9.2.1.1 Design Requirements

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

Table 9-1. Design Requirements

DESCRIPTION

VALUE

Single supply voltage

VS = 5.0 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

ADC supply voltage

VADC(fs) = 5.0 V

Full-scale range of ADC

VOUT = 0.25 V to 4.75 V to avoid saturation

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 + VCMzero

(1)

where

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

(2)

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

VCMMAX =150 mV2+ 2.5 V=2.575 V

(3)

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

VCMMIN =−150 mV2+ 2.5 V=2.425 V

(4)

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

respect to the required VOUT swing of maximum 4.75V to avoid saturating the amplifier and the ADC running at

5V supply.

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

voltage:

G= VOUT

VDIFFMAX = 2.25 V

150 mV=15 V/V

(5)

Considering, INA351A is an INA in G = 10, an additional gain stage in G = 1.5 is added.

Next, let us make sure that the INA351A 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.

INA351A

SBOSAG2 – JUNE 2023

www.ti.com

24

Submit Document Feedback

Copyright © 2023 Texas Instruments Incorporated

Product Folder Links: INA351A