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Old 29th August 2007, 04:18 PM   #1
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Default CS4329 DC offset

I have a CS4329 based DAC and there is a small amount of DC offset on each channel. I can measure the offset on the input and output of the opamp.

I found this schematic for a different CS4329 DAC board and it looks VERY similar to mine. Could resistor tolerance mismatch be the cause?

http://www.quadesl.com/pdf/cs4329_dac.pdf

Click the image to open in full size.


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Old 29th August 2007, 05:36 PM   #2
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Time to get one of those fancy voltmeters that actually provide readings in numbers

Are you are measuring the same offset at the ouput of the dac (between the two outputs)?
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Old 29th August 2007, 06:01 PM   #3
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When you say a small amount of DC offset, the OPA2132 does give +- 2-4mV in itself which is quite acceptable, what opamp are you using?

Cheers George
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Old 29th August 2007, 06:17 PM   #4
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Very sorry guys.

OPA2134PA....

Left input: 1.258v
Right input: 1.267v
Left output: 20mv
Right output: 9mv

CS4329 outputs....

Pin 13: 2.194v - Pin 14: 2.188v = 6mv
Pin 17: 2.206 - Pin 18: 2.205 = 1mv

After closer examination the above circuit is identical to my DAC. The parts values differ a bit though.
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Old 29th August 2007, 06:51 PM   #5
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Yes, resistor mismatch appears to be problem.
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Old 29th August 2007, 11:00 PM   #6
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Everyone shares the blame: DAC, opamp, mismatched resistors. You should certainly match the resistors as this has an effect on distortion. It is not so obvious what to do with the remaining offset.
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Old 30th August 2007, 03:51 AM   #7
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Thanks guys, I'll try replacing all the resistors with hand matched values and see what I come up with.
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Old 30th August 2007, 08:31 AM   #8
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A quick thought...... If I replace the resistors and there is still some offset is there any way to build a trim circuit to null it? A pot across the 5v supply connected to the input of the opamp?
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Old 30th August 2007, 09:20 AM   #9
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Quote:
Originally posted by theAnonymous1
A quick thought...... If I replace the resistors and there is still some offset is there any way to build a trim circuit to null it? A pot across the 5v supply connected to the input of the opamp?


There will certainly be offset even with perfectly trimmed resistors.
What you propose is possible but not without issues. The trim circuit will also unbalance the differential gain so other resistors will need adjustment. Using the 5v supply is certainly not a good idea though - it will be injecting ps noise.

Cap coupling is almost tempting.
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Old 30th August 2007, 01:30 PM   #10
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Are you telling me I'm SOL? Is there nothing that can be done?

I went through great pains to get rid of the caps on the amplifiers input, this offset problem is the only thing keeping me from DC coupling (my goal).

I'll do anything it takes!

EDIT: I think I've got it!

When I put a 1Mohm resistor across R17 the offset goes from 20mv to 48mv, if I put it across R15 it goes down to -7mv. Those two resistors measure 14.93k and 15.01k. A significant mismatch no?

Thanks!
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