AN135 データシートの表示（PDF） - Xicor -> Intersil

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AN135

Sensor Circuits and Digitally Controlled Potentiometers

Xicor -> Intersil 

Xicor Application Note

AN135

Ordinarily, a ~10mV/psi pressure-proportional strain

gauge signal is output differentially on pins 2 and 4 of the

sensor, superimposed on the ~50 times larger bridge

excitation voltage. The standard method for separating

the bridge's differential pressure signal from the common

mode bias voltage would be to use an expensive, high

performance differential amplifier. Figure 3 employs a

different scheme. The bridge is current-biased and two

amplifiers and two digitally controlled potentiometers

provide for zero and full scale (gain) adjustments. The

output of A2 drives the bottom of the bridge until pin 4

output of the bridge is at a programmable voltage near

zero volts. With pin 4 at zero volts, pin 2 can be treated as

a single ended or ground referenced voltage. Pin 2 of the

bridge output is then amplified by the noninverting A3

amplifier circuit.

In the detailed circuit of Figure 4, the A2 circuit provides

for the precision adjustment, via DCP1, of any

transducer initial null offset error. To accomplish this, the

bridge excitation voltage is programmably attenuated by

the R2, R3, R4, R5 network and applied to DCP1. The

range for the zero adjustment voltage is from +22mV to

–22mV. The resolution is 172µV and is proportional to

the bridge excitation voltage, thus improving the

temperature stability of the zero adjustment. Boosting the

~10mV/psi bridge signal by 100x to a convenient 1V/psi

output level is the job of the A3 noninverting amplifier via

its feedback and calibration network consisting of R7

through R9 and DCP2. The gain of A3 can be varied

from 75 to 125 with a resolution of 0.2. Bridge bias is

provided by the A1 circuit which uses voltage reference

D1 and current-sense resistor R1 to generate a

constant-current bridge drive of I = 1.225V / 2KΩ = 612µA.

I BIAS

A1

4

–

+

A2

VZERO

Zero Adjust

3

2

1, 8

A3

VO = 1V/1PSI

Gain Adjust

Figure 3. Pressure Transducer Signal

Conditioning Circuit – Simplified

R6

51K

–5V

A1 +5V

2– 4

3+ 1

11

–5V

R4

R2

1KΩ

62KΩ

4

+5V

X9258T

Serial

Bus

A2

6–

7

5+

100K

Zero Adjust

R3

–5V R5 62KΩ

DCP1 1KΩ

All RS = 1 %

All Amplifiers = 1/4 LMC6064

LM4041-1.2

R1

2KΩ

Sensor

Lucas NPC-410-005

3

0-5 PSI

A3

2

+

–

VOUT (1V=1PSI)

1, 8

R9

1.5KΩ

DCP2

100KΩ

R8 1KΩ

R7 182KΩ

Full Scale

Adjust

Figure 4. Pressure Transducer Signal Conditioning Circuit – Detailed

The net result of the combination of transducer and the Figure 4 circuitry is a signal conditioned precision

AN135-3

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