Electronics > Metrology

Valhalla 2720GS

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chuckb:
The data sheet for the ICL7652 lists the current noise at 0.004 pa / rt Hz. This number is most likely not a measurement. It may be a shot noise calculation based on Bias current.
The LTC2057 looks like a good replacement with 0.2 uVpp noise. I have not calulated the output noise from the opamp current noise of 0.12pa / rt Hz. But the LTC2057 has the lowest current noise, of the low voltage noise chopper opamps.
The ouput noise could be from residual PWM current spike noise or another grounding issue. My latest project took several revisions to get the grounding correct. Just going from memory I believe the Datum 4910 PWM filters had more sections.
Good luck!

Kleinstein:
There are OP-amps with less current noise than the LTC2057. It still has a good combination of current and voltage noise, especially among the higher voltage ones.
Chances are the OPA186 should be lower current noise (at least that is what the data-sheet says), but is also higher voltage noise.
The ICL7652 should be low current noise, maybe not all the way to 4 fA/sqrt(Hz), but it may not be that much worse.
Current noise specs with AZ OP-amps are anyway not very reliable: quite some false ones, calculated from the bias current and even the real value can depend on the conditions of use. There is a good chance to have correlation in the current noise for the 2 inputs - so a single number is not enough to describe it fully and there are different ways (resistor at one side of both) to have the resistance to test.

The reported noise still looks higher than expected for the ICL7652 - so there are likely other sources.
There are different stages that could be followed to see if the higher than expected noise is alread there at the reference, the PWM DAC or only at the output stage.  Ripple in addition to noise a possibilty.

branadic:
The 0.1 - 10 Hz at the 6.5 V output appears to be similar to the noise of the 10 V output (13 V range). So one of the things to look at is the noise of the averaged reference voltage.
Someone suggested to stop the DAC by pulling the jumper PL201 connected to the 16.777216 MHz clock at the reference generator.

-branadic-

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