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Old 5th March 2007, 09:28 PM   #1
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Join Date: Dec 2006
Default passive filter + non-inverting amp for output stage..

Hi all,

I'm new to the forum and not exactly an expert in electronic..

I need to improve the analog stage of a DAC (the output of the DAC is a voltage source). Here is my idea: using a passive second order filter followed by an op-amp in a noninverting amplifier configuration. For the opamp I was thinking of the AD825.

Also the DAC is differential, but I need an unbalanced signal. Should I just "ignore" the inverted signal from the DAC or employ it for common mode noise rejection?

Since the DAC is differential, should I just use a transformer?

Every comment/suggestion is GREATLY appreciated.


Luigi S.
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Old 6th March 2007, 04:13 AM   #2
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Location: Adelaide
You probably need to step back a moment and think about what it is you want to achieve. Initially identify what it is you think is wrong that you want to improve. The DAC output may not be the prime source of trouble, or the most influential place that you can usefully improve.

As to the DAC output - OK - it is voltage - so it almost certainly has a transconductance amplifier inside (which isn't a great start - but what you have.) And it is differential. Ignoring one side of the output has the unfortunate effect of dropping your signal to noise by 6db (as a first approximation anyway.) So you really don't want to do this. There is nothing to say that you can't put a passive filter on each leg, and then drive a differential receiver. But think about the above first.
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Old 6th March 2007, 06:34 AM   #3
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Hi Francis,

thanks a lot for your answer. Well, the DAC is that of a behringer DCX2496 (a digital crossover). This is a PA unit and the power supply was modified, voltage regulation improved, the output was simplified but it still uses the same opamp of the stock behringer. So the output now is: the opamp which performs filtering and also "merge" the two out pf phase signals from the DAC + a buffer (this was not originally on the Behringer).

And this is the reason why I think the analog stage with the opamp is the problem..

What is a differential receiver?

thanks again,
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Old 12th March 2007, 03:09 PM   #4
rtd is offline rtd  France
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Location: RP
Look at to use a complete passive output stage for DCX2496 (if you haven't read).
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