Re: Leakage Current in Photodiode Circuit

Jarvis wrote:
Hi Folks -

I've got a question that is really more about electronics but probably only
a scientist skilled in electro-optics may appreciate. My question concerns
practical and realizable volume and surface resistivities seen in FR-4
printed circuit material.

Here is a description of the problem...

I'm building a ratiometric optical sensor. It has two channels of optical
signal that are divided with a beamsplitter, spectrally filtered, and
incident on two separate InGaAs photodiodes. The photodiode current then
goes to a charge integrating 20-bit A/D that has an input at "virtual
ground".

I'm finding that the ratio of the output of the two sensors is varying as a
function of overall light intensity. This is very bad as it indicates that
spectral ratio is changing when it isn't actually doing so.

For instance, when the overall light intensity drops by a factor of 2, the
ratio changes by 3%. When the light intensity drops by a factor of 13, the
ratio changes by 20%.

I'm presuming that the photodiodes are linear and the A/D is linear. At
least the spec sheets would suggest so.

I've pretty much determined that the problem is (mainly) due to leakage
current. I'm typically dealing with photodiode currents of about 2500 pA to
250 pA or lower. My leakage current would appear to be about 75pA.

I think that I've found a layout issue with the board that I would like to
blame but things still don't quite add up yet. This a four-layer board and
it has a power plane that runs under a 3 cm length of a 0.5 mm (20 mil)
sensor trace. The leakage current to the power plane suggests that the
leakage resistance is about 66 GOhm. By using the geometry and the
published minimum resistivity of FR-4 (8e7 MOhm*cm), I estimate the leakage
resistance should be 600 GOhm.

There are also four thru-holes along the 3cm length of trace that also go
through the power plane in four spots. At this point, there is potential
for the surface resistivity between the surrounding power plane and plated
through hole to be an issue. There is a 0.5 mm (20 mil) separation between
the hole and the power plane. Using a published figure of 5 MOhm for
surface resistivity, it looks as if I could get about a 50 GOhm leakage
resistance. Hmmm, this is low enough to be relevant and account for
observation.

My questions are...
1) Is the minimum volume resistivity for FR-4 of 8e7 MOhm*cm practically met
in finished boards?
2) Is the minimum surface resistivity for FR-4 of 5e5 MOhm met for interior
layers? (Seems like this would be very specific to board vendor and their
attention to cleanliness.)
3) Actually both photodiodes have similar current paths and the same number
of thru-holes. I'm a bit baffled about why one side would "leak" more than
the other. This is the part that doesn't quite add up. I would appreciate
any thoughts on this.

Ultimately, I need to re-layout the board to get rid of this issue but would
like to do it from an informed viewpoint.

My apologies for the long post but I hope it is succinct.

Sincerely,

John




Why is the signal level so low? Fluorescence or SHG or something like that? (My prejudice is always to try for more light first.) Chasing board leakage is a miserable business. Running the board through a nice clean domestic dishwasher is a good start, if there's nothing that'll die if it gets wet. Conformal coating, ideally using a Parylene machine, is a good next step, because your leakage will depend strongly on humidity otherwise.

Cheers,

Phil Hobbs
.

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