Trimmed or chopped: How do you like your op amp?

Depending on the application you’re working on, smaller offset voltages do not always mean higher precision or better DC performance. First, you need to determine the most dominant source of error. If indeed it is the input offset voltage then chopper-stabilized (zero-drift) operational amplifiers (op amps) come in handy. They give you the lowest input offset voltage … and yes, the lowest drift … and yes, practically no 1/f noise. They are extremely useful in high-gain circuits and wider temperature range applications.

In applications with a narrow temperature range such as medical instrumentation, the very low offset drift may not buy you a lot – at least not when compared to the input bias current of your amplifier coupled with the source impedance. This could be a major concern with a zero-drift device, as their input bias current can be orders of magnitude higher than a standard complementary metal-oxide semiconductor (CMOS) or field-effect transistor (FET) input device.

For narrow temperature range applications, you’re better off using a well-trimmed device such as the OPA376 instead of the OPA333, for example. The difference in the initial offset voltage is 15µV, but the difference in input bias current is 190pA! With a source impedance of 1MΩ, your error is 190µV, a much larger value than the initial value for input offset voltage of 10uV of the OPA333.

Another advantage of non-zero-drift op amps is their ubiquitous use in precision measurements. Chopper-stabilized amplifiers can have limitations depending on the application and circuit configuration.

If you’re considering a zero-drift device to use as a buffer, you may want to consider adding a simple filter at its output to avoid the glitches (chopping) that typically reside in the unity-gain bandwidth of the op amp.

For advice on more complex filters, check out this blog on active filtering.

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