COD - Current Output DAC

[BrianDonegan]

Originally Posted by dirkwright

When do you expect to have the IVY boards back in stock?

Now-ish...

[dirkwright]

I have a question about the PCM1794a. As I read the datasheet, there is a -6.2mA offset at the output pins. How are we supposed to handle that? In the demonstration circuits shown, there doesn't appear to be anything done about it. The outputs from the DAC goes directly to the inverting pin of the opamps. Do opamps not care about a DC current offset? I'm confused.

[Russ White]

It's all about the magic of feedback.

[dirkwright]

Originally Posted by Russ White

It's all about the magic of feedback.

OK, so you're saying I think that the opamps have enough "compliance" or what ever it's called to handle a DC current offset presented to the negative input pin when it's arranged as a transimpedance amplifier with feedback, which is the way they are typically used in I/V converters. Is that right?

What about using a discrete op amp instead of the chips? If performance isn't any better, at least it would be more fun. I don't want to turn into a "chip jockey".

[Russ White]

Legato 3.1 works wonderfully for the task, and it is discrete. Just use double the stock output swing.

[dirkwright]

Originally Posted by Russ White

Legato 3.1 works wonderfully for the task, and it is discrete. Just use double the stock output swing.

Thanks. I looked at the schematic and I don't understand it. It appears to be highly paralleled mosfet's in common gate configuration, is that right?

There is no loop feedback, so I am unaware as to how the DC current offset of this DAC is handled by the Legato. In my sim's, the input for a common gate/base configuration is highly sensitive to DC offset.

I think this particular DAC is pretty hard to work with, and maybe not worth the effort.

[Russ White]

No its pretty easy to work with really.

Legato circuit is actually a bit more than that. There are parallel mosfets, but that is only to reduce their impedance in their linear region.

The Legato is a simple balanced output I/V stage. At the balanced outputs it will have common mode DC offset (same on both phases), but you will use output caps if your balanced gear can't handle that (most can without any issue). Differential offset is adjusted out with a pot and can be brought to zero. If you need SE output Legato uses a separate BAL/SE stage that also has a pot to adjust to 0 offset. One bonus is that the SE output is buffered and can drive headphones.

Not hard at all

If you don't want to adjust anything use IVY-III instead. It will work as stock, and if you like you can double the output swing. With IVY-III all outputs will swing around ground, because it does use global feedback (super symmetrical at the I/V stage) at all stages.

[dirkwright]

Thanks Russ! I think for the COD I'm going to settle for a Lundahl transformer. I read through Analog Devices AN-912 and I think in this case a transformer is a good idea. I need to read some more though before deciding what to do.

[dirkwright]

I'm still flopping about like a fish out of water about this. Does anyone have distortion readings for the passive I/V stage used on the COD? Does the distortion go down if the I/V resistor is made smaller? How does the distortion compare to using an active I/V stage with virtually nil input impedance?

Also, is there much of a common mode signal output on the Iout+ and Iout- pins from this DAC? I have read that at least some DAC's have significant common mode noise in their outputs.

Can the DC blocking capacitor be used without the I/V resistor? The DAC has a DC offset of -6.2mA which can cause problems with some types of I/V stages.

[Zero Cool]

with a 90 ohm IV resistor there is about 1v of DC offset out of the DAC. you need the caps to block this DC from a following Buffer stage or use some form of compensation like a servo etc. The distortion is very low using a resistor. I would bet an active stage would have comparable results but a different harmonic structure to the distortion.

I know the active vs passive thing has been debated to death here. but I can tell you first hand a passive IV stage with a jfet buffer sounds WONDERFUL! that is of course MPO on the subject. and YMMV but it is an easy thing to experiment with and see what works for you!


Zc