I/V stages have been a passion for me for a rather large number of years but only very recently have I approached I/V stage design from the point of view of getting the lowest possible noise. In the past so long as my estimate of the I/V noise was 6dB or more below the best recording noisefloor (say -92dB on 16bit material) then I considered job done. Now though my curiosity has been awakened for pursuit of 'how low can I go?' in relation to I/V stage noise.
The first port of call in an I/V stage has always been an opamp in transimpedance mode. Indeed lots of DAC chips are purpose designed for this kind of I/V and even many have on-chip opamps to facilitate this application. The best opamp I/V I've heard to date was from AD811. So I opened LTSpice and downloaded the AD811 spice model from Analog Devices, pasted it in and I figured I was all set to admire AD811's impeccable noise performance as I/V stage.
But it turns out the rabbit hole goes a bit deeper. Setting the I/V resistor to 600R (as AD811 DS recommends for -1 gain) I got an almost perfectly flat response over the audio bandwidth of 5.3nV/rtHz. That's a noise density figure, to get the full audio band noise, multiply by 141.4 (which is the square root of 20,000). Answer = 0.75uVRMS. To get the SNR plug in the signal level - the DAC is PCM56 with +/-1mA output so the signal is 1.2V p-p or 424mV. Crunch the math and out pops an astoundingly good result of 115dB SNR, even ignoring that the output signal needs gain to bring it up to the 2V standard CD level. But I was suspicious of this as based on my experience of modelling CFB amps on paper, they're kinda noisy and 115dB isn't at all noisy, especially with a flat line down to 20Hz where normally noise would be rising due to 1/f.
So I needed some kind of sanity check for this result. I figured I'd find the lowest noise LT CFB opamp (which require no download of subckts, the LT parts are built in to my LTSpice) and do a comparison. The LT part I settled on is LT1227 - its a bit noisier than AD811 (3.2nV vs 1.9nV on voltage and 32pA vs 20pA on current) but close enough for a sanity check. And the result was interesting : 21nV/rtHz at 1kHz and rising reassuringly below that frequency. A huge difference that could not be accounted for by the DS noise difference. That's 12dB worse and just 103dB SNR if we ignore that the density figure more than doubles by 20Hz.
I'm no expert in deciphering Spice subckts but I decided to have a look to see if the AD811 subckt models the noise. Since I have no idea what to look for I opted to compare with the subckt I downloaded for AD797. I reasoned that because low noise was AD797's big selling point they'd be bound to include that in the model. And sure enough, in AD797's subckt there are helpful comments which show the part of the circuit which sets the noise parameters. There are models (DIN,DEN) which are referred to right at the bottom. I'm guessing they're diodes. Any subckt without these won't model noise and sure enough, AD811 lacks them. As does AD844 which I also tried. Since I already had the subckt for it, I wondered - how does AD797 fare as I/V ? (You need to remove the decompensation node in the definition to make it compatible with a standard 5 pin opamp).
AD797 turns in a very impressive 14.3nV/rtHz figure (using 2.8k feedback resistor) giving an SNR of 120dB. LT's competitor LT1115 fell a fraction of a dB short of the AD797, at 15.1nV/rtHz but LT1128 beat it out by about 1dB, at 13.1nV/rtHz. LT1028 gives the same result as LT1115. Oddly, the 1128 only differs from 1028 by virtue of its compensation, the IPS (and hence noise) should be the same. So the last dB of performance seems an area of uncertainty, but clearly CFB opamps aren't the winners in the SNR stakes.
Going back to good old fashioned pencil and paper, how does AD811 do? Its crucial noise parameter is the inverting input current noise which sees 600//1200 (the latter inside the DAC) so 400R. 20pA * 400ohm gives 1.1uV in 20kHz. The +ve input noise at 1.9nV gives 269nV. The noise gain is 1+600/1200 = 1.5 so I'm getting 1.7uV with 424mV output or 108dB SNR. This excluding the intrinsic noise of the resistors themselves so the real result will be slightly poorer. Rather than do that math, I'll estimate back from the LT1227 result : as -ve input current noise dominates and that's 4dB poorer on LT1227 than AD811, I figure the AD811 result with resistors will be 4dB better than with the LT1227, i.e. 107dB.
