3 Improving CMRR with Precision Matched Resistor Divider Pairs, RES11A-Q1

The RES11A-Q1 is a precision matched thin-film resistor divider pair optimized for high common-mode rejection and gain accuracy. This device consists of two precision matched resistor dividers, R_G1/R_IN1 and R_G2/R_IN2, in a small SOT-23 package. The CMRR of a difference amplifier stage is dominated by the ratiometric mismatch between the two resistor dividers. The RES11A-Q1 data sheet specifies the matched ratio tolerance, t_m, of the resistor dividers, which can be used to directly determine the minimum and typical CMRR by Equation 9. The detailed derivations for Equation 9 are found in Section 6.

Figure 3-1 RES11A-Q1 Precision Matched Resistor Divider Pair

Where,

• G is the nominal differential gain in V/V
• t_m is the matched ratio tolerance between resistor dividers R_G1/R_IN1 and R_G2/R_IN2 in Ω/Ω

Figure 3-2 shows the minimum CMRR of a difference amplifier using the RES11A-Q1 precision matched pair compared to standard discrete resistors.

Figure 3-2 Minimum CMRR of Difference Amplifier with RES11A-Q1 vs Discrete Resistors
OPA387, Gain = 1 V/V

The effective CMRR can be further improved by increasing the differential gain of the diff-amp stage. The RES11A-Q1 is available in various ratios between 1 and 10, which allows for a variety of fixed-gain configurations. Figure 3-3 shows the minimum CMRR of a difference amplifier configured with common RES11A-Q1 gain ratios.

Figure 3-3 Minimum CMRR of Difference Amplifier with Various RES11A-Q1 Gain Options, OPA387

Table 3-1 shows the various gain ratios of the RES11A-Q1. These ratios can be configured inversely to achieve attenuation by simply rotating the device package 180°. The nominal gain ratio, or attenuation ratio, is used in Equation 9 to determine the minimum and typical CMRR of the RES111A-Q1 when configured in a difference amplifier circuit.