This op-amp, although it has a high GBW of 10MHz, is meant to be used at much lower frequencies. This family of parts use autocorrelating zeroing techniques, which involves an internal chopper clock. This clock operates at 70kHz. In high gain applications, especially when these op-amps are cascaded, a 140kHz periodic signal (first harmonic of chopper clock) can be seen on the output.
There are a couple ways to limit the amplitude of this unwanted signal, but it cannot be eliminated completely without heavy filtering.
- A low pass filter (or notch at 70kHz and 140kHz) will do a lot to limit this periodic "noise". The corner frequency should be selected so that the 140kHz and 70kHz are attenuated to an acceptable level. Note that this does limit your bandwidth as well. Filtering can be applied either on the output or in the feedback loop.
- If cascading these op amps, reduce gain of the second (or third) op-amp. High gain propogates noise. The first opamp produces the 70kHz signal inherently and if the second op amp has a high gain (for example, x20), the unwanted signal will be 20 times larger on the output of U2 than on the output of U1. If a third op amp is cascaded, also with x20 gain, the output of U3 will be 400x larger than U1. You can see how this can quickly lead to issues. Depending on the application, using this part with low gain may be sufficient to operate correctly as the 140kHz "noise" remains low.
When using this part above 70kHz, it is very hard to completely eliminate the chopper clock artifacts. Therefore, it is only recommended for low frequency applications (<<70kHz). If a higher bandwidth is needed, the MAX44242 can be used instead. This op amp is the same as the MAX44251, but without the autocorrelating zeroing technique, and therefore without the 70kHz artifacts.