If you were an op amp, instrumentation amplifiers (INA) are like the sibling you never wanted. That’s because in key applications, such as current sensing and sensor signal conditioning INA’s are more powerful and do a better job. INA’s also don’t require as much external assistance and they don’t run open-loop. They are, however, not as versatile and usually a little more expensive compared to op amps, so don’t give up hope.
A key function of INAs is to condition small differential signals in the presence of large common-mode voltages and DC potentials. INA’s are designed to reject the common-mode voltage (VCM) and only gain up or condition differential voltage (VDIFF). How much error is passed to the output by the common-mode voltage is determined by the common-mode rejection ratio (CMRR) specification. Figure 1 defines what common-mode voltage is for an INA and shows the referred-to-input error voltage that changing the common-mode voltage can cause.
Figure 1: Representation of common-mode voltage for INA
INAs are essentially derivatives of the difference or subtractor amplifier with high impedance buffers on the input. So when we talk about INA’s it is best to start with the difference amplifier. As seen in figure 2 the difference amplifier should only amplify the differential-mode signal and reject the common-mode signal. This is true for ideal amplifiers with ideally balanced resistors. Practical applications have to take into account not only the common-mode rejection of the amplifier, but also the error associated with resistor mismatch.
Figure 2: Difference amplifier
Resistor matching is extremely important. In the figure above the matching of R2 to R1 is a major factor in the CMRR of the difference amplifier. The table below shows the potential worst case CMRR for difference amplifier for varying tolerance resistors. Laser trimming monolithic difference amplifiers and INAs , like the INA149 and INA826, achieve greater than 0.01% matching. If you like data as much as me and want play with the resistor values and tolerances to see the impact on CMRR then refer to the spreadsheet in this link.
For a common 3-op-amp INA like the INA128, (see figure 3) the input amplifiers will also contribute to the CMRR, but the resistor matching on the output stage difference amplifier usually dominates this specification. In addition to the high impedance inputs you can also gain up the differential signal in the first stage and reduce the impact of CMRR error.
Figure 3: Three-op-amp INA and its voltage nodes
So now that we know what impacts CMRR what does it matter?
CMRR is important in applications where you are trying to condition small differential signals present across large common-mode voltages or in noisy environments where common noise, such as 50/60 Hz power line noise, exists at each terminal. The CMRR error can impact your measurement if you don’t choose a part that is right for your application. Example applications include current sensing or sensor signal conditioning (See figure 3).
Figure 4: Application examples
CMRR will also vary with frequency for INAs similarly as it does for op amps.
For more info on INAs, watch my Engineer It video on how to determine the ins and outs of instrumentation amplifiers.
Stay tuned for my next blog on how CMRR varies versus frequency as well as the benefits of special topologies of instrumentation amplifiers. And, if you missed it the first time around, check out my post on what you need to know about CMRR in op amps for more info.
Thanks for reading!