Best opamp for I/V conversion? (DAC)

[Elso Kwak]

Originally posted by Charles Hansen
Elso -
You really should try one of his circuits...

I did. Even posted kind of merger of Jocko's and Jan Diddens and Rudolf Broertjes circuit though I never built it. Will see if I can find that back and post again saving you to use that lousy searchengine.
Found it!
Its a Sziklai input folded cascode design. Or two times common base. It is drawn somewhat awkward due to that stupid schematic capture program or my computer blindness.

[Charles Hansen]

Originally posted by Elso Kwak
Hi Charles, I put in a 0dB 1kHz tesignal into my IV and got about 0.01 V peak to peak signal at pin 2 of the opamp. Could not measure anything on pin 3 as it should. So I don't know where that 400 mV value comes from as I get 10mV p-p.

My estimate is just that -- an estimate. There is nothing that substitutes for a real-world measurement, and even more important, real-world listening tests. However in this case, I would recommend using a square wave rather than a sine wave. This will more closely approximate the transients that may be present with music. Also, it is important to make sure that your measuring device have sufficient bandwidth to see what is going on. Please let me know what you find out.

Originally posted by Elso Kwak

I did. Even posted kind of merger of Jocko's and Jan Diddens and Rudolf Broertjes circuit though I never built it. Will see if I can find that back and post again saving you to use that lousy searchengine.
Found it!
Its a Sziklai input folded cascode design. Or two times common base. It is drawn somewhat awkward due to that stupid schematic capture program or my computer blindness.

I'm a little unclear from your post if this is the circuit you listened to. If so, I would expect it to sound quite a bit better than any op-amp I-V stage. I'm not so fond of the Sziklai pair, as in my experience, this local feedback loop will color the sound. However, Jocko has found that the lowered input impedance here will increase bass "slam" and authority, presumably because of the lowered input impedance presented to the DAC output.

The rest of this circuit is zero feedback, which is something of a double-edged sword. This makes things much more transparent, but also much more revealing of the actual implementation. In other words, if you use (for example) a bad sounding capacitor with a feedback circuit, the coloration produced by the feedback tends to mask the problems of the capacitor. But in a zero feedback circuit, you will much more clearly what the capacitor is doing to the signal.

[Jocko Homo]

Elso:

Your implementation of the CFP is not quite right......

You have to be very careful to keep it from oscillating. Lower the gain some.......for starters.

As for transparency:

Yes, it must have a clean supply. And guess what......Charlie and I tend to use open loop supplies.

Or something similar....that has constant Z vs frequency. I am firmly convinced that a large part of my sucess is a result of the total implementaion, not just coming up with a common-base, or common-source circuit.

Jocko

[Elso Kwak]

Originally posted by Jocko Homo
Elso:

Your implementation of the CFP is not quite right......

You have to be very careful to keep it from oscillating. Lower the gain some.......for starters.

As for transparency:

Yes, it must have a clean supply. And guess what......Charlie and I tend to use open loop supplies.

Or something similar....that has constant Z vs frequency. I am firmly convinced that a large part of my sucess is a result of the total implementaion, not just coming up with a common-base, or common-source circuit.

Jocko

Hi Jocko!,
It was just a proposal. I haver never built this one. I built something simpler from Rudolf or Thijs for real life testing. Will try to find it back. I know you are using open loop supplies.

[ojg]

Originally posted by Charles Hansen
Just to give an idea of the problems that can happen when using an integrator (ie, op-amp) as an I-V converter:

a) Looking at the data sheet for the TDA1545 (sorry, I don't have the DS for the '1541), we can see that the analog output settling time is 0.2 uS. This translates to a 5 MHz signal.

b) The open-loop gain of the OPA604 at 5 MHz is approximately 5x. (Remember, it's really an integrator.)

c) Let's assume that you have the op-amp set to give 2 V output. With a gain of 5x (at 5 MHz), that means that the "virtual ground" inverting input is not a virtual ground at all. Instead, expect to see up to 400 mV of signal present there.

This is why you have to include a capacitor in parallel with the feedback resistor to roll of the frequency response of the I/V converter. If you choose a sensible -3dB point of 50kHz then there will be 100 times attenuation at 5MHz and the voltage at the inverting input drops to 4mV. Now if you choose the AD825 (as recomended by Walt Jung in the article you pointed to) it has a gain of 15 at 5MHz and so this voltage is reduced further to 1.3mV.

I ran it through LTSpice and it confirms this, here is the netlist:
I1 in 0 SINE(0 1.2m 5Meg) AC 1.2m
X§U1 0 in N002 N001 out 1pole Avol=3k GBW=40Meg Slew=125Meg ilimit=25m rail=0 Vos=0 en=0 enk=0 in=0 ink=0
V2 N002 0 15
V3 0 N001 15
R2 out in 1.66k
R3 0 out 10k
C1 out in 2000p
.tran 2m
;ac oct 10 100 100Meg
.lib 1pole.sub
.backanno
.end

Can you tell it is a slow day at work

[ojg]

I couldn't resist some plots...

Top:
OPA627: Avol=10Meg GBW=16Meg Slew=55Meg
Bottom:
AD825: Avol=4k GBW=40Meg Slew=125Meg

V(in) is inverting input, V(out) is output of opamp. Zf is 1.66kohm || 2nF.

Notice how, in the audio-band, the inverting input is 180 degrees out of phase with the current when using AD825 while it is 90 degrees out of phase when using OPA627. My gut feeling tells me this difference affects the DAC performance in some way. Inverting input voltage @5MHz is ~-50dBV = 3mV.

[Pedja]

Ojg,

Just checked this with MicroCap. Model provided by ADI says at 5MHz the inverting input (closed loop; output impedance of the DAC set to 2k and I/V resistor is 1k - doubled value of the I/V resistor will double the inverting input’s impedance) has impedance of about 150 Ohm. Both AC and transient analysis show this. Since the open loop response of the model looks a bit shorter (11MHz) than shown in the datasheet (25MHz), and the phase shifts earlier, I would expect lower value in reality, maybe about half this value, but not less than that.

Btw, you are misquoting Socrates.

Pedja

[fmak]

Originally posted by Charles Hansen
The best op-amp for an I-V converter is no op-amp. As Barrie Gilbert (one of the top analog designers of all time) explains, an op-amp is in reality just an integrator. This causes all kinds of problems with input stage overload when presented with the step output of a DAC. Anything (transformer, resistor, discrete, etc.) will sound better than an op-amp for an I-V converter.

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I tried the Sowter transformer recommended by Doede Douma and didn't like the sound at all! Distortion was also barely -78dB at 1 kHz. The opamp route sounded better.

[Marlowe]

Originally posted by till
what is a transimpedance amp?

IIRC, the transimpedance amp equals the current feedback amp, as AD811 etc ?

[Christer]

Originally posted by Marlowe


IIRC, the transimpedance amp equals the current feedback amp, as AD811 etc ?

Maybe I'm tired, but wouldn't you have to swap the inputs?
That is, making the inverting input high impedance and the
non-inverting one low-impedance, which of course you cannot
do with a chip unless you design it.

Edit: Hm, maybe not if you use it in invertin config?? Guess it
is to late to think about it now.