Hi all,


I designed a simple 3-opamp instrumentation amp (ins-amp) in order to buffer the output of a low-IF receiver front-end, but when measuring the output noise of the ins-amp, there was a lot of noise between the frequency of 1 MHz and 5 MHz (See the attachment below). This noise did not appear in simulation, and the stability of the ins-amp seemed fine in simulation also. Does anybody have any clues as to where this noise originated, and how I can take care of it in future?


thanks,
Aaron

Hi Tosei,


thanks for the reply. It is not a chopped design. It was actually quite a quick and dirty design of just three two-stage miller op-amps. The ins-amps were designed to contribute negligible noise to the actual design so I did not fabricate them separately. However, they were powered by a separate supply from the main DUT, and when I powered down the main DUT so that only the ins-amp was on, I got the noise PSD I showed above. The only blocks which were powered were the op-amps in the ins-amps. Since the ins-amp was designed to drive 50-ohm, there was nothing of significance in between the ins-amp and the spectrum analyzer (a DC-block with 1 kHz corner and a cable...).

The first thing that came to mind was some kind of instability, but I figured that if it was really unstable I would see a single sharp and high peak.


thanks,
Aaron