatstone Resistive

Bridge Sensors, Application Report . . . . . . . . . .sloa034

Related Web siteswww.gesensing.com

Appendix A. CalculationsThe following values and equations were used in this article. Values in bold are calculated. All entered values are non-bold.

Given:InputMIN = 0.0 psi OutputMIN = 1.25 V

InputMAX = 1.0 psi OutputMAX = 3.75 VSensitivity = 0.004 mV/V/psi

Excitation voltage = 5 VFull-scale span = 0.02 V R8 = 3.22 k

Nominal Gain = 125 R9 = 1.43 kR1 = R4 = R7 = R6 = 20 k

R5 = R2 = 2 kVIN = 0.02 V

VOUT = 2.5 VR3 = 3478

1% valueR3A = 3304 3320

Pot. valueR3B = 348 500

C4 = C6 = 0.01 F C3 = C5 = C7 = 0.01 FR14 = R15 = 10 k R1 = R4 = R6 = 10 k

FC = 1592 Hz FC = 796 Hz

R3a

2 k

2 k

C1

100 nF

R4

VIN2

+5 V

C310 nF

R2

Q1

NPN

20 k

1430 VREF

TLV2474

6

57

3220

20 k

U2

BRIDGE

+5 V

R14

VOUT

10 k

R5

R15

10 k

U1a

TLV2474

2

31

4

11R13

R7

R8

3320

C610 nF

R9

C210 nF

500

+5 V

VIN1

U1b

+

+

+

R3b

R120 k

C510 nF

U1cTLV2474

9

108

+

+

R6

C710 nF

20 k

TLV2474

13

1214

U1d

+

10 k

Continued from previous page

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