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Old 7th May 2011, 07:42 PM   #1
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Default Using PGA23XX with dac with dc offset

I forgot i had a pga23XX setup ready to go...

Ok here is my setup:
amb gamma2 dac
PGA2320 with a +-12v supply (it works from 5-15v)
PGA is jumpered for zero gain mode
Output will have dc blocking caps.

The gamma2's output buffer is not bipolar so its power supply is 5v with a 2.5v dc offset. It is normally cap coupled to the next device. My question is can i run the output directly from the gamma2 to the PGA, and then use dc blocking caps at the PGA's outputs?

I know this wouldnt work in gain mode since the dc offset would be amplified, and after 3-5db of gain the signal would hit the rails. In my mind this setup is just a couple of opamps in series buffering a 2.5v dc signal. This isnt a case of running dc through a pot or anything.
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Old 9th May 2011, 11:07 AM   #2
Bonsai is offline Bonsai  Taiwan
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Why use the PGA then?

If you have to, then I would capacitively couple the INPUT and not do it the way you propose.
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Old 9th May 2011, 11:27 AM   #3
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Quote:
Originally Posted by nightanole View Post
I forgot i had a pga23XX setup ready to go...

Ok here is my setup:
amb gamma2 dac
PGA2320 with a +-12v supply (it works from 5-15v)
PGA is jumpered for zero gain mode
Output will have dc blocking caps.

The gamma2's output buffer is not bipolar so its power supply is 5v with a 2.5v dc offset. It is normally cap coupled to the next device. My question is can i run the output directly from the gamma2 to the PGA, and then use dc blocking caps at the PGA's outputs?

I know this wouldnt work in gain mode since the dc offset would be amplified, and after 3-5db of gain the signal would hit the rails. In my mind this setup is just a couple of opamps in series buffering a 2.5v dc signal. This isnt a case of running dc through a pot or anything.
Well, since the PGA is single ended, the best way is to send the DAC balanced signal into a balanced-to-single ended opamp stage (see DAC datasheet). The output of that stage is at 0V DC and can be directly coupled into the PGA and you can use the PGA as it is meant.

Otherwise, as Bonsai said, why bother with the PGA?

jan didden
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Old 9th May 2011, 01:02 PM   #4
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I have some amps that need to be ac coupled due to their own dc offset. Im asking if its ok to dc couple my balanced to se converter that has a 2.5v offset, to my PGA.
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