7.2.1.2 Detailed Design Procedure

This section provides basic calculations to design the instrumentation amplifier circuit with respect to the given

design requirements.

One of the key considerations in resistive-bridge sensors is the common-mode voltage, VCM. If the bridge is

balanced (no pressure, thus no voltage change), VCM(zero) is half of the bridge excitation (VEXT). In this example

VCM (zero) is 2.5V. For the maximum pressure of 12psi, the bridge common-mode voltage, VCM(MAX), is calculated

by:

VCMMAX = VDIFF2 + VCMzero

(2)

where

VDIFF=SMAX × VEXT × PMAX = 2.5  mVV× psi × 5V × 12psi = 150mV

(3)

Thus, the maximum common-mode voltage applied results in Equation 4:

VCMMAX =150 mV2+ 2.5V=2.575V

(4)

Similarly, Equation 5 calculates the minimum common-mode voltage.

VCMMIN =–150 mV2+ 2.5V=2.425V

(5)

The next step is to calculate the gain required for the given maximum sensor output voltage span, VDIFF, in

respect to the required VOUT, which is the full-scale range of the ADC.

Equation 6 calculates the gain value using the maximum input voltage and the required output voltage:

G= VOUT

VDIFFMAX = 3.0 V150 mV=20V/V

(6)

The INA122 has a gain range from 5V/V to 10000V/V. To set the gain to 20V/V, set RG to 13.3kΩ to provide the

maximum output signal swing for the ADC.

Next, make sure that the INA122 can operate within this range by checking the Input Common-Mode Voltage vs

Output Voltage curves listed in the Typical Characteristics section. The relevant figure is also in this section for

convenience. Based on Figure 7-4, an output signal swing of 3V is supported for the input signal swing between

2.425V and 2.575V, thus allowing linear operation.

Figure 7-4. Input Common-Mode Voltage vs Output Voltage

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SBOS069A – OCTOBER 1997 – REVISED DECEMBER 2024

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