SBOSA75B July   2021  – November 2021

PRODUCTION DATA

1. Features
2. Applications
3. Description
4. Revision History
5. Device Comparison Table
6. Pin Configuration and Functions
7. Specifications
8. Detailed Description
1. 8.1 Overview
2. 8.2 Functional Block Diagram
3. 8.3 Feature Description
4. 8.4 Device Functional Modes
9. Application and Implementation
1. 9.1 Application Information
2. 9.2 Typical Applications
1. 9.2.1 Resistive-Bridge Pressure Sensor
2. 9.2.2 Supporting High Common-Mode Voltage in PLC Input Modules
10. 10Power Supply Recommendations
11. 11Layout
12. 12Device and Documentation Support
1. 12.1 Device Support
1. 12.1.1 Development Support
2. 12.2 Documentation Support
4. 12.4 Support Resources
6. 12.6 Electrostatic Discharge Caution
7. 12.7 Glossary
13. 13Mechanical, Packaging, and Orderable Information

• D|8
• DGK|8

#### 9.2.2.2 Detailed Design Procedure

The gain of the IA is calculated so that the circuit operates at unity gain, where VOUT = VDIFF.

The single-ended input impedance, Rin(SE), of the circuit is the sum of the scaling resistors (Rf + Ri). To minimize the error that is caused by the tolerance of the scaling resistors, keep Rin > 1 MΩ.

Ideally, choose the resistors so that Rf / Ri = Rf' / Ri'. In the real world, designers have to trade off between the mismatch of ratios that degrades the common-mode rejection ratio (CMRR) and the acceptable cost for the design.

The following text describe how to estimate the CMRR performance of the external resistor scaling approach. In the calculation of CMRR, the following factors are considered:

• Take into account the number of resistors, which is estimated by √n, where n is the number of resistors applied. In this case, this estimation results in a factor of 2.
• ΔR / R is the resistor matching ratio. The resistor tolerance for all four resistors is 0.1%.
• Take into account that a normal production distribution of the resistor value with a standard deviation of ±3 σ (99.7%). In this case, the assumption results in a factor σ = 1/3 = 0.33 into the equation.

Equation 9 calculates the common-mode rejection ratio with given factors:

Equation 9.
Equation 10.

The scaling ratio G1 is calculated by:

Equation 11.

where

• Rf is variable
• Ri is fixed at 750 kΩ.

Figure 9-6 shows a comparison between the CMRR performance at worst-case (α neglected) and considering normal distribution for different gain settings of G1.

For more details about the calculation of CMRR, see the Difference amplifier (subtractor) circuit analog engineer's circuit.