DDC114: Photodiode reverse bias

Hi Daniele,

There is actually nothing specific about reverse biasing the PD related to the DDC. The only thing you need to make sure is that the current flows into the device (see figure 3) as the negative swing is limited to 0.4% of the full-scale. That the source of current is a PD, biased or not, or some other source, doesn't matter. The DDC does not know the difference and figure 3 is just showing one potential sensor/configuration that can be connected to the DDC.

So, as you are already pointing on your email, you can simply take the cathode of the device on that figure and raise its voltage (to whatever voltage 2 to 9V). Again, the DDC does not care. It will keep its input as virtual ground (anode of the PD) as long as it doesn't saturate, effectively applying a reverse bias to the diode.

Of course, this means that you'll have a dark current (current in the absence of illumination) which will flow into the DDC (this didn't happen with no bias). So, you have to take into account that, but this has nothing to do with the DDC. It will just measure it, like any other current flowing into it. Our experience is that many customers do not reverse bias the PD so that they do not have to deal with this dark current and its variations with temperature. But if you take these into account in your calibration routines, you should be fine.

Hope this helps,

Eduardo

Hello Eduardo,

thank you very much for your detailed answer.

I have wrongly stated to set the cathode voltage to negative values while you correctly point out to apply a positive voltage.  Also your consideration about the dark current is right. We are using a photodiode that should better perform with a small reverse voltage: we'll test if we actually have benefits. I only had one doubt with regards to the anode potential but I realize now that the current is always integrated by either the A or B converter, and therefore the IN pin is always tied to virtual ground.

Best

Daniele