Gfyuki

I am wondering why the feedback resistor values specified in the datasheet of the AD797 are so small. My understanding is that the low feedback resistances keep the noise small, but isn't it also not ideal to have large currents flowing through the feedback network? My understanding was that feedback resistors should be >1k.

Here is a link to the datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/ad797.pdf

And a picture of an example application:

26.1 ohms seems like far too small a value for feedback resistors.

This op-amp boasts input noise of 0.9nV/rt.Hz, which is roughly equal to the Johnson noise of a 50 Ω resistor over the human audio bandwidth. If you aren't putting resistors smaller than that around it, you're wasting some of this op-amp's performance, and probably should be buying something cheaper.

A graph in the datasheet of the opamp shows it clipping with an output voltage swing of only plus and minus 2V into the 26.1 ohm resistors which might not be enough.

If you refer to figure 34, the total noise (assumed to be the white noise region for both en and in) is approximately 8x better at 10 ohms source resistance compared to 1K.

Remember that it’s not only the noise voltage and the Johnson-Nyquist noise from the feedback resistors but the input noise current multiplied by the resistance seen looking out from the inverting input.

The transistors at the input are run at very high currents so the voltage noise is quite low, but there is a cost in current noise.

They all add in quadrature, of course, since they are typically uncorrelated.

The resistors as shown are tolerable for smallish output swings and if you don’t care too much about accuracy of gain.

Datasheet states that with +/- 5 volt supplies minimum load is 30 ohms. At +/-15 volts minimum load is 200 ohms. Output resistance is 3 milliohms. GBW at G = 10 is 8MHZ. This is a awesome op-amp. It is not the fastest or has the lowest noise for audio purest, but for a GP op-amp is it much better than just 'ok'.

Due to a fast slew rate the board layout needs to be as good as if this was an RF amp IC. 100nF bypass capacitors right at the supply pins, or within 1/4" or 7mm. Close by should be 4.7uF MLCC caps, plus within a few inches 100uF aluminum caps for wide band filtering, and a stable supply when driving 50 ohm or even 600 ohm loads.

The resistors shown in the schematic are the lowest safe value possible, and still maintain somewhat normal operation. The idea here is to get the resistor noise levels to a minimum. Many op-amps could not tolerate such low resistor values, but that resistance range is on the datasheet. It is implied on the datasheet that at such low resistance levels supply voltage should be +/- 5 volts.

With honor to Walt Jung (of ADI, et al), low values of resistors can cause detectable THERMAL distortion. Drive the opamp with 20Hz and 2,000Hz; use a spectrum analyzer, and you'll see the 2,000Hz output with some 20Hz sidebands.

Which means what? Use physically larger resistors. Or experiment with resistors having different resistive element structures? The very thin metal-file-spiral-trimmed has a very fast timeconstant for heating/cooling, as heat is dumped into the ceramic/clay core.