DAC with built in Feedback resistor and PGA

[cciarleg]

Hi,

I have a DAC with a built in feedback resistor (the AD7538) and I would like to make a PGA with it (and an op-amp of course). (unipolar-although I would be curious if you could do this with a bipolar output as well)

It is a current output DAC. I'm still learning a lot of this so I have some questions about how to implement the circuit. The attached picture is the recommended PGA config.



I would also like to make sure there is an RC filter on the output of the opamp.
So, I have my analog GND attached to the (+) of my opamp and my opamp output fed back directly to VREF.

Then I get a little confused-I can connect the current output to the (-) of the opamp. But if RFB is my gain resistor, than I also connect that to my (-)? If this is the case than my VREF is the VIN as referred to in the PGA diagram? (which cannot be right)

And to get the RC filter in, I connect the cap from the opamp output to the (-) opamp terminal, and then I'm unsure where to place the resistor since RFB and IOUT are tied together. Is it correct to place it after those lines are tied together?

Also, in the unipolar circuit given, they have R2 (10 ohm) placed from VOUT to RFB and I'm not sure what that is for. Is this the 2nd resistor you usually see to set the gain? And do I need it if I have a PGA? Since my VREF is not fixed, I feel like I should have that in there somewhere. If someone could explain briefly the purpose of R2 in the context of

I think that would really help

My last bit of confusion is the voltage to power the opamp. I can either hand it +/-5V or +/-15V. What would be the criteria I would use to determine which one is ideal?

I made a little sketch to help show what I'm confused about and the configuration I currently have it in.


Its a really interesting DAC. I'm just very confused as to how to set it up and how to use the built in resistor. Suggestions and explanations would be much appreciated. Thanks in advance.

 

[FvM]

You want to make a PGA, so you'll use the first schematic.

I presume you understand the basic difference in transfer characteristic between PGA and regular multiplying DAC operation?

PGA Vout = -Vin*c/DACWORD

regular DAC Vout = -Vin*DACWORD/c

Regarding R2, it's complementary to R1 that allows a gain fine tuning of a few 0.1 %, usefull for applications where a nominal gain should be achieved because no digital gain correction is provided in the DACWORD generation.

OP power supply should be chosen to allow the intended signal swing with sufficient margin. E.g. +/-10 V signal swing mostly uses +/-15 V supply, at least +/- 12 V, depending on the OP specification and output load.

There's no signal input in the third schematic. What should it be used for? Connecting RFB to IOUT seems pretty useless.

 

[cciarleg]

Thanks for the reply.

I've seen the equations in regards to this DAC and some others I've looked at. So that means, because this is a PGA, that there is inherent digital gain correction so no R2 needed. On the other hand, how do you tell if digital gain correction is provided (or enough is provided)? The DACWORD does modify Vout. Is this considered gain correction? If so, why is R2 needed in the first circuit and how do I know the value is correct? (Is there an equation that might be similar to or refer to the use of a resistor like R2)

I think this is the big part of my confusion-what then does one feed into RFB in the PGA config? If DACWORD changes the Vout in both cases, then how does the PGA help additionally? (I'm doubting that I need one now) If I give RFB a constant voltage, would that make it essentially the same as a constant PGA? (and I may as well go back to the normal unipolar binary operation) Is the key to needing a PGA being able to vary the voltage into RFB and so have a much wider gain range?

I appreciate the help-challenging to learn on one's own.