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Old 26th April 2009, 04:11 PM   #1
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This is also in the Digital forums, but it can be quite quiet over there and I figure this is as analogue as it is digital.

Are there any special requirements when using this opamp in an I/V circuit?

Currently I've got mine working with a PCM1794, in the circuit illustrated on page 21 of the data sheet.

http://focus.ti.com/lit/ds/symlink/pcm1794.pdf

When connected like this the measured distortion is rather high -63dB 2nd order, - 67dB 3rd order and -77dB 4th order at all frequencies.

This is when driven at 0dB according to the computer, arriving at the target of 4.5V RMS on the output of the differential. The peak to peak voltage after the I/V opamps is 6.4 Volts with no signs of clipping anywhere in the system. And although the distortion is rather high, nothing sounds wrong.

If I lower the signal given to the DAC, down from digital zero, such that the peak to peak output of the I/V opamps is 2 Volts. I end up with 2nd, 3rd and 4th orders at ~ -85dB. The differential's are OPA627.

Is this typical behaviour for this chip?

Looking at the data sheet I'm tempted to say it's not and that something is amiss. Everything points towards this chip having low distortion at audio frequencies, although we're not really shown it's abilities at high voltages and "low" frequencies.

As a comparison I've had this DAC built on another PCB for a while that allowed IC swapping in DIL sockets (this board is surface mount) and when using OPA627 for I/V conversion I got better then -95dB and the THS performed worse then it is here, although again, sounded fine.

Should I alter the gain of the I/V stage to output ~2V p-p and alter the input of the differential to 1k as this would appear to give the best results according to the data sheet for the THS?

The opamps get quite hot to the touch, but you can keep your finger on them, its uncomfortable, but doesn't get hot enough to really burn.

I have a 50mghz scope, if these are oscillating, what exactly should I look out for?

As an update, I had a fiddle with some of the settings.

I can arrive at -95 2nd order -110dB 3rd on one channel.

The other one manages -85dB 2nd and -85 3rd.

This is still highly governed by the level the THS are asked to deliver. People really seem to like the way this opamp sounds handling I/V conversion, however have people actually measured their DACs performance with them in place? They do sound absolutely fine, great in fact, when driven hard, but just perform badly.

What I'm asking here is

1) Is this normal behaviour for this chip?
2) I really like the way they sound, so should I redesign the I/V to max out at ~2V p-p, then alter the differential's gain accordingly.
3) Use a different chip for I/V altogether.

Thanks in advance

Matt.
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Old 26th April 2009, 04:24 PM   #2
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I'm using THS4131 in my dac driven by mono mode PCM1794A.. I have not done comprehensive thd measurements on my dac.. A couple of things to note -

1. They don't drive capacitance very well and this usually manifested by high levels of thd.. If you are using a capacitor across the feedback resistor to effect a 1st order roll-off add a small resistor in series, 22 ohms or so - note obviously this does reduce the ultimate attenuation of ultrasonic non audio noise to what ever the ratio is between this resistor and the feedback resistor. So if you need more use a passive roll-off externally.

2. See above, but should interpose 22 ohm resistors between the THSxxxx and the external load, again to isolate capacitance from the device.

3. When doing these sorts of measurements if you do not have a very good LPF incorporated into the dac design you MUST band limit your measurements externally or your thd +noise will mostly reflect the noise present in the system. Use a high order 22kHz LPF for CD based measurement bandwidths. Obviously not the case with FFT based spectral analysis, but an FYI for those using conventional analyzers.

4. The open loop gain in the THSxxxx series is lower than the typical audio op-amp which by inference might result in higher thd depending on open loop linearity.

IMO Other sources of distortion in your system are likely to mask these levels of distortion.

Hope this helps. I really like the performance of the THS4131s I am using in my dac, but the rest of the system is tube based and a fair smattering of transformers in the signal path does help to knock down the out of band noise. (Transformer in dac balanced to unbalanced, line stage transformer coupled, SE amplifier with output transformer. )
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Old 26th April 2009, 06:26 PM   #3
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Thanks for that, I'll try giving it a go.
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Old 29th April 2009, 03:20 PM   #4
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Thanks for the tip, those 22ohms in series with the feedback cap solved the issue.

I'm now getting better then -95dB across the board.

Also the 4031's now run cooler, seems they really didn't like that capacitance at all.
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Old 30th April 2009, 12:28 AM   #5
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Glad to hear it worked, those are great numbers and probably near the limit of what is possible for this chip given the available open loop gain. Looking at the signals on a scope you will observe a substantial increase in out of band noise - as much as 20dB, but in practice this hasn't caused a problem in my application. I noticed an immediate improvement in sound quality when I added those resistors.
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