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What's the best dual I/V opamp these days???
What's the best dual I/V opamp these days???
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Old 10th April 2019, 07:12 PM   #21
diyiggy is offline diyiggy
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excellent...

I'm looking forward to know if the op will find any better oaps than the AD844 or OPA861 for that I/V purpose. I'm confident of his hearing for the final choice.
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Old 16th April 2019, 03:16 AM   #22
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by abraxalito View Post
LM6172 has long been one of my recommendations for I/V ever since I tried it in an Asus soundcard many years ago. I think that card was using PCM1792 so probably its well suited to 1796 too.

I'm working on a DAC using AD815, first gen PCBs have some errors, hopefully after a re-spin I'll share gerbers.
Just got around to changing the OPA2132 I/V for the PCM1796 dac to the LM6172 for I/V duties, and all I can say abraxalito it's far better in every respect, I'm still using a OPA2132 as the output buffer/filter though, it seem to be a great pairing for I/V and buffer for the PCM1796.

What a great I/V opamp the LM6172 is, I would have never thought it would be this good.

Cheers George
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Last edited by georgehifi; 16th April 2019 at 03:19 AM.
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Old 16th April 2019, 09:04 AM   #23
georgehifi is offline georgehifi  Australia
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sorry that should be OPA2134 not 32

Cheers George
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Old 16th April 2019, 09:11 AM   #24
dreamth is offline dreamth  Romania
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Quote:
Originally Posted by georgehifi View Post
What a great I/V opamp the LM6172 is, I would have never thought it would be this good.
Cheers George
This was one of the most important evidence produced by Abraxalito a while ago here and i fully agree with it.
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Old 16th April 2019, 09:26 AM   #25
abraxalito is offline abraxalito  United Kingdom
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If you look at the simplified schematic of LM6172 you'll see its essentially the same as an AD844 but with a buffer feeding the -ve input pin which gives it the characteristics of a VFB opamp. I reckon the PSRR of this buffer is likely its weakest link, most likely its a diamond (as shown in various LT amps with the same architecture). Incidentally has anyone tried those Linear Technology offerings as I/V, I mean LT1818/9 and LT6274/5 ?
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Old 16th April 2019, 09:37 AM   #26
dreamth is offline dreamth  Romania
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The psrr is good enough...we have fast regulators anyway, what really troubled me was the current noise as i would have wanted to use it with 7x lower iout dacs of the past.THS4631 would probably qualify for that purpose better than lt 1818/6274. It has about 800v/us and much lower current noise while the voltage noise is just about the same .Its serious drwback is the quiscent current though.At least its pad is connected to gnd not at v- as in ada4898.

Lt6274 might have that same buffer as 6172 as it shares the low psrr.In its datasheet is a diamond buffer indeed...maybe the missing part of the lm6172 simplified schematic.
I had a serious reason to get back to the audio op-amps of the past(30 years old) with my own circuits as analyzing all these new op-amps they never have it all in one package.

Last edited by dreamth; 16th April 2019 at 09:52 AM.
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Old 16th April 2019, 09:57 AM   #27
abraxalito is offline abraxalito  United Kingdom
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Current noise doesn't look to me to be a problem but then I'm only using a 16bit DAC. For LT1818 its 1.2pA/rtHz which means you'd need to have a 5k FB resistor to get to the same magnitude as the voltage noise (6nV/rtHz).

With JFET opamps it pays to watch how hot they're getting - I agree the THS4631 is a tad on the toasty side. JFETs when warm have considerably higher current noise.
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Old 16th April 2019, 10:02 AM   #28
chris719 is offline chris719  United States
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What's the best dual I/V opamp these days???
LM6171 was really designed for driving stuff, it's unity gain bandwidth isn't that impressive, but the high slew rate lets it achieve that bandwidth with large output swing.

It's a bit noisy and distortion in the audio band could be better, as you pointed out.

THS4631 is a nice op-amp, at least you get a FET input for all that voltage noise. If you're going to go with a single and you want a JFET input, then might I suggest OPA828.
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Old 16th April 2019, 10:18 AM   #29
dreamth is offline dreamth  Romania
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opa828 looks good, the price though...not up to my liking...I dream at 0.5 dollars each...
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Old 16th April 2019, 10:20 AM   #30
abraxalito is offline abraxalito  United Kingdom
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On Taobao AD8065 are less than $0.50 each.
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