Op-amp requirements for a I/V converter? - diyAudio
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Old 26th January 2012, 04:41 PM   #1
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Default Op-amp requirements for a I/V converter?

Imput noise density, slew rate, gain bandwith, THD + N, settling time or open-loop gain, which of this parameters really matters when choosing op-amps for an I/V converter of a DAC?
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Old 26th January 2012, 05:28 PM   #2
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Old 26th January 2012, 05:42 PM   #3
jcx is offline jcx  United States
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Quote:
Originally Posted by jcx View Post
if you put a C acrosss the feedback R giving a single pole low pass then there is little need for excessive slew rate - can be part of the anti-image/reconstruction filter

ideally higher frequency error V at the op amp input would then be reduced by ~ fc/GBW

with the feedback C there is a "feedthrough zero" that gives a small input Vdiff glitch at current steps due to the op amp output Z - decades old I/V app notes described the advantages of a low impedance output buffer inside the loop for op amp I/V

the op amp slew rate spec can still be a proxy for input diff pair linear operating range with degeneration, fet inputs improving both

but the low Vnoise parts can't have much degen

a compleltely new "highly linear" diff pair is used in AD8099/ADA4898 family parts - I think the ADA4898 is the best combination of specs for audio DAC I/V available today

you would need to look with a very good 'scope to see if ccs bias of the output to either rail helps - the ADA4898 has a 50 mA "linear current range" spec so you could bias at several times the fs current of most audio DAC chips
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Old 26th January 2012, 09:23 PM   #4
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Let me place the question in a different way. Among the listed parameters, which are the most relevant?

I need to choose a pair of dual op-amps for I/V convertion but they must be electrically compatible with NE5532. I searched about this subject but all recommendations where based mostly on subjective tastes. I would like to get a technical explanation about the matter.

Last edited by Audio Maniac; 26th January 2012 at 09:30 PM.
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Old 26th January 2012, 09:35 PM   #5
wahab is offline wahab  Algeria
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http://www.ti.com/lit/ds/symlink/pcm1794.pdf
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Old 26th January 2012, 10:05 PM   #6
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The PCM1794's datasheet informs gain bandwith, settling time and slew rate affects the audio dynamic performance of I/V section.

So imput noise is less important for this application?
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Old 26th January 2012, 10:18 PM   #7
jcx is offline jcx  United States
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Default why trade off when you can get "very good" or "best" in all categories at once

you might want to read the Hawksford paper I refence in the 1st post: The Other Hawksford IV (nested loop op amps)

I show a sim of glitch at the input of the I/V op amp - Hawksford shows that you should want to minimize a function like the V^2 time integrated error V

GBW is the biggest help, low output Z as I mentioned makes the feedback C work better

the function to be integrated also depends on the linearity of the input stage - the ADA4898 uses a new linearized BJT input stage that is unmatched by previous monolithic op amp tech
if I've done the calc form the patent correctly the input stage gm is linear to -120 dB with |4 mV| input differential - the same number for a low noise bjt op amp is ~100 uV

so the ADA4898 has high GBW >50 MHz, high "linear output current" spec f = 100 kHz, SFDR = −70 dBc, RL = 150 Ω 40 mA, >50 V/uS slew rate, low 1 nV/rtHz noise - all together - and avaiable in duals


I/V S/N is limited by DAC I_fs and the larger of op amp Vnoise/Rfdbk or op amp Inoise - so op amp Vnoise can be reduced in importance by increasing R, increasing output V swing

Last edited by jcx; 26th January 2012 at 10:34 PM.
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Old 26th January 2012, 10:35 PM   #8
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I just got today some THS4032 (TI). Noise: 1.6nV/sqrt Hz; Slew Rate: 100V/us; Settling Time: 60ns, THD: -90dB at 1MHz; BW: 100MHz!
Another contender that I have at hand is ADA4897-2 (AD). Noise: 1.0nV/sqrt Hz; Slew Rate: 85V/us; Settling Time: 96ns, THD: -115dB at 100kHz, -93dB at 1MHz; BW: 230MHz!

I am planning to use them as I/V stage after a PCM61P - it has a settling time of 350ns. I am planning to get rid of the SR limiting capacitors in the I/V stage.
I just need to wait to get the adaptors from SMD to DIL.

Last edited by SoNic_real_one; 26th January 2012 at 10:40 PM.
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Old 26th January 2012, 10:44 PM   #9
jcx is offline jcx  United States
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the ada4897 is new to me - the Vnoise looks very good but the chip has a max |Vsupply| of only 10 V so +/-5 V supply would be its limit

pkg is msop - 20 mil pin spacing is challengening to hand solder - but the size is probably required to keep lead/layout inductance low - I am suprised they didn't use a better pinout

I really wouldn't try to use a 230 MHz op amp on an adapter

Last edited by jcx; 26th January 2012 at 10:50 PM.
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Old 26th January 2012, 10:47 PM   #10
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The ADA4897-2 is also new to me. I'm amazed with its great specs, but unfortunately its power supply operating range is 3 to 10V. I need an op-amp which supports 15V.
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