# Can a digital pot and op-amp challenge the performance of a programmable gain amplifier

Currently I'm using a Microchip PGA for applying gain factors of 1, 2, 4, 8, 16 and 32. It's used in a design of strain gauge amplifier. The part number is MCP6S21 and I am happy with its performance in these areas listed below: -

1. Noise - it doesn't seem to introduce any but there is a large gain stage preceding it. It is specified as having 10nV / $\sqrt{Hz}$ but like I say, the circuit isn't challenging in this area.
2. Input offset voltage is quite good at +/-275uV and I expect, at the output this could be up to 32 times higher on maximum gain. Again, this is easily acceptable for the product.
3. Gain error is +/-1% - providing this is a stable gain error with temperature then no problem. The MCP6S21 is specified as having a gain drift of ±0.0004% per deg C
4. Bandwidth and slew rate are flat easily beyond 100kHz at all gains.

The input to the PGA is from an op-amp and the PGA feeds an ADC via a low-pass RC filter i.e. the output isn't feeding a low-load and the input impedance could be a few hundred ohms without too much bother.

There may be better PGAs that are rail-to-rail input/output devices but does a digital pot combined with an op-amp come close to challenging this "moderate" level of performance?

• Alternatively consider an 8-bit MDAC and op-amp; certainly capable of good performance. Mar 11 '14 at 15:05
• @BrianDrummond Yeah looked at this but I can't seem to find an MDAC that is small enough on footprint - I have to fit 16 channels into a small area and by the looks of digipots I can get 4 channels in 24 pin devices. I may not have looked hard enough though!! I can only find one dual device (LTC1654) that has separated Ref pins. Mar 11 '14 at 15:12
• Could be. I used to use the TLC7528 (dual, separate ref, SOP20) which at 6*6mm for the PW pkg may still be too big for you. Mar 11 '14 at 15:53