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Old 29th March 2006, 05:49 PM   #1
00940 is offline 00940  Belgium
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Default Balanced receiver and offset adjustment ?

Balanced line receivers' schematics are easily found. I didnt' find much however with regards to trimming offset due to mismatch in the common-mode voltage.

My application is at the output of a pcm1798 DAC. It has differential, current-out outputs which need to be turned to unbalanced. Digital silence is at -3.5ma, which means offset after the I/V opamps. Most of it will be cancelled but a few mv of offset could still be there. I would like to avoid a coupling cap at the output of the DAC so what's the best way of getting rid of the possible offset left ?

The INA134 has an offset adjustment circuit but with a range limited to 300V. And it is directly referenced to the power rails, something I'd prefer to avoid.

If anyone can point me to something, it'd be much appreciated.
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Old 29th March 2006, 09:36 PM   #2
moamps is offline moamps  Croatia
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Default Re: Balanced receiver and offset adjustment ?

Quote:
Originally posted by 00940
...... It has differential, current-out outputs which need to be turned to unbalanced. Digital silence is at -3.5ma, which means offset after the I/V opamps. Most of it will be cancelled but a few mv of offset could still be there. I would like to avoid a coupling cap at the output of the DAC so what's the best way of getting rid of the possible offset left ?
....
Hi,
The offset at the output of the I-V converter opamp is defined by the Iout and resistor Rf. If you are using standard resistors with 1% tolerance, the offset between the positive and negative outputs can be arround 40mV for Rf=820 ohm +/-1% at worst. This offset appears at the output of the diff. opamp (gain=1). So, IMO, the best and simplest solution is to trim the Rf resistors in the I_V converters to obtain zero DC at the diff. opamp's output.

Regrads,
Milan
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Old 30th March 2006, 06:24 AM   #3
00940 is offline 00940  Belgium
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I will use resistors hand matched to more like 0.1%. Still, this can give me something like 4mV, which is still too much for my taste (feeding it to a headphone amp with a gain of 11 would give me 44mv at the output, which is too much). And then on top of this, there is the offset of the opamps themselves (typically 1 or 2ma).

Trimming the I/V resistors means to put a trimmer in the I/V position and it's something I'd like to avoid since I plan to use high quality resistors in this particular position.

If I wanted to put a servo integrating the offset at the output, where should I feed it back ?
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