[sdiy] buffer opamp configuration [was: 1/8th inch questions]

[Seb Francis]

Grant Richter wrote:

> >> Unity gain (non-inverting) buffers can be problematic since opamps are
> > less stable at unity gain, especially when driving capacitive loads.  There
> > seem to be two common configurations to give stability:
> >> (a) Put a capacitor and resistor in series between the opamp inputs, and a
> > larger resistor in series with the input signal
> >> (b) Put a capacitor and resistor in series between the opamp inputs, and a
> > larger resistor in the feedback loop.
> >> Note that both these ways increase the AC noise gain, and decrease the
> > buffer bandwidth.
> >
> > Method (b) is normally used to compensate a pole that comes from the opamp's
> > *input* capacitance. It's always recommended if you have large feedback
> > resistors.
>
> Which is why you see a 5 pf cap across every feedback resistor in the LM318
> data sheet.

Aah, now this is a useful datasheet .. it has exactly the circuit I'm trying to work out .. a S&H cap with unity gain non-inverting opamp.  It has a 10pF cap and a 5K resistor in parallel as the feedback loop.  Split the 5K resistor into a 4K + 1K and you have a current limited output point.

So I think I was wrong in my previous mail to Jürgen - going by this datasheet it seems having a dual feedback path can help even in a buffer.  Probably something to do with the subtleties of resistors, caps and AC which I've still yet to completely get my head around.

Anyway, I'm going to stop spamming this list with mails for bit and get back to programming this PIC ...

Seb