Question of MT-072 Precision Variable Gain Amplifiers Tutorial

Hello Taksh,

I think the basic reason is stability of the AD797 in voltage follower configuration. If you read the datasheet of AD797, you'll find the follwing statement on page 13:

"With the AD797 , additional input series damping is required for stability with direct output to input feedback. A 100 Ω
resistor (R1) in the inverting input (see Figure 37) is sufficient;"

So you need at least 100 Ohm in the feedback path, 1kOhm is on the safer side. The 20pF capacitor in the feedback path serves the same purpose according to MT-072 (besides the holding of output voltage during switch transitions).

The 1kOhm will contribute sigificantly to the offset of the amplifier - but only in gain=1 configuration, where the offset is not amplified. The 1 kOhm could in principle be reduced to smaller values. I assume the combination of 1kOhm and 20pF was choosen to achieve a good compromise of stability, bandwidth and current noise contribution.

best regards

Achim

Hello Taksh,

I agree with Achim, the damping resistor helps with the stability if you use voltage follower config (unlike other gains where we have feedback resistor). MT-072 probably used 1kohm resistor since the supply stated there is +/-15V. Value of damping resistor can be lower as long as you are within the output current of AD797(30 mA minimum). So using 1Kohm at +/-15V supply will have max of 15mA at the output when your op amp rails on the positive side.
100 ohm resistor can be used but you have to operate at 5V supply.

Hope this helps.

Best Regards,
Kenneth