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Will my input stage blow up?
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void_error:
Well, not actually blow up but stop working in any way with signal applied and powered off...

I've been looking for an answer to this but all I could find is this and a simplified schematic of the op amp I'm using - AD8065.

You can find the schematic here, input signal is labeled SIG_IN, second page. Something that's not up to date is the power rail sequencing I'm currently working on, so you can safely assume that every rail on the page will be disconnected by MOSFET switches.

Anything pointing me towards an answer would be greatly appreciated.
Zero999:
The diode connected transistors Q201 to Q204 should protect the input stage of the op-amp from input voltages outside its supply range, so having the input connected, when the circuit is unpowered won't cause any damage.
T3sl4co1l:
FYI, as shown you've got a +/-1.2V or so clamp, which isn't a very wide range.  Also, the >= 1M input impedance into the ~20pF B-E capacitance forces a maximum bandwidth of 8kHz.  You certainly won't need to spend all that money on a 120MHz opamp!  But I'm not sure that this would be all that practical for a frequency counter.

To get 120MHz BW into 20pF, you need a "speed-up" cap, of the same value as the clamp, divided by the attenuation ratio.  Unfortunately because your attenuation ratio is 1 (i.e., it's not dividing anything), there's no room for a "speed-up" cap in it (or, pedantically speaking, your only solution to the equation requires a negative capacitor -- not an outright impossibility, but hardly a practical one).

You probably need a capacitor in parallel with R209 for stability, and with R206 to maintain bandwidth, for precisely the same reasons but working against U205's input capacitance (2-5pF plus strays) this time.

If you need a wide range, high bandwidth, very high impedance, very low capacitance input circuit, consider the bootstrapped JFET follower circuit that oscilloscopes typically use.  You will need to sacrifice gain for bandwidth, using speed-up caps on attenuators, to compensate for their resistance and load capacitance.  You can also use the opportunity to bootstrap the clamp/ESD diodes, so they have less effect on the signal.

Tim
void_error:

--- Quote from: T3sl4co1l on June 17, 2019, 02:42:33 pm ---FYI, as shown you've got a +/-1.2V or so clamp, which isn't a very wide range.  Also, the >= 1M input impedance into the ~20pF B-E capacitance forces a maximum bandwidth of 8kHz.  You certainly won't need to spend all that money on a 120MHz opamp!  But I'm not sure that this would be all that practical for a frequency counter.

--- End quote ---
Oh, forgot to change the value of that, it'll definitely be a lot lower. I'll play with that bit in LTspice.


--- Quote from: T3sl4co1l on June 17, 2019, 02:42:33 pm ---To get 120MHz BW into 20pF, you need a "speed-up" cap, of the same value as the clamp, divided by the attenuation ratio.  Unfortunately because your attenuation ratio is 1 (i.e., it's not dividing anything), there's no room for a "speed-up" cap in it (or, pedantically speaking, your only solution to the equation requires a negative capacitor -- not an outright impossibility, but hardly a practical one).

--- End quote ---
I'm aiming for roughly -6dB @ 100MHz.


--- Quote from: T3sl4co1l on June 17, 2019, 02:42:33 pm ---You probably need a capacitor in parallel with R209 for stability, and with R206 to maintain bandwidth, for precisely the same reasons but working against U205's input capacitance (2-5pF plus strays) this time.

If you need a wide range, high bandwidth, very high impedance, very low capacitance input circuit, consider the bootstrapped JFET follower circuit that oscilloscopes typically use.  You will need to sacrifice gain for bandwidth, using speed-up caps on attenuators, to compensate for their resistance and load capacitance.  You can also use the opportunity to bootstrap the clamp/ESD diodes, so they have less effect on the signal.

Tim

--- End quote ---
Thanks for the tip again Tim.
As for the scope-like input circuit, I might be biting off more than I can chew.
void_error:
Possibly a dumb idea... but it might just work: use a relay to disconnect the input. Something like this one?
That way I could increase the clamping voltage to something like +/-5V, and supply the AD8065 with something that would make the +/-5V input be within its input range.

 :-//
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