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georgehifi 5th December 2012 09:30 PM

I/V using opamps
While waiting for smms73 to get his discrete I/V kit up and going I would like some opinions on opamps for I/V use for the PCM1704.

It seems that most feel that the OPA627 is the favoured choice for I/V use, if one must use an opamp, the important specs for i/v seem to be the following.

Bandwidth 16mhz
Settling time 550ns
Slew Rate 55v/us

The AD825 correct me if I'm wrong looks much better for I/V use on the data sheet
Bandwidth 41mhz
Settling time 80ns
Slew Rate 125v/us

Am I missing something, and is there even better opamps for this I/V use?

Cheers George

jcx 5th December 2012 11:20 PM

PCM1704 - not recommended for new design - $75 ea?? and mono??
hard to argue against the OPA627 specs, but the price... well looking a the DAC price tag, maybe not an issue

AD825 Vnoise spec sucks - likely would degrade the DAC noise performance

slew rate is not so critical if you follow the PCM1704 datasheet circuit and use a feedback C for the 1st pole in the I/V
I'd lower fc enough to be one of the anti-image/reconstruction filter poles
but then the Cload may want a buffer in the loop

at least Class A bias the op amp output with a pull down

looks like the OPA637 could be used with a noise gain C to gnd at the input - has been suggested as a good idea for op amp I/V - shunts some of the fastest edges/glitches directly to gnd

much cheaper options exist, or faster - new linearized SiGe input bjt - but most options today are smt, the best even samller than soic-8

georgehifi 10th December 2012 08:54 PM

Thanks for that JCX, another one that looks interesting is the LT1028 but there's no mention of settling time on their data sheet, very low noise though, what is your opinion of this opamp.

Cheers George

otherside 10th December 2012 09:46 PM

Hi George,

I've tried a few different op-amps with my PCM1792 design.

The main contenders in my opinion are the OPA627, LME49710, AD797, NE5534 and LT1028.

The result with each one is slightly different but not so evident. I'm not going to offer any strong opinion on the sound of each one as it's down to personal preference.

I've gone for the LME49710 metal can case. They are also very expensive (the metal can) but worth it.

The NE5534 sounds horrible compared with the others. The AD797 is sweet and the clarity is sublime. The OPA627 sounds very good but not worth the money (specially for a 1792 design which you need a bunch of).


otherside 10th December 2012 09:53 PM

jcx, do you have a specific smt one in mind?

dirkwright 10th December 2012 09:59 PM

If you are into SMD and can design a really good board, then look into the LME49990. If you want a low impedance feedback loop, than include an LME49600 with the opamp.

jcx 10th December 2012 10:15 PM

if GBW, loop gain, speed are good for I/V it looks like the AD8067 must be great
low noise bjt have large Inoise at the input, this causes V noise when multiplied by the I/V feedback R

with the low output I of the PCM1704 the I/V R is kOhm - most low noise bjt Inoise will cause more I/V output noise than the best fet input op amps Vnoise

OPA211, OPA1611 have low Inoise for this class of bjt op amps - the latter speced "for audio", recommended by Bruce Hofer

at the low Ipk-pk of the PCM1704 I would more likely go with fet input op amp I/V

FET inputs are also expected to be more immune to RFI or DAC switching noise/glitch

op amp Vnoise ~ <6 nV/sqrt(Hz) about equals the noise of the 2.5 kOhm I/V R in the datasheet circuit

fet input meeting the noise requirements like opa627/637 and the more recent parts clearly aimed at replacing them would be my 1st try for something simple
AD8610 AD4627/4637 OPA827

at lower cost but still with good audio specs the OPA164x parts look fine too

for something "sporty", a design challenge (and an excuse to get a faster 'scope) you could see if you can noise gain stabilize a AD8067 - >1/2 GHz GBW, 640 V/us - at a slight audio noise penalty

otherside 10th December 2012 10:25 PM


If you are into SMD and can design a really good board, then look into the LME49990. If you want a low impedance feedback loop, than include an LME49600 with the opamp.
Yeah i had my eye on the LME49990 for a while just never got around testing it.
Do you have any example schematics with what you describe, i've never tried this approach.

georgehifi 10th December 2012 11:04 PM

This guy/guys seem to think highly of the LT1028 I don't know what their creds are like?
Cheers George

ClaveFremen 10th December 2012 11:37 PM

Hi George,

I've tried several opamps as I/V converters (subjective evaluation) and my favourite is TI's OPA827, OPA627 cheaper (and better sounding, IMHO) successor.

OPA827's applications include transimpedance amp,

Also LME49710/720 is a very good sounding opamp as I/V, just a bit less musical than OPA827.

Also OPA211 is a good one but inferior to 827 and 49720.

OPA211 is superior, IMHO, to its SoundPlus counterpart OPA1611

Compared opamps: OPA2132, OPA2134, OPA211, OPA1612, OPA1643, OPA827, OPA627, OPA2107, THS4032, LME49720, NE5532, LT1028, OP275, others I can't remember now.

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