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9.2.1.1 Design Requirements

For this application, the design requirements are as shown in Table 9-1.

Table 9-1. Design Requirements

DESCRIPTION

VALUE

Single supply voltage

VS= 5 V

Excitation voltage

VEXT = 2.5 V

Occlusion pressure range

P = 1...10 psi, increments of p = 0.5 psi

Occlusion pressure sensitivity

S = 2 ± 0.5 (25%) mV/V/psi

Occlusion pressure impedance (R)

R = 4.99 kΩ ± 50 Ω (0.1%)

Total pressure sampling rate

Sr = 20 Hz

Full-scale range of ADC

VADC(fs) = VOUT = 4.5 V

9.2.1.2 Detailed Design Procedure

This section provides basic calculations to lay out the instrumentation amplifier 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 1.25 V. For the maximum pressure of 10 psi, the bridge common-mode voltage, VCM(MAX), is

calculated by:

VCMMAX =VDIFF2+VCM(zero)

(3)

where

•

VDIFF=SMAX×VEXT×PMAX=2.5 mVV×psi×5 V×10 psi=125 mV

(4)

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

VCMMAX =125mV2+1.25V=1.375V

(5)

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

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

The following equation calculates the gain value using the maximum input voltage and the required output

voltage:

G = VOUT

VDIFF(MAX) = 4.5 V125 mV = 36 V/V

(6)

The following equation calculates the gain-setting resistor value using the INA823 gain equation shown in

Equation 2:

RG = 100 kΩG − 1 =2.86 kΩ

(7)

Use a standard 0.1% resistor value of 2.86 kΩ.

Next, make sure that the INA823 can operate within this range by verifying the Input Common-Mode Voltage vs

Output Voltage curves or by running the values in the Analog Engineer's Calculator.

Applying VCM(Max), VS, and gain results in a maximum valid output voltage of VOUT of 3.6 V.

INA823

SBOSA75A – JULY 2021 – REVISED JULY 2021

www.ti.com

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