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Old 13th June 2017, 11:47 PM   #1
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Default Calling all I/V gurus

Can this paralleled circuit be done for negative input for I/V stage instead of positive? Any advantages doing this, lower input impedance, distortion, better drive, headroom ?
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Last edited by georgehifi; 14th June 2017 at 12:01 AM.
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Old 14th June 2017, 12:21 AM   #2
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Reason why is, I have this dac that has paralleled PCM1704's with a dual opamp I/V that only uses one, the other is not used, and I've replaced with the dual AD826 which I can parallel the other half as above.

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Old 14th June 2017, 12:41 AM   #3
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Quote:
Originally Posted by rayma View Post
If you use the right scaling resistors ( ~5.4k//110pF from each - input to output),
and ground the +Vin terminals.
There will be lower common mode distortion for the inverting mode operation.
This sound promising, when you say the right scaling resistors, couldn't the 2.7k already in the second pic with the 221pf filter cap across it be used?
Any more suggestion to guide me through this?

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Old 14th June 2017, 12:45 AM   #4
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Quote:
Originally Posted by georgehifi View Post
This sound promising, when you say the right scaling resistors, couldn't the 2.7k
already in the second pic with the 221pf filter cap across it be used?
Possibly, if the current output can drive it to the right level. The data sheet says
only 1mA peak max output current though. To get the nominal 2V out, you'd need
to double the feedback resistors, since the current is halved into each op amp circuit.
Also I'd try breadboarding the circuit. There may be some parasitics to deal with,
for example with stopper resistors in series with the -inputs, etc.

Last edited by rayma; 14th June 2017 at 01:01 AM.
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Old 14th June 2017, 12:54 AM   #5
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Each 1704 dac is 1kohm output impedance and +-1.2mA output, and there are 2

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Old 14th June 2017, 01:04 AM   #6
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Quote:
Originally Posted by georgehifi View Post
Each 1704 dac is 1kohm output impedance and +-1.2mA output, and there are 2.
Right, you may be able to make the 2Vrms level with those values.
1.2mA x 2.7k = 3.24Vpeak or 2.29Vrms.

Last edited by rayma; 14th June 2017 at 01:10 AM.
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Old 14th June 2017, 01:25 AM   #7
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How does this look, treat me as a dummy, I like guidance.

Cheers George
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Old 14th June 2017, 01:27 AM   #8
rayma is offline rayma  United States
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Originally Posted by georgehifi View Post
How does this look.
That's the idea, but you need to see how well it actually works in operation.
Do you have a 100MHz scope to see hf oscillations? These 50MHz wide bandwidth
op amps will almost certainly require a ground plane to work cleanly.

Last edited by rayma; 14th June 2017 at 01:36 AM.
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Old 14th June 2017, 01:44 AM   #9
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Yep, got a good scope, and there is a good ground for it, I'll get around to it in a few days and report back.


BTW are these the advantages?? (lower input impedance, better distortion, better drive, headroom?}
And will gain remain the same?

Cheers George
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Last edited by georgehifi; 14th June 2017 at 01:48 AM.
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Old 14th June 2017, 01:47 AM   #10
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Quote:
Originally Posted by georgehifi View Post
Yep, got a good scope, and there is a good ground for it, I'll get around to it in a few days and report back.
Use SMT parts if possible, with a compact layout, solid ground plane, and good hf bypassing.
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