However, if the feedback pole is less than approximately 6 to 10 times the ideal −3dB frequency, connect a

feedback capacitor, CF, between the output and the inverting input of the op amp. This condition can also be

stated in terms of the amplifier low-frequency noise gain. To maintain stability, a feedback capacitor is probably

be needed if:

RFRIN+1 ≤ 6×2π×GBW×RF×CS

(2)

where

RFRIN+1 istheamplifierlow-frequencynoisegainandGBWistheamplifiergainbandwidthproduct.An

amplifier low-frequency noise gain is represented by the following formula, regardless of whether the amplifier is

being used in an inverting or noninverting mode:

RFRIN+1

(3)

A feedback capacitor is more likely to be needed when the noise gain is low, the feedback resistor is large, or

both.

If the previous condition is met (indicating that a feedback capacitor is probably needed), and the noise gain is

large enough that:

RFRIN+1 ≤ 2GBW×RF×CS

(4)

the following value of feedback capacitor is recommended:

CF= CS

2 RFRIN+1 

(5)

If

RFRIN+1 < 2 GBW×RF×CS,thensetthefeedbackcapacitorto:

CF= CSGBW×RF 

(6)

These capacitor values are usually significantly smaller than those given by the older, more conservative

formula:

CF=CSRINRF 

(7)

CS consists of the amplifier input capacitance plus any stray capacitance from the circuit board and socket. CF compensates for the pole

caused by CS and the feedback resistor.

Figure 6-2. General Operational Amplifier Circuit

LMC660, LMC662

SNOSC51D – MARCH 1998 – REVISED FEBRUARY 2024

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