OPA380: Can OPA380 used to amplify the photodiode which is illuminated by a DC light

Jonathan Navor

Part Number: OPA380

Hi Guys,

Good day. We need your help to check our customer's concern.

I have attached OPA380 TIA circuitry that was used in two cases from our customer. Kindly review the circuit.

Currently they are using the OPA380(single-side power supply +5V, pin-4 is to GND) to convert and amplify the Photodiode's photocurrent. The PD is illuminated by a LED biased with a DC voltage. However, they can only get a constant voltage around 3.4V with a 1 Mohm feedback resistor and no feedback capacitor each time under different LED light intensities. The output voltage is supposed different because the PD photocurrent is different under different LED intensity.

When they use OPA380 to amplify the PD' photocurrent which is generated by shining a AC modulated LED (e.g. a squre wave), the photocurrent in that case is AC and OPA 380 works well there and can generate a square voltage output.

In the first case from customer photocurrent is DC. Some Transimpedance Amplifier (TIA) does block the DC current (so-called DC current rejection). Kindly confirm if it is true that OPA380 also blocks the DC current if PD is shined by a light source with constant light intensity? Or there is something wrong with the connection.

Looking forward to your response.

Best regards,

Jonathan

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Jonathan,

is the photodiode connected to the OPA380 with the correct polarity?

What photodiode are you using?

Kai

0 Yuefei Cai over 4 years ago in reply to kai klaas69

Prodigy 30 points

Yes, the Photodiode is SFH2430. The anode is grounded and cathode is connected to the -IN (Pin2). The cathode and anode of photodiode are confirmed both by the I-V test by ourselves and the chip printout of its datasheet. Another further confirmation is that OPA380 can get a amplified square-wave photovoltage when shining the Photodiode with a LED modulated by square waves. My question is whether the OPA380 can be used for amplify a DC photocurrent?

0 kai klaas69 over 4 years ago in reply to Yuefei Cai

Guru 140200 points

Hi Yuefei,

the SFH2430 has a huge detector capacitance of 1000pF! This results in instability:

yuefei_opa380.TSC

Add a small phase lead cap:

Also, you might want to decrease the feedback resistance a bit to prevent overriding effects due to too high effeciency of photodiode. Take 100k in parallel with 22p and mount a 100R isolation resistor at the output of OPA380:

Sorry, I cannot post a picture here

Kai

0 kai klaas69 over 4 years ago in reply to kai klaas69

Guru 140200 points

See this thread for the picture:

https://e2e.ti.com/support/amplifiers/f/amplifiers-forum/992087/opa380-i-cannot-post-a-picture-from-one-moment-to-the-other

Kai

0 Yuefei Cai over 4 years ago in reply to kai klaas69

Prodigy 30 points

Hi Kai,

Thanks very much for your kind reminding. Yes, I agree with you that a stable DC point is critical and precondition for a normal operation. I will add a larger feedback capacitor and try different feedback resistor again. (Previous CF=1.5 pF maybe too small)

Thanks!

Yuefei

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OPA380: Can OPA380 used to amplify the photodiode which is illuminated by a DC light