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?
As a point of interest, when tested with a 1khz sine wave and connected to a spectrum analyser the performance gets worse as the sampling frequency increases, there are also a lot of junk spikes, beyond 20khz, looking like a standard graph for jitter, centred on around 50khz. Kind of like this.
http://www.audio-components.de/seiten/ac/info/musicalfidelity/mf_cdpre_24/Cdpfig13.jpg
It's being driven by an Envy24HT chip on a sound card mind you, so this might be entirely normal.
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?
Thanks in advance
Matt.
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?
As a point of interest, when tested with a 1khz sine wave and connected to a spectrum analyser the performance gets worse as the sampling frequency increases, there are also a lot of junk spikes, beyond 20khz, looking like a standard graph for jitter, centred on around 50khz. Kind of like this.
http://www.audio-components.de/seiten/ac/info/musicalfidelity/mf_cdpre_24/Cdpfig13.jpg
It's being driven by an Envy24HT chip on a sound card mind you, so this might be entirely normal.
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?
Thanks in advance
Matt.
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.
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.
I had the same issue using a THS4131 on that DAC. Also, the THS got pretty hot as well.
It sounded ok, but in the end I gave up and used something else for I/V
It sounded ok, but in the end I gave up and used something else for I/V
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