Hi,
An application question for the DAC7562 12-bit version.
Any suggestions for the datasheet bipolar output circuit (page 44) to work with +/-5V supplies and output -/+1V? The DAC is at 3.3V supply so I would figure the output voltage goes 0-3.3V (or pretty close to it). This is to drive the Vg pin of the LMH6503 VGA. I am using the OPA140 opamp.
thanks for any help.
Hi Dennis,
I used TINA-TI to simulate your question and the circuit I found was:
I tried to make the circuit line up with the bipolar circuit on page 44 that you mentioned. If R1 and R3 are my standard R values, then R2 (top) is R/2.5 and R4 (bottom) is R/1.282.
Thank you,
Lucas
Hi Lucas,
Another follow up question. In that linked app note from my last message there is a compensation capacitor used across the output to the inverting input. The app note lists a value of 150pF and it seems good. Does raising this value any higher have bad effects? I can get rid of the output peakiness by increasing this capacitor to 470pF but I am not certain of the downsides to doing so.
Hi Lucas,
I think my questions are answered with respect to any instabilities. I don't usually use TINA simulator as I find it to be tedious and it did not fail me this time. In any case using ltspice I was able to see instabilities in the opamp design if there is no Ccomp across the output to INV input. If you load the output of the opamp with 0.01uF capacitor you can generate an oscillation. If you add a Ccomp of 25pF or more this oscillation goes away. Going to 150pF does not seem to be problematic. I don't know if I will put in a 0.01uF load cap but it will be a placeholder on my board for further testing.
With a 1pF Ccomp, I see this:
With Ccomp of 150pF I see this:
I do want to thank you again for the application circuit.
dennis brown
