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 = 5.5 V

Excitation voltage

VEXT = 5.0 V

Occlusion pressure range

P = 1 psi to 12 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 = 3.0 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 2.5 V. For the maximum pressure of 12 psi, the bridge common-mode voltage, VCM(MAX), is

calculated by:

VCMMAX =VDIFF2+VCM(zero)

(5)

where

•

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

(6)

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

VCMMAX =150mV2+2.5V=2.575V

(7)

Similarly, the minimum common-mode voltage can be calculated as,

VCMMIN =−150mV2+2.5V=2.425V

(8)

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.

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

voltage:

G =  VOUT

VDIFF(MAX) = 3.0 V150 mV = 20 V/V 

(9)

Considering, INA351 is a selectable gain INA with gain options of 10, 20, 30, 50, the INA351ABS with GS tied

high enables G = 20 maintaining the maximum output signal swing for the ADC.

Next, let us make sure that the INA351 can operate within this range checking the Input Common-Mode Voltage

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

convenience. Looking at Figure 9-5, we can confirm that a output signal swing of 3 V is supported for the input

signal swing between 2.425 V and 2.575 V, thus making sure of the linear operation.

INA351

SBOSAD5 – DECEMBER 2022

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