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Best opamp for I/V conversion? (DAC)
Best opamp for I/V conversion? (DAC)
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Old 28th November 2019, 02:05 AM   #431
Terry Demol is offline Terry Demol  Australia
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Quote:
Originally Posted by bluealbin View Post
Here is a LTspice sim, and I got a thd of -90db.
That's better.

I suggest doing open loop circuit sims at 10kHz or 20kHz. This will tell real
story of linearity when all the non linear capacitances come into play.

Also you will need to take care of OP offset.

Having said all of the above, this circuit will probably sound quite good, maybe
really good.

PS, you also should account for DAC OP Z. In the case of 1794 I use 200k or so.
Apparently it is even higher than that but 200k is safe. It will affect the distortion
to some degree.


TCD

Last edited by Terry Demol; 28th November 2019 at 02:17 AM.
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Old 28th November 2019, 02:19 AM   #432
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by bluealbin View Post
Here is a LTspice sim, and I got a thd of -90db.
If the best SQ is your aim don't just sim for THD, also check PSRR by setting the supply to produce an AC voltage (with the source turned off) and sweep it across the frequency range monitoring the output.

<later> Since you provided the .asc I ran a quick check - the basic PSRR is fine, worst case about -67dB @20k on the +ve rail. -ve rail is better. Next step is to check how the circuit modulates its own supply rails and turns out that's not quite so good, there are very likely changes you can make to make the current draw more invariant of signal. But listen to it first before going on to that. I agree with @Terry, it'll probably sound very good.
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Last edited by abraxalito; 28th November 2019 at 02:34 AM.
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Old 30th November 2019, 02:42 AM   #433
bluealbin is offline bluealbin
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Very thanks.

I starts to make my PCB for this circuit, and wish a success
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Old 6th December 2019, 05:56 PM   #434
Jon Lord is offline Jon Lord  Germany
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Did anybody so far try the quite new OPA828 as I/V for PCM1704 or the like ?
It is marketed as an modern upgrade for OPA627/637 and 827 at a fraction of their costs.

The design has special speedup circuitry in the J-FET input stage to avoid slew rate limiting and as OPA627 also has a huge differential input voltage range (no antiparallel protection diodes in input, which TI calls "MUX friendly").

This is so far the lowest distortion J-FET OPamp I know of (Almost OPA1611 performance), but with the advantage of much better settling time and I/V friendly non saturating J-FET-input stage. The noise perfromance should be more than good enough for a PCM1704 or PCM179x.
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Old 6th December 2019, 06:51 PM   #435
DPH is offline DPH  United States
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^ thanks for pointing out this new part, Jon!

30 mA output, depending on the DAC, might run into some issues (which is crazy to think about, but that's how they're getting those crazy high SNR's nowadays), but I would imagine the OPA828 would work well on the front end of a composite with a fast CFA line driver if you're going for that crazy level of specmanship. JFET/CMOS front end, I think, helps with glitch energy (less likely to rectify RFI?), but I could be misremembering things. Done it a few times in my life.

There's also the OPA1656 which is acquitting itself nicely in the low distortion oscillator circuit. Fast, low noise CMOS front end, with a beefy output stage (and bias) seems like a ticket for some of these newer DACs as well.
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Last edited by DPH; 6th December 2019 at 06:58 PM.
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Old 7th December 2019, 01:43 AM   #436
Joseph K is offline Joseph K  Hungary
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In fact, I had put in Yanasan's dual 4499 dac the OPA1656 as I/V converter and the OPA828 as diff/ SE summing. The result is quite spectacular.
I also use since it came out, the 828 as front end in Penasa' s Evolution amp, a composit power amp with dual 3886 buffer (current pump) in output.

Ciao, George

Last edited by Joseph K; 7th December 2019 at 01:48 AM.
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Old 8th December 2019, 10:35 PM   #437
Spartacus is offline Spartacus  United Kingdom
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Quote:
Originally Posted by Joseph K View Post
In fact, I had put in Yanasan's dual 4499 dac the OPA1656 as I/V converter and the OPA828 as diff/ SE summing. The result is quite spectacular.
I also use since it came out, the 828 as front end in Penasa' s Evolution amp, a composit power amp with dual 3886 buffer (current pump) in output.

Ciao, George
Hey George, is it simple to compensate when used for the Penasa amp?
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Old 8th December 2019, 11:36 PM   #438
Joseph K is offline Joseph K  Hungary
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Nice to see You!

It's not complicated, but some parts has to be changed.
We can discuss it!

Ciao, George
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Old 24th November 2020, 05:49 PM   #439
ztwindwalker is offline ztwindwalker
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Quote:
Originally Posted by Jon Lord View Post
Did anybody so far try the quite new OPA828 as I/V for PCM1704 or the like ?
It is marketed as an modern upgrade for OPA627/637 and 827 at a fraction of their costs.
See also opa820/2810.
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