operational-amplifiervoltage-reference
I am designing a linear voltage source. I have a precision volatge reference (REF5050 by TI), and before I do anything with its output I buffer it. However, the datasheet of the amplifier I use (OPA197) suggests the following:
This brings up the question – why are there 2 feedback paths and why is the isolation resistance needed? A unity gain opamp should have only 1 feedback path, as I understand.
Thanks in advance!
How about this? You do give up very high impedance input, but the resistors do not have to all be 1K.
Cipocip - the phase of the loop gain does not assume a value of 180deg between those two frequencies. You must not look at the asymptotic lines only. The real magnitude of the loop gain does not have a roll-off of -40dB/dec. This is true because (a) the open-loop gain of the opamp and (b) the rise of 1/beta both never reach 20 dB/dec (corresponding to a 90deg shift). As you know, each first-order system has a phase shift of 90 deg for infinite frequencies only. In the present case, the situation is even less critical because the 1/beta function has a pole (following the zero). Hence, the maximum phase shift will be (perhaps) only 75...80 deg. - dpending on the distance between both frequencies.
More than that, a clear picture of the stability margin can be gained looking at the NYQUIST plot for the loop gain. The critical point never is encircled.
Best Answer
The op-amp is a real op-amp with output resistance, so heavily loading the output with a capacitor will add phase shift which will reduce the phase margin. Most op-amps will oscillate like a banshee with much, much less than 10uF on the output (sometimes less than 1nF is enough).
The isolation resistance allows the AC feedback to be separated from the DC feedback so you get a voltage across the capacitor that is eventually very close to the input voltage, despite loading, whilst the circuit remains stable.
If you don't care about small slow changes due to loading, you can eliminate the DC feedback path and just put the resistor in series with the capacitor on a voltage follower.
With many op-amps, especially low power types, the resistor will have to be higher than 37 ohms, maybe 100 ohms to 1000 ohms.