Electronic – Design constraints when dealing with very high resistances in op amp’s feedback loop

high pass filteroperational-amplifierpcb-designresistancesurface-mount

I have designed a PCB for a charge amplifier to handle a piezoelectric sensor, the first prototype is working properly. However, now coming down to the specific working frequency range, since the feedback loop is effectively a high-pass filter, with a cutoff frequency of 0.1 Hz, and there is a constraint on the capacitor's magnitude, with it being inversely proportional to the gain, I need to use very large resistance(s), in the range 3-10 Gohm, so I came to this question regarding the PCB design.

I have ordered SMD resistors, 5, 1, 0.5, 0.1 Gohms, all with the dimension of 0805 (inches), and the material of the Board of the PCB is FR4, the data sheet of which can be found here: https://www.farnell.com/datasheets/1644697.pdf, it's gonna be just the plain board with no coating or anything else after etching the copper.

I assume there might be some complications when it comes to such high resistances, stuff that I should be aware of, some design tips, and I just want to make sure there is no fatal flaws in my selection of components, or some properties that render them unusable for my application, like for instance the resistivity of the FR4 material, and the shortness of 5Gohm resistor. I have researched about aspects that seemed relevant to me, but I am still not so sure, I thought of everything.

So, if you see anything fishy or if you care to shed a light on anything related to this design, I'd very much appreciated.

By the way, is there any drawback to installing a number of resistors in series in the feedback loop instead of one gigantic one, maybe noise issues?

Thanks very much!

Albukhari

Best Answer

I used to do condenser microphone stuff professionally, which was very much this kind of thing (10G ohms with maybe a 30pF source).

Cleanliness is VITAL, a bit of skin oil in the wrong place and you will get weird popcorn noise, very annoying (An ultrasonic cleaner can be your friend here).

I would absolutely advocate solder resist to keep the potential for surface contamination down, but really the way to go is to use a teflon standoff for the high impedance node, and think carefully about the possibility of using a discrete jfet for the input buffer.

Consider guard tracks and also consider cutting a slot in the board under the resistor, extending the guard tracks to a pour on a layer under the resistor is also not a bad plan.

The problem with multiple resistors is they expose more places for contamination, and also give more capacitance to internal layers, neither of which is good.

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