Low drift opamp for output of DAC? (to drive frequency control of VCO)

[danjel]

I am using a 12bit DAC (TLV6518) which is referenced to 2.048V (so the max output is double this).
The output of the DAC is fed to an opamp in non-inverting configuration that has a trim pot as part of the resistor network for adjusting gain.
The desired output range of the OpAmp is a very stable (i.e. temperature stable, low drift) 0-10V

This voltage is being used to control the frequency of an oscillator so once the trim pot has been set to calibrate the gain it would be ideal for this setup to remain stable.

Question:
1. What lowdrift, dual supply opamp would you recommend using in this situation?
(preferably dual package that is pin compatible with a TL072)

2. What other considerations should I have for the design and pcb layout?
(e.g. extra cap in op amp feedback? tempco resistor? guard traces around the non-inverting pin?)

 

[crutschow]

Stable is a relative term. What gain drift and offset drift can you tolerate? Without those value I would only be guessing as to what you need.

 

[danjel]

Good question: something close to 5uv/C would be ideal.

 

[danjel]

Looks like this might be ok: MC33078DR2G

It has 2uV/C , operates on dual supply (pin compatible with Tl072) and is pretty cheap.

 

[crutschow]

Two things to be aware of with that amp:

The 2µ/C offset is typical, not maximum. If you need a guaranteed maximum limit, then you must buy an amp with such a limit stated.

The input bias and offset currents must also be considered. These translate into voltage offsets when multiplied by the equivalent resistance at the plus and minus op amp inputs.

And the variation of the gain resistors with temperature must be determined. A typical 1% resistor may have 50ppm/C change which can translate into significant voltage changes, as determined by the gain changes this causes and the voltage output of the amp. You may need to consider using high stability resistors (such as 0.1% units). Also consider the stability of the trim pot.

Unless the circuit resistances are above a megohm, you shouldn't have to worry about guarding the summing junction or any other unusual layout considerations. Just good normal layout with adequate decoupling capacitors to a ground plane, and short trace lengths, especially to the summing junction.