OPA855: OPA855 TRANSIMPEDANCE

Parveen beg

Part Number: OPA855

Hi Team

I am trying to make a balanced detector. In the balanced detector I measured ac current through TIA stage and DC current through current sense amplifiers. I am struggling how can I place the photodiode model in the schematic. I have placed two photo diode models in series as shown in the schematic. Can you please suggest the right way of placement of the series photodiode models in the simulation such that I can measure both ac and dc currents simultaneously. The way I have currently connected its DC simulation shows a very high voltage at point A which is 500MEGV.

TWO SERIES PHOTODIODES DETECTOR.TSC

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Parveen,

remove C4 and replace it with a short circuit

Kai

0 Parveen beg over 3 years ago in reply to kai klaas69

Prodigy 60 points

Hi Kai,

Thank you for your reply. I need a coupling capacitor at the input stage to process only the AC signal. Can you please suggest any another alternative for this?

0 kai klaas69 over 3 years ago in reply to Parveen beg

Guru 140200 points

Hi Parveen,

but you must allow the photodiodes' DC currents to flow, otherwise the photodiodes' currents will destroy the DC biasing of photodiodes. Replace the cap by a short circuit, allow all the photodiodes' currents to flow into the virtual ground of TIA and remove the DC information afterwards, behind the TIA.

Kai

0 Parveen beg over 3 years ago in reply to kai klaas69

Prodigy 60 points

Hi Kai,

Thank you for your quick reply. But still a few things which are not clear. For example, if photodiode generates 1mA dc and noise of 40uA (ac), then at the output I will get only the saturation voltage because the 1mA dc generates a voltage of 2V at the TIA output. Noise is not detected at the output. Can you please clarify this?

8103.TWO SERIES PHOTODIODES DETECTOR.TSC

0 kai klaas69 over 3 years ago in reply to Parveen beg

Guru 140200 points

Hi Parveen,

then remove the DC component by the help of high pass filter at the output of TIA and amplify the AC component with an additional stage.

To keep the photodiodes operating linearily, all photodiodes' currents must be able to flow into the virtual ground ground of TIA, all DC and AC components and from both photodiodes.

Kai

0 Parveen beg over 3 years ago in reply to kai klaas69

Prodigy 60 points

Hi Kai,

Thank you very much for such a helpful explanation. You really solved a major problem for me and I really cannot thank you enough .

0 kai klaas69 over 3 years ago in reply to Parveen beg

Guru 140200 points

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

Amplifiers

Amplifiers forum

OPA855: OPA855 TRANSIMPEDANCE