Can we be sure that the HP35665A is as quiet at those low frequencies as the very high quality parts under test?
That's a great question. See the attached file.
I first started evaluating the amplifiers with a gain of 60 dB. Then I saw that all three amplifiers had the same low frequency noise. So I increased the gain to 80dB. I only use metal thin film resistors for this low level work. The 10 and 100,000 ohm resistors in the feedback network are
5 10 ppm/ deg C, 0.1% accuracy parts. So the gain is stable. The temperature is also stable to minimize thermocouple and potential issues with op amp offset voltage drift.
The downside of using that much gain is that it limits the high frequency performance of the op amps because of GBWP limitations. I collect data out to 1KHz but because of two different high frequency roll off issues I only report performance out to 100 Hz. The OPA189 with a 14MHz GBWP has much better performance at 1 kHz than other op amps with a 1 or 3 MHz GBWP.
Part of this research is to decide which chopper to use as a preamp for my DSA. Just for fun, I attached is a photo of the 1.5F coupling cap for that preamp. There are a lot of 22,000 ufd caps in that box! It uses a triax connector to connect with the preamp. The switch adds a 10k resistor in series to limit charging current.