I'm very interested in measuring the the precise amount of amperage coming out of a Si photodiode for an experiment of mine while under illumination. We use Gamry potentiostats for these measurements. I'm looking to purchase a photodiode from thorlabs, such as those listed here. The FDS100 and FDS1010 are both attractive candidates for my application, but for the FDS100 there is an anode, cathode and "case." Should I treat "case" as a ground and wire it to my potentiostat's ground? Why doesn't the FDS1010 have a "case"?
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My guess is that the case has to be grounded when the TO-5 package isn't mounted to a chassis, in order to protect from ESD. – Shamtam Jun 07 '12 at 01:58
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Why doesn't the FDS1010 have a "case"?
Case provision in the FDS100 is more a matter of happenstance than necessity.
The case is provided for convenience of mounting.
As they have the case there they make provision to connect it electrically, but a case is not an inherent necessity for photo diode operation.
Should I treat "case" as a ground and wire it to my potentiostat's ground?
The case can be grounded.
In the FDS100 the case is not electrically connected to the diode in any way but can serve as an electrostatic shield when the diode is used in a "floating" application where neither terminal is grounded.
For example, diode response speed and linearity can be improved by applying reverse bias. In this case the diode "floats" above ground and the 3rd lead is grounded.
As shown below (diagram from datasheet).
The FDS100 and FDS1010 are both attractive candidates for my application
It's worth noting that as well as increased area, the FDS1010 offers better response linearity with wavelength (but less response at the very deep UV end of the range). The FDS1010 has about 7 x the active area and ~= 5 x the sensitivity, but frequency response is about 8 MHz compared to typically 35 MHz for the FDS100. Graph from datasheet -
FDS100 FDS1010.jpg
Russell McMahon
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The case, for the FDS100, is simply the metal shell surrounding the device. Electrically, this is not connected to anything, but tying the case to a low noise point such as ground would be a good idea. There is a chance that the metal case has a substrate connection to the device, and it could have electrical impact on the device.
For the FDS1010, there is no metal can surrounding the device. The photodiode die is directly mounted to a copper-laminated ceramic substrate. Through some brief reading of the product material, I believe the primary reason that there isn't a standard case for the FDS1010 is because it is very large. There probably isn't a good existing package with a window available that can house a 10mm x 10mm device, which is why you have a different packaging approach.
W5VO
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