Not all architectures are created equal. Just like you wouldn’t pick a single tool to build a house you shouldn’t assume all instrumentation amplifiers (INA) operate optimally in all applications.
Common mode rejection ratio (CMRR) and common mode rejection (CMR) measure the ability of a differential input amplifier, such as an op amp or an INA, to reject signals common to both inputs. In other words, as the common-mode voltage differs from how it is specified in the data sheet, an offset voltage appears at the input. This offset voltage is in addition to the initial input offset voltage and also amplified by the differential gain of the device or circuit!
The technical definition for CMRR is the ratio of differential gain to common mode gain. It’s measured by changing the input common mode voltage and observing the change in output voltage. This change is referred to the input by dividing by the gain and is thought of as an input offset voltage variation. CMRR is typically reported in decibels (dB) for easy interpretation and comparison. There is not an industry standard and the CMRR and CMR are often interchanged.
Common-mode gain, and thus CMRR, is dependent on a few amplifier design factors, including:
CMRR variation with frequency is illustrated in the INA128 curve below.
Figure 1: INA128 CMRR vs frequency with a gain of 1, 10, 100, 1000V/V
There are a few different ways to create INA’s, including:
Figure 2: 3 op amp INA
Figure 3: 2 op amp INA
Figure 4: Current mirror INA
Modern design techniques and process improvements have drastically improved the CMRR in low gains and across frequency as you can see with the PGA281, an INA with programmable internal gain. The PGA281 utilizes proprietary architecture techniques and precision process matching to improve unity gain CMRR by 30dB compared to traditional INAs.
Figure 5: PGA281 CMRR vs Frequency in unity gain compared to traditional instrumentation amps
For more information on how to determine the ins and outs of instrumentation amplifiers check out my Engineer It video. Or, read my first post in this series on what you need to know about CMRR- The op amp.
Thanks for reading!
I think the △Vos/△Vcm should be represented by CMR , not CMRR
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