Know of any op amps with low input **capacitance**???

[Cerebus]

I've ordered some LMC662's which we will drop on the board in place of the existing (rather low spec) units and see if that buys us some improvement.

I assume you are aware, but just in case you aren't, that these are such high input impedance devices that you might have to step up your guarding and shielding techniques  to avoid problems with tiny leakage currents that wouldn't bother lower spec devices. There's a video kicking around somewhere showing the rig that Dave Grohl Paul Grohe and Bob Pease built to characterise its more expensive cousin the LMC6001 which is most instructive.

[Alex Nikitin]

For low input capacitance and less than 5V supply voltage have a look at  OPA355, OPA356 and OPA358 with 1.5pF/1.5pF input capacitance.

Cheers

Alex

[Zero999]

How about the input impedance and bias currents?

If you just want a low input capacitance, the old uA741 has an input capacitance of only 1.4pF.
http://www.ti.com/lit/ds/symlink/ua741.pdf

[xygor]

Do you suppose you could use a bridge circuit, which would cancel the nonlinear part?

[Marco]

People are getting a little too scared of discretes ...

[IDEngineer]

I assume you are aware, but just in case you aren't, that these are such high input impedance devices that you might have to step up your guarding and shielding techniques  to avoid problems with tiny leakage currents that wouldn't bother lower spec devices.

Indeed, and we already have careful guarding and shielding in the design for exactly that reason. We also treat the PCB as an active component and spend a LOT of time working with the PCB Designers to optimize the layout for this type of measuring.

Good reminder, thanks!

[T3sl4co1l]

Put a bootstrapped buffer in front of it?

Phil Hobbs did this for a microchip probe jig, and posted and discussed the circuit.  The basic skeleton is this: Probe --> JFET follower (CCS loaded) --> BJT follower --> (output) --> positive voltage offset (zener) --> BJT follower --> JFET drain.  The CCS-loaded JFET has DC gain very close to 1, the BJT has a low output impedance and fairly high gain (0.99ish), and the follower in the drain circuit bootstraps out Cgd.

Actually I think he used a PHEMT or something, not a JFET, but you might not need that.  BF862 is the usual favorite, though it's EOL now; there's an On Semi replacement out there.

Also place the output node under/around the probe node, to bootstrap the capacitance to space.

He ended up with a fairly small fractional pF input capacitance, which easily met the application requirement.

Tim

[Cerebus]

BF862 is the usual favorite, though it's EOL now; there's an On Semi replacement out there.

2N/MMBF 4117 would probably work nicely here as there's no requirement for a high f_t and it's got a tiny junction with concomitantly low C_iss (3pF versus 10pF for the BF862).

[T3sl4co1l]

'4117 is embarassingly small (kohms Rds(on)), I might be worried about output impedance / drive capability instead.  Sure, there's probably something inbetween, a few mA versus the 8-20mA of a BF862.

A 2N3819 or 2N5486 probably won't give very good noise at these low frequencies... but I suppose if an LM358 is in play, there's no care about noise here.

Which might give the OP some reason to think about how to improve the sensitivity of this circuit: if it's not in the noise floor, it's not really at the limit, is it?

Tim

[BrianHG]

No one has yet recommended to additionally add a series resistor & cap to GND on the - feedback input to create a filtered compensated output response adaptively nullifying some of the sluggish response due to the capacitive load of the + input.

[Henrik_V]

The LTC6268 spec is  .450pF ...
http://cds.linear.com/docs/en/lt-journal/LTJournal-V25N2-00-df-LTC6268-GlenBrisebois.pdf