Re: Summing-Junction Snooping

At that level of accuracy, beware thermocouple effects.

If you are compensating a slow drift in offset, chop slowly and sinusoidally, then the 'spikes' will matter less.

It's all on one board, and the power level is low, so that shouldn't be a huge issue, I don't think. Gradients on the board should be way under

I'm not the one doing the chopping--the spikes come from the CMOS switches inside the chopamp.

The noise canceller works by splitting a larger photocurrent using a BJT diff pair, and adjusting the split ratio until the current in one arm exactly cancels a smaller photocurrent derived from the same laser.

There are various fine points, but because the diff pair is a highly linear current splitter, the fluctuations split the same as the DC, so by adjusting the DC to zero, one in principle obtains cancellation of the fluctuations at all frequencies. A slow servo loop lets you do AC-coupled measurements down at the shot noise even with noisy lasers.

With a bit of math, you can use the delta V_BE of the diff pair to do the same thing inside the feedback loop bandwidth.

An offset voltage in either the TIA or the integrating servo amp causes the cancellation to be in error by

delta I = V_os / R_F.

With a 5k ohm R_F, a millivolt of offset makes 200 nA of current imbalance. With a 100-uA photocurrent, that limits the cancellation performance to

Amax = 20*log(100uA/500nA) = 54 dB.

It's better than that at higher photocurrent, but I'm chasing an honest

Cheers

Phil Hobbs

I think several hertz is the sweet spot, because there are situations such as turn-on where it’ll start way out of whack and have to recover.

It’s more the fixed part of the offset than the drift that’s the problem, except at extreme temperatures.

Cheers

Phil Hobbs

Trimpot!

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Am 30.01.26 um 10:17 schrieb Liz Tuddenham:

I once had a chopper where it was enough to balance VCC/VEE (which were not equal!). Gate switching levels were fixed and came from a Xilinx Coolrunner2.

Gerhard

The snooper is cheaper, more accurate, and (I think) will cause no troubles.

Cheers

Phil Hobbs

The snooper’s main rationale is that it’s not in the main signal path—it’s just a bag hung off one side to balance out the offsets of the otherwise very nice AD822 TIA and AD8605 servo/driver.

The signal paths are much faster, up to 10 MHz.

Cheers

Phil Hobbs

The canonical way to do that is by grounding one end of an inductor and switching the other end between VCC and VEE. The duty cycle sets the ratio.

Good for rail splitting, too.

Cheers

Phil Hobbs