AD8066 buffer oscillates with inductor on input

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:

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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":

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Other than that I think this is not a commonly-known characteristic of op-amps. I vaguely recall encountering it in simulation, years ago with an on-chip op-amp that I designed, within part of a SOC.

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

Reply to
Chris Jones
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The inductor is probably resonating with stray capacitances. A shunt resistor might kill the oscillation. It would be interesting to know the critical value.

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

A circuit I have used is a X2 amplifier with the 50 Ohm series termination resistor outside the feedback path. This allows you to put whatever compensation cap across the feedback resistor to get the stability you want.

For driving large signals into 50 Ohms, my solution is a suitable current- feedback op amp (such as LT1229) followed by an HA9P5002 buffer. In this case, the feedback resistor value is selected to get the desired gain bandwidth.

Jon

Reply to
Jon Elson

I don't think that it would help much with this problem (other than that the loop gain would be 6dB lower).

Ok thanks for the part numbers, that might be useful later, but the problem I was having was with the *input* of the op-amp oscillating, not with driving the 50 Ohm cable. I did really want the FET input as I had hoped for high input impedance (but I didn't bargain for negative resistance!).

Reply to
Chris Jones

The thing is that I wanted to be able to use it as a general purpose lab amplifier like a FET active scope probe. You don't have to worry about putting shunt resistors on your circuit board to de-Q the traces when you use a normal FET active scope probe, that would be a pain. What I did instead was to put a series resistor and shunt cap right at the op-amp, as shown in red on my schematic. That can at least be done once and for all inside the amplifier/probe, and doesn't involve messing with each device that you want to probe with it.

Reply to
Chris Jones

Did you notice the resistors in the feedback of the G=+1 and G=+2 circuits in the datasheet? Put 25 ohms in series with both inputs of the unused section too.

I think you can remove the shunt C at the positive input, but keep the series resistor. You'll need a shunt resistor too, say a Mohm or so, if you want the amplifier to behave with its positive input not connected to anything else. FET bias current is small, but not zero.

The picture doesn't seem to match the schematics. Take the feedback straight from the op-amp output, not from the far end of the output series resistor.

Don't try to insert buffer amplifiers in the loop. They'll make you miserable.

Jeroen Belleman

Reply to
Jeroen Belleman

I saw something similar in the past... I think (like you) I added some series resistance.

George H.

Reply to
George Herold

Yes I saw that in Fig. 42. I thought it was for the reason in the paragraph: "For the best settling times and the best distortion, the impedances at the AD8065/AD8066 input terminals should be matched. This minimizes nonlinear common-mode capacitive effects that can degrade ac performance.". They don't bother with the resistor in Fig.55 or Fig.57. The value of 25 Ohms is probably too low to stop it oscillating anyway, I needed around 400 Ohms in some cases, without the shunt capacitor anyway, and with some inductive impedances that was still not enough.

That one didn't misbehave (as the wire from the non-inverting input to the ground plane is too short to allow it to oscillate) so I don't see the need. If the wire were 50mm or longer then I would expect it to oscillate though.

Yes, I tried that first. There are some inductanctive source impedances for which it will oscillate even with 1k series resistance. I didn't try higher series resistors, at some point the resistance starts to affect noise and bandwidth. With the added shunt C, much less series resistance is needed and it prevents oscillation more reliably.

Yes, but in the intended application 1MOhm would disturb the measurement so instead I warned the user to make sure the input is connected. If I had more time, I might add a resistive divider to the output of the op-amp and connect a 100M resistor from the non-inverting input to a tap at about 0.9 of the output voltage. I could also add a clamp as part of the resistive divider which would allow me to raise the positive supply voltage with less risk of damaging the instrument that receives the output signal. That would improve the positive input voltage range of the op-amp.

Sorry for the low-resolution picture, but it does match the schematic. The only change is that I have replaced the trimpot with a fixed resistor in the final construction.

Yes, I did that.

Generally, yes I agree, much extra testing would then be required, and trouble is likely unless the buffer is much faster that the op-amp whose loop it has been inserted into, which would not be easy with this fast op-amp.

Reply to
Chris Jones

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