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.
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