What you need to know about CMRR – Instrumentation amplifier topologies (Part 3)

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:

• Design process variation of:
• Source and drain resistor matching
• Gate-drain capacitance
• Forward transconductances
• Gate leakage currents
• Output impedance of the tail current source
• Changes with frequency due to tail current source’s shunt capacitance

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:

• The three op amp INA: A difference amplifier with buffered inputs like the INA826. For more information on the difference amplifier topology check out my previous blog on what you need to know about CMRR- The instrumentation amplifier. This topology resolves the low impedance limitation of the difference amp. The input stage is used to gain up the differential voltage improving signal to noise ratio and common mode rejection. The down side is larger size and cost.

Figure 2: 3 op amp INA

• The two op amp INA: This simpler INA, like the INA126, has fewer high precision resistors to contend with and fewer op amps. This can translate into lower cost, smaller packages, and potentially lower quiescent current. The pitfall of topology comes with the differences in noise gain of the two op amps. This makes CMRR degrade faster across frequency than the three op amp INA.

Figure 3: 2 op amp INA

• Special topology INAs: New architectures are coming out that are pushing the boundaries of INAs. Current mirror based designs, like the PGA281, convert the input signal to a current.  Current-mode signal processing   providrejection of common-mode input voltage and power-supply variation without accurately matched resistors.

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.