I'll go on to talk discrete I/V in a later post.
The first port of call in an I/V stage has always been an opamp in transimpedance mode. Indeed lots of DAC chips are purpose designed for this kind of I/V and even many have on-chip opamps to facilitate this application. The best opamp I/V I've heard to date was from AD811. So I opened LTSpice and downloaded the AD811 spice model from Analog Devices, pasted it in and I figured I was all set to admire AD811's impeccable noise performance as I/V stage.
But it turns out the rabbit hole goes a bit deeper. Setting the I/V resistor to 600R (as AD811 DS recommends for -1 gain) I got an almost perfectly flat response over the audio bandwidth of 5.3nV/rtHz. That's a noise density figure, to get the full audio band noise, multiply by 141.4 (which is the square root of 20,000). Answer = 0.75uVRMS. To get the SNR plug in the signal level - the DAC is PCM56 with +/-1mA output so the signal is 1.2V p-p or 424mV. Crunch the math and out pops an astoundingly good result of 115dB SNR, even ignoring that the output signal needs gain to bring it up to the 2V standard CD level. But I was suspicious of this as based on my experience of modelling CFB amps on paper, they're kinda noisy and 115dB isn't at all noisy, especially with a flat line down to 20Hz where normally noise would be rising due to 1/f.
So I needed some kind of sanity check for this result. I figured I'd find the lowest noise LT CFB opamp (which require no download of subckts, the LT parts are built in to my LTSpice) and do a comparison. The LT part I settled on is LT1227 - its a bit noisier than AD811 (3.2nV vs 1.9nV on voltage and 32pA vs 20pA on current) but close enough for a sanity check. And the result was interesting : 21nV/rtHz at 1kHz and rising reassuringly below that frequency. A huge difference that could not be accounted for by the DS noise difference. That's 12dB worse and just 103dB SNR if we ignore that the density figure more than doubles by 20Hz.
I'm no expert in deciphering Spice subckts but I decided to have a look to see if the AD811 subckt models the noise. Since I have no idea what to look for I opted to compare with the subckt I downloaded for AD797. I reasoned that because low noise was AD797's big selling point they'd be bound to include that in the model. And sure enough, in AD797's subckt there are helpful comments which show the part of the circuit which sets the noise parameters. There are models (DIN,DEN) which are referred to right at the bottom. I'm guessing they're diodes. Any subckt without these won't model noise and sure enough, AD811 lacks them. As does AD844 which I also tried. Since I already had the subckt for it, I wondered - how does AD797 fare as I/V ? (You need to remove the decompensation node in the definition to make it compatible with a standard 5 pin opamp).
AD797 turns in a very impressive 14.3nV/rtHz figure (using 2.8k feedback resistor) giving an SNR of 120dB. LT's competitor LT1115 fell a fraction of a dB short of the AD797, at 15.1nV/rtHz but LT1128 beat it out by about 1dB, at 13.1nV/rtHz. LT1028 gives the same result as LT1115. Oddly, the 1128 only differs from 1028 by virtue of its compensation, the IPS (and hence noise) should be the same. So the last dB of performance seems an area of uncertainty, but clearly CFB opamps aren't the winners in the SNR stakes.
Going back to good old fashioned pencil and paper, how does AD811 do? Its crucial noise parameter is the inverting input current noise which sees 600//1200 (the latter inside the DAC) so 400R. 20pA * 400ohm gives 1.1uV in 20kHz. The +ve input noise at 1.9nV gives 269nV. The noise gain is 1+600/1200 = 1.5 so I'm getting 1.7uV with 424mV output or 108dB SNR. This excluding the intrinsic noise of the resistors themselves so the real result will be slightly poorer. Rather than do that math, I'll estimate back from the LT1227 result : as -ve input current noise dominates and that's 4dB poorer on LT1227 than AD811, I figure the AD811 result with resistors will be 4dB better than with the LT1227, i.e. 107dB.
I'll go on to talk discrete I/V in a later post.
Last edited: