Guard for a photodiode signal

[kender]

signal of photo diode

Colleagues,

Could you recommend a technique for guarding a signal from a photodiode in photovoltaic mode? The photodiode signal will be close to DC. The photodiode will be connected with a 15cm cable. Environment is not expected to have a lot of EMI. I’m expecting photo currents in nano-amperes, I’m afraid that leakages will distort my readings, and I would like to add a guard to the photodiode signal.

The plan is to have a shield around cathode line, which is driven with a cathode voltage through a buffer (guard buffer).

Do you think this can work? Any suggestion and insight is appreciated!

- Nick

 

[dick_freebird]

An HP instrument I have, uses triax cables with a
similar scheme. But if there's only a nanoamp of
photocurrent, there ain't nothing to see and maybe
you don't care.

 

[kender]

An HP instrument I have, uses triax cables with a similar scheme. But if there's only a nanoamp of photocurrent, there ain't nothing to see and maybe you don't care.

What's the minimum current that your HP instrument can see?

 

[dick_freebird]

Long integration time and clean fixture, down to pA.

 

[jorgito]

Hello kender,

Rigid cables will help a lot.
If you need a flexible cable you may need to consider triboelectric charge generated during flexing.

Regards!

 

[FvM]

If active guard is feasible or if it makes sense at all, also achievable current resolution can't be answered without specifying the intended bandwidth.

"Photoconductive" mode b.t.w. implies to favour speed against low dark current. For highest sensitivity at low or medium speed, "photoelement" mode is usually preferable.

 

[kender]

If active guard is feasible or if it makes sense at all, also achievable current resolution can't be answered without specifying the intended bandwidth.

Signal from the photodiode will be DC.

"Photoconductive" mode b.t.w. implies to favour speed against low dark current. For highest sensitivity at low or medium speed, "photoelement" mode is usually preferable.

Thanks! I actually meant photovoltaic. The OP was corrected.

 

[rfsystem]

Guard drive is especially for DC important because the shield cabel could shield the electric fields but if there is a mechanic impact on the cable it could generate serious noise in low frequency.

Some more compressible cables are used as microphones for military submarines where there they have hundreds of cables sensing the accustics.

So if you have a guard drive the stored charge in the cable is zero. So ther charge change of the cable capacitance change because of mechanic impact is zero.

The second advantage is that the active gurard drive minimize the virtual capacitance and extend the bandwidth.

 

[kender]

Colleagues,

One more question on the subject of guard traces. Suppose there's a guard trace in the PCB layout. Would it better to leave it covered with silk screen or to make it exposed copper?

Cheers,
- Nick

 

[FvM]

Silk screen offers at least some protection against enviromental influences, e.g. moisture. It should be always used if no electrical contact is intended to the traces. I can't imagine a serious reason to remove it over guard traces.

Guard drive is especially for DC important.

. Yes, if cable microphony is an issue and a DC voltage is present between signal and screen. Otherwise, it doesn't reduce microphony.

Furthermore, microphony also occurs with zero DC level across the dielectric caused by "frozen" charges. Unfortunately, it's even amplified by a driven guard screen.

 

[kender]

Silk screen offers at least some protection against enviromental influences, e.g. moisture. It should be always used if no electrical contact is intended to the traces. I can't imagine a serious reason to remove it over guard traces.

Here's my concern. Suppose there's a place where the signal trace has to be exposed (e.g. a pad). If the guard trace is covered with solder mask in that area, then the impedance between the source of leakage and the signal will be smaller than that between the source of leakage and the guard. It should be the other way around.

 

[FvM]

The problem also arises on surfaces where you can't have guard traces, either coated or uncoated, e.g. at parts. If leakage currents are that critical, conformal coating is the obvious solution.

But if it seems appropriate to you to expose part of the guard trace, why not?