Dear all,

I have simulated the PSRR of a bandgap which uses a two-stage amplifier with the first stage chopped and a large off-chip compensation capacitor connected to the supply.

If the bandgap is unchopped, the PSRR looks ok. If the chopper is actived the low frequency PSRR is reduced and is peaking around a frequency which depends on the large off-chip capacitor and the chopping frequency.

I can imagine that the chopping function will affect (modulate) the supply disturbance and consequently influence the PSRR. But what the exact mechanism behind it? Can somebody explain? References?

Cheers

Hi Berti,

You should be careful when considering PSRR of a chopped amplifier, since the chopped amp is not an LTI system, and as such the PSRR concept cannot be directly applied since - as you suggested - intermodulation products will arise.
However, if we consider PSRR of a chopped circuit as the transfer function of output components at a certain frequency generated by means of a disturbance on the supply voltage at the same or any frequency then the definition is consistent.
I assume that  your lower PSRR at DC with the chopper ON is the result of a pac analysis. Following the definition for PSRR I described (which actually is what a pac analysis measures as I´m sure you know) then such result makes sense: assuming your circuit is unbalanced (otherwise at least for a differential circuit the PSRR is infinite) then you will have a finite PSRR and therefore the supply noise will be converted into differential signal by means of a certain transfer function that depends on the topology of your circuit. If such noise gets injected in between the input and output chopper switches, and if it happens to have a frequency content AT the chopper frequency or one if its harmonics, then the result at the output of your opamp will look like a DC shift. That means offset is created at the output due to a supply noise injected into the chopper.  synchronized with the chopper frequency. The result is a reduced PSRR at DC.

For the same reason is that you see a high rejection at the chopper frequency: any supply noise injected into the chopper after having been converted into a differential signal will not create a noise component at the chopper frequency, unless such noise is a "DC noise" or simply offset.

Sorry for the long answer. Hope it helps
Best
Tosei

The peak is probably a result of the fallof of the loopgain of your amplifier+feedback circuit. Does the peak shift when you change the loopgain?

By falloff I intended to say the high frequency behavior of the loopgain. The graph looks familiar, although it might be better to get rid of the spike u have in your PSRR. But again, I don't know the numbers or application, so maybe it is not a big deal.

The basic explanation of "chopping decreases the gain of your amp" is a bit too simplified, and imho incorrect.