Calculating BW for DAC based PGA??

Right, most mdacs are fairly slow from the reference to the output. Unless you can get a really good Spice model, you should just breadboard it. Opamp loop stability could go either way. Note the variation of Cout as a function of code; expect variations in feedthrough versus code, too.

Also, don't expect accuracy to be very good at high gains, namely DAC codes of just an LSB or two. TC can be terrible down there, too.

Oh, don't use the dac as a simple feedback resistor. You have to include its internal Rfb in the gain equation if you want good accuracy. That probably implies two opamps and a tricky loop.

John

Adding poles to the feedback loop is bad. For fast amps, even more than a few pF on the -in node can cause instability. This is worsened by your adjustable gain, which probably means the pole will shift all over the place, too. That's hard to compensate.

I don't have a clue what kind of transfer function a DAC has, or how it varies with setting. Maybe there's capacitance from Vref to out as you suggest, but there's inevitably capacitance to GND and +V as well. Does it have a buffer on Vref? That'll cause delay as well (even more poles..).

Tim

-- Deep Friar: a very philosophical monk. Website:

The 'output capacitance' is specified to different limits, depending on the input code (all zeros and all ones gives different values). A lot of that channel capacitance is resistor-coupled to your Vref input pin. So your feedback has R-C-R type impedance that is somewhat numeric-input-value dependent.

I appreciate the info.

The configuration uses the MDAC to provide both the input and feedback paths of a traditional inverting amp configuration. The path in the MDAC through Rfb acts as the input resistor and the path through the R2R ladder to current source acts at the feedback resistor. So for counts of 1 - 4095 the idealized gain ranges from 4096 - ~1 respectively.

This is a retrospective analysis so the hardware exists but it's not readily available to actually make measurements on it (yet...). In the mean time, I think I'll see if I can get some spice models from the MDAC and op-amp vendors.

It bugs me that I can't figure out (using the datasheets) any analysis to either support or refute my intuition... In particular, what bugs me is that I intuitively think that the effective bandwidth of this PGA configuration will be less (when at high gains) than the Vref to Iout bandwidth when the part is used as a traditional DAC (and at the same settings) but I can't come up with one wit of support for my intuition (so maybe it's totally wrong...).

Steve.

Oh, it'll be less, all right; look at the recommended designs, there are feedback capacitors there. That's because your 'multiplying' DAC is actually multiplying by a value less than 1, so additional feedback capacitance is required; the negative feedback is too small for stability, otherwise.