Hello,

@andreas, i have same "pop corn" problem before, and i didnt quite know the way to solve it.

if i tap on the SOT23 lightly with a insulator stick, it can trigger/reproduce the noise.

Just for noise testing even a more normal resistor would be OK to test a different current. Its allways the possiblity to have a certain current or temperature the zener diode does not like. So it might be interesting to do a slow temperature ramp (e.g. 1 degree), while doing a longer noise measurement.

I did several tests on LTZ#5:

Knocking on critical components -> no additional events.

Changed current through Zener: 1K resistor in parallel to R1=120 R -> no change in behaviour.

Additionally changed temperature setpoint: 22k resistor in parallel to R5 = 1K -> the first measurements showed no popcorn noise events. Then I did my "last noise measurement". Result: extreme number of popcorn noise within this measurement.

The following measurements showed again numerous popcorn noise events.

So the problem was not gone. (only a "statistical" improvement).

I decided to built the 2 further references immediately to check wether the chopper (LTC2057) is the guilty.

So the 2nd device with LTC2057 (LTZ#6) showed no popcorn noise.

Whereas on LTZ#4 (with LT1013A) showed 1 single (small 0.35uV event = within 3 sigma limits) during 15*100 sec measurement time.

I tried to differentiate between OP-Amp and zener by measuring noise at the OP-Amp Output (the gate of the BF245C FET).

The interesting result is that the noise at this point is a factor 2.9-7.6 higher than at the zener.

The average relative amplitude on LTZ#5 between noise is factor 2.9

The average relative amplitude of the popcorn noise jump heigth is factor 2.0

So the probability is higher that the Zener is the guilty.

But anyway: I think I will try exchanging the current regulator OP first. (since it is cheaper).

With best regards

Andreas