JAJSS97 December 2023 RES11A
PRODUCTION DATA
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In this simple difference amplifier configuration, the nominal CMRR is calculated as:
The term AD is the differential gain of the circuit, and the term ACM is the common-mode gain of the circuit. These are defined as the following:
Therefore,
When this expression is evaluated with the definitions given in Section 7.3.1, assuming the worst-case scenario of the most unbalanced divider matching possible,
Because tRx2 << 1, the worst-case CMRR is approximated as
By definition, the parameter tM describes the effective error that is otherwise equivalent to 4 × tx for an unmatched divider network, and so the maximum value of tM can be used to calculate the same worst-case result. Likewise, the typical value of tM can be used to approximate the typical CMRR.
For example, the worst-case CMRR for a RES11A40 device with G = 4 is approximately 74.0 dB, with a typical CMRR of approximately 95.4 dB. In comparison, implementation of a comparable G = 4 difference amplifier with unmatched 0.1%-tolerance resistors results in a worst-case CMRR of approximately 62 dB.
In a difference amplifier configuration, the CMRR of the op amp contributes error as well. The op-amp CMRR is considered in parallel with the CMRR of the resistor network, as per the following equation:
Additional mismatches in the divider end-to-end resistances reduce the effective CMRR of a difference amplifier. While the low absolute tolerance span of the RES11A (235 ppm typical) helps reduce these concerns, parasitic trace resistances can lead to additional mismatches that impact the CMRR specs. Bench results from a difference amplifier implementation of the RES11A40 and the OPA210 are presented for various deliberate input-impedance mismatches.