Hi all,
Someone at work wanted a buffer to drive a coax cable with a signal from a high impedance device. I thought this would be a quick job so I built a buffer amplifier, using half of an AD8066 configured as a unity-gain buffer. I know that op-amps don't like capacitive loads, so I connected a 50 Ohm resistor between the cable and the buffer amplifier and also terminated the other end of the cable in 50 Ohms, so that the op-amp sees a purely resistive 100 Ohm load. When I tested it, the first thing I did was to short the input to ground with a clip lead, and it oscillated vigorously at about 50 MHz.
Here is a diagram and a photo of the construction:
Using a x10 passive probe on an oscilloscope I could see 2V p-p at the non-inverting input of the op-amp. Moving the shorting clip-lead, or connecting different inductances across the input of the amplifier would change the frequency. With low inductances it could be made to oscillate at up to 116MHz, or with 19 turns of hookup wire wound aroud a 60mm diameter, it oscillated down at 6.5MHz, sometimes still able to oscillate with 1kOhm in series with the inductor.
It seems like the input impedance of this buffer has a negative real part. If someone has a VNA handy, I'd be interested if you made any measurements of this chip configured as a buffer. Anyway, as a general-purpose lab amplifier, or as a FET active scope probe, this op-amp seems fairly undesirable, since one generally wants it to be stable regardless of what is being probed at the time.
The datasheet of the AD8066 gives no warning of this kind of misbehaviour. There is a linear tech app note that makes a brief mention of this sort of phenomenon for op-amps in general, under the heading "strange impedances":
As a very hand-wavy explanation, I suspect that there is a fair bit of capacitance between the two input pins, and that in combination with the inductor, this causes the differential input voltage of the op-amp to lag in phase behind the output voltage of the op-amp, leading to excessive loop phase shift at frequencies where the gain is still more than unity.
By trial and error with a variable resistor and some capacitors, I managed to mostly fix this buffer amplifier by adding some shunt capacitance to ground from the non-inverting input pin, and some resistance in series with the input connector. The 3.3pF shunt capacitor really helps to reduce the amount of resistance needed. It is not a very satisfactory fix because it increases the input capacitance, and also because I can't prove that there is not some other source impedance which, when applied to the input, would make it oscillate again.
If anyone knows a fast fet-input opamp or buffer that certainly won't oscillate with arbitrary passive impedances connected at its input, I'd be interested to know.
Chris