7.2 Typical Application

7.2.1 Resistive-Bridge Pressure Sensor

The INA122 is an instrumentation amplifier that measures small differential voltages while simultaneously

rejecting larger common-mode voltages. The device offers a low power consumption of 60µA (typical) and

is designed for portable applications where sensors measure physical parameters, such as changes in fluid,

pressure, temperature, or humidity.

The pressure sensor is made of a piezo-resistive element that can be derived as a classical 4-resistor

Wheatstone bridge. Changes in the strain gauge resistance on one leg of the Wheatstone bridge (R + ΔR)

induces a differential voltage VDIFF.

Figure 7-3 shows an example circuit for a pressure sensor application. The signal chain connected to the bridge

downstream processes the pressure change and can trigger an alarm.

GND

+

–

INA122

VOUT

VREF

VEXT = 5 V

0.1 F

VDIFF

ADC

Pressure

Occlusion

Sensor

µC

A

B

C

D

REF

1 F

VS = 5 V

1 F

R1

RG

Figure 7-3. Resistive-Bridge Pressure Sensor

Low-tolerance bridge resistors must be used to minimize the offset and gain errors.

Given that there is only a positive differential voltage applied, this circuit is laid out in single supply mode.

The excitation voltage, VEXT, to the bridge must be precise and stable; otherwise, measurement errors can be

introduced.

7.2.1.1 Design Requirements

For this application, the design requirements are as provided in Table 7-1.

Table 7-1. Design Requirements

DESCRIPTION

VALUE

Single supply voltage

VS = 5V

Excitation voltage

VEXT = 5.0V

Pressure range

P = 1psi to 12psi, increments of P = 0.5psi

Pressure sensitivity

S = 2 ±0.5 (25%) mV/V/psi

Pressure impedance

R = 4.99kΩ ±50Ω (0.1%)

Total pressure sampling rate

Sr = 20Hz

Full-scale range of ADC

VADC(fs) = VOUT = 3.0V

INA122

SBOS069A – OCTOBER 1997 – REVISED DECEMBER 2024

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