Misunderstanding of data sheet of photodiode C30954EH

[ggeorgiev]

Hi all,

I am a bit confused about the datasheet of the photodiode C30954EH https://www.excelitas.com/de/de/file-download/download/public/60161?filename=Excelitas_Si_APD_C30954EH_datasheet.pdf  from Excelitas.

As shown in the picture below, they have written Anode (Negative Lead) and Cathode (Positive Lead). But isn't that actually wrong?
Isn't the anode usually always the positive lead and the cathode the negative one?

Or why have they written it that way?

Thanks in advance for the help!
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[ataradov]

The diode itself does not have "positive" or "negative" side. Photodiodes are typically operated in a reverse-biased configuration, so I guess they stressed this fact by marking C and A that way.

[ggeorgiev]

The diode itself does not have "positive" or "negative" side. Photodiodes are typically operated in a reverse-biased configuration, so I guess they stressed this fact by marking C and A that way.

I want to use the diode in a reversed-biased configuration. So connecting it as shown in the picture below, basically Anode and Case to the ground and Cathode to the negative input of the OPAM, must be correct. "Without being confused by negative and positive.."

[ataradov]

Yes, this is the typical connection. So, go with the Anode/Cathode nomenclature and ignore those "helpful" notes.

[RoGeorge]

Anode (the arrow side of the diode symbol) and Cathode (the bar side of the diode symbol) of any diode are dedicated terms.  'Positive lead' and 'Negative lead' are ad-hoc terms, probably used to denote that the photodiode should be kept polarized in reverse in the detection circuit.

[RoGeorge]

In the schematic you posted both the anode and the cathode of the diode are connected to the same potential, the signal ground, because the positive input of the opamp is also at the signal ground.  A and B inputs of the opamp are at the same potential.

To polarize the diode in reverse, you'll have to keep the in+ of the opamp at a higher potential than the anode of the diode.