Hello Ian,
sorry but the info from your side is a bit vague for me.
How did you measure the tens of uV change? (Oscilloscope or multimeter)
Which bandwith/NPLC settings did you use?
Wiring capacitance of your setup? (long coax-line?)
What was the other part number with no issues? (bipolar or CMOS?)
Is your output stage a voltage follower or a inverting cirquit?
I also had some problems with some uV drift measured on DMM for the LTC2057 in voltage follower cirquit.
I blame it on the CMOS protection diodes at the negative input.
But as Ken already described this can be solved with a output filter capacitor and a capacitive isolation cirquit.
It is generally a good idea to have some kind of isolation cirquit on precision outputs.
With best regards
Andreas
Hi,
Yes a bit vague because it was a few months ago and I don't remember much about the exact nature of the noise. I do remember though that it wasn't at the chopper switching freq, it was a lot less.....I think at the time I thought it was picking up noise from something else on the pcb (lcd, Atmel uC etc)......and that I was a bit surprised to see it given I had tested quite a few other chopper op-amps and none of the rest exhibited the same issue......at all!
To answer your questions:-
- Not drift, but glitching. I have never had a problem with drift using any of the choppers I tested.
- Oscilloscope (Rigol 2202) and using a ground clip on the standard Rigol scope lead.
- The 2202 is a 200MHz scope, I tried with the 20MHz limit in & out (as I usually do).
- Output not connected to any DMM or any other leads used when testing noise with the scope.
- Voltage follower x2 gain (5ppm/degC resistors)
- My circuit has a similar setup by way of series resistor to isolate, and a snubber etc.
- The feedback cap is not in my circuit as standard, but only because it made no difference. My circuit is quite noise free (well, as much as the noise floor of the Rigol will allow). I remember trying the feedback cap on the LTC2057 but it made no difference.
I have tried a couple of other precision choppers.
LTC2050 - Limited to 11.5vdc supply max, nice op-amp, but output relatively easily damaged. No current limit or short circuit protected.
LTC1250 - Advertised as a bridge amplifier, but works great in all respects. The best all round solution for me, albeit the most expensive op-amp. Robust output.
I tried a bunch of others.......don't have the part numbers to hand.
Hope that helps.
Ian.