OPA858: AC Coupling APD to OPA858

Dharani Jade

Prodigy 75 points

Part Number: OPA858
Other Parts Discussed in Thread: LMH32404, LMH13000

Hi,

I have designed an APD based TIA amplifier circuit using the OPA858, with these connections-

APD cathode connected to positive HV bias

APD anode connected directly to OPA858 IN- input

OPA858 IN+ input connecte to a 3.5V reference

Feedback resistance of 22k

The circuit produces the expected pulse shape and amplitude. However, it is intended for a LiDAR system, and I’m concerned that DC photocurrent from ambient light will reduce the available output swing and potentially saturate the TIA.

How can I effectively block the DC photocurrent from enterting the OPA858? My circuit currently looks like this:

3 days ago

0 Evan Kirkbride 3 days ago

TI__Expert 3220 points

Hi Dharani,

This app note outlines a popular scheme for ambient light cancellation:

https://www.ti.com/lit/an/sboa324a/sboa324a.pdf

There are also schemes to AC couple but that has its own considerations. How much DC current are you looking to cancel?

Thanks,

Evan

0 Dharani Jade 2 days ago in reply to Evan Kirkbride

Prodigy 75 points

Thanks very much for the response and App note. I did have a look at this before but I have some form factor restraints, i.e, Im looking to use the OPA858 in die form for a 4 quad APD (so 4 OPA858s, one for each channel) all fitted in a TO-8 package. I was looking to see if there are any other simpler ways of doing it. The exact value of the ambient light hasn't been quantified yet unfortunately, but could you elaborate on other schemes to AC couple which may only involve passives?

0 Evan Kirkbride 2 days ago in reply to Dharani Jade

TI__Expert 3220 points

Hi Dharani,

If space is a constraint then I would recommend the LMH32404 instead - which is also available in die form. It is 4-channel, 20kohm gain and has built in ambient light cancellation. Much easier and smaller than going discrete.

https://www.ti.com/product/LMH32404

If you are looking for something on the Tx side as well the LMH13000 is a good choice.

Otherwise I was thinking of something like the first proposition in this thread where the APD has a resistor to ground and is AC coupled after. However depending on the amount of DC current this can forward bias the APD. AC coupled photodiode amplifier: differences on 2 different circuits

Thanks,

Evan

Amplifiers

Amplifiers forum

OPA858: AC Coupling APD to OPA858