9.2.2 Supporting High-Voltage Common-Mode in PLC Input Modules

Figure 9-5 showcases a high common-mode voltage circuit that is commonly required for programmable logic

controller (PLC) analog input modules. This circuit uses a resistive scaling network in front of the IA.

RG

INA823

VOUT

VREF

0.1 μF

ADC

+15 V

+5 V

750 k

750 k

47 nF

47 nF

33.2 k

+

–

+

–

+

+

V

CM

V

V

R

i

R

R

R

f

C

f

C

─ 15 V

0.1 μF

Figure 9-5. High Common-Mode Voltage PLC Input Module

For a detailed description of the passive scaling approach and more, see the Supporting High-Voltage Common

Mode Using Instrumentation Amplifier application brief.

9.2.2.1 Design Requirements

Table 9-2 lists the requirements for this design example.

Table 9-2. Design Parameters

PARAMETER

VALUE

Supply voltage

±15 V

Common-mode voltage

+36 V / –43 V

Input differential signal

1 V

1 V/V

Minimum dc CMRR

65 dB

9.2.2.2 Detailed Design Procedure

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 our 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 our case, the assumption results in a factor σ = 1/3 = 0.33 into the equation.

INA823

SBOSA75A – JULY 2021 – REVISED JULY 2021

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