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I/V using opamps
I/V using opamps
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Old 11th December 2012, 07:18 AM   #11
Shinja is offline Shinja  Japan
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ADA4627/4637 must be the competitor of OPA627/637.
it seems a successor of AD8610/8620.

ADA4898-1/ADA4898-2 seems competitor of OPA211 and LM49990 and so on, modern opamps.

you should not think little of offset spec of opamps.
the output impedance of binary weighted DAC varies along with input binary, especially at higher order bits.
if the output Z varies, then the noise gain varies. and the offset voltage will be modulated and become distortion.

It is more important ordinary R2R DAC, but it may be less at PCM1704 because it is sign magnitude DAC.
(sign magnitude DAC do not switch highly weighted element at bipoler zero)
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Old 11th December 2012, 11:40 AM   #12
dirkwright is offline dirkwright  United States
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Originally Posted by jcx View Post
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
Ah, very interesting! Thanks for sharing. I was actually able to follow what you said and I agree. For low output current DACs, an fet input opamp would produce lower noise than a bjt input opamp.

Well, there is the option of adding a discrete fet follower inside the feedback loop of the bjt opamp. It's something to play with anyway, and has been done before.
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Old 11th December 2012, 10:50 PM   #13
smms73 is offline smms73  Portugal
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Originally Posted by jcx View Post

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
Jcx, Can you please explain what you mean by " a noise gain C to gnd at the input"
and if possible the opamps that you suggest in smt , Thanks
SÚrgio Santos
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Old 12th December 2012, 02:22 AM   #14
Joachim Gerhard is offline Joachim Gerhard  Germany
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The OPA1611 is a SiGe BJT.
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Old 12th December 2012, 02:37 AM   #15
Shinja is offline Shinja  Japan
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these documents are informative.
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Old 12th December 2012, 02:44 AM   #16
georgehifi is offline georgehifi  Australia
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Hi JCX, I have done a mod which has me falling in love again with the OPA627 as the I/V.
My cdp has as you know a PCM1702K for each channel, I was always bothered that they gave dc offset to the OPA627 I/V. With the help of abraxalito, I got rid of it (circuit attached)
This is the post I sent to him and I would like your opinion as well.
"thank you very much for the dc offset nulling circuit it worked a treat, instead of 17mV left and 50mV right comming out of the OPA627 I/V's now I have +- .1mV I don't know why but the sound has improved very much overhaul esspecially in the bass. Is this possible, that the OPA627 as an I/V does not like to see any dc comming into it from the dac/s???"

Cheers George
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Old 12th December 2012, 04:01 AM   #17
jcx is offline jcx  United States
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shinja's refs are good for noise gain compensation - allowing the use of decomp op amps at unity gain - like op amp I/V

the question with sims is the model accuracy - output Cload effects in particular - so beware - the sim below doesn't seem too sensitive adding ~50 Ohm in the op amp outputs so may not be too bad - but as speed goes up, op amp macromodels are worse

Analog Devices in particular is frustrating in printing one of the better macromodeling app notes ever - and not using it for their own Spice models - even posting too simple models with impossibly ideal sources for its most recent, highest performance op amps where output Z, even bond wire L is critical to real performance - or outright failure to even grossly indicate stability

below is a simmed comparison of the PCM1704 datasheet I/V on the left and the noise gain compensated ADA4637 with higher feedback C

I plotted the V at the I/V op amp inputs - Hawksford makes the point that a quality criteria for I/V quality can be the magnitude, and extent in time of this error

so I also plotted the integral of the input Verror squared - note the blue trace for the noise gain circuit on the right has 100x extra gain to be seen on the same scale

(to run the sim you need to download Analog's op amp Spice .cir models to the same directory as the .asc)
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Last edited by jcx; 12th December 2012 at 04:29 AM.
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Old 12th December 2012, 04:40 AM   #18
jcx is offline jcx  United States
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as an example of ADI Spice modeling "excellence" (not) - I just stuck the AD8067 model in the above sim - even better dynamic error

but the sim's integrated error plot is spoiled by the AD8027 Spice model ~74 dB open loop gain - where the datasheet clearly claims 100 dB min, ~120 dB typical
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Old 12th December 2012, 04:51 AM   #19
EUVL is offline EUVL  Europe
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If it has to be opamp then this is the one I shall build, using OPA1641 :


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Old 12th December 2012, 05:35 AM   #20
qusp is offline qusp  Australia
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yeah the 1641 is a great chip, needs a buffer though. or VERY light loading.

Last edited by qusp; 12th December 2012 at 05:40 AM.
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