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Technical Tidbit - January 2014
Analog Designer's Notebook Part 2 - The Importance of Good Power Bypassing

Non-inverting opamp circuit with parasitic inductance

Figure 1.
Non-inverting Op Amp Circuit with Parasitic Inductance in Power Leads

Abstract: Power bypassing is more important than one would think for analog ICs like op amps. Even low frequency op amps need high quality power bypassing to prevent the op amp from breaking into oscillation at a much higher frequency than the unity gain bandwidth of the device. The reason for this and recommendations are presented.

Figure 1 shows a typical inverting op amp circuit with parasitic inductance in the power connections. This inductance might be only a few tens of nanohenries and still cause problems, even for low frequency 1 MHz unity gain bandwidth op amps. Also shown in Figure 1 is the CRFI capacitor described in the November 2013 Technical Tidbit for increasing immunity to external EMI.

A problem occurs with inductance in the power leads because the inductance essentially adds another feedback loop from the output to the power pins, through the last stage or two of the op amp back to the output. The bandwidth of this feedback loop can be much higher than the bandwidth of the complete op amp. I have seen 1 MHz unity gain op amps oscillate at frequencies of over 20 MHz in response to inductance in the power leads!

This unwanted feedback loop occurs because driving current to the output causes drop across the power lead inductance which then feeds high frequency energy into the power pins (essentially another input). The op amp rejection of noise at the power pins is generally very low or non-existent at 20X the unity gain frequency of the op amp. Feeding capacitive loads can increase this effective feedback signal even further.

Many circuits utilizing 1 MHz unity gain op amps are constructed on two layer PCBs to save costs as they are perceived to be low frequency circuits, not needing the power and ground planes. Thus, the inductance between the op amp power pins and the nearest bypass capacitor is increased compared to multi-layer PCBs, aggravating the effect.

When designing op amp circuits, even low frequency ones, be sure to provide bypass capacitors right at the power pins, making as small a loop as possible between the + and - power pins.

Summary: Parasitic inductance in the power leads of op amp circuits can cause oscillation at frequencies well above the unity gain bandwidth of an op amp. Always provide bypass capacitor(s) between the power pins as close as possible to those pins to insure low parasitic inductance, even for low frequency circuits.

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Copyright 2013 Douglas C. Smith