OPA857EVM-978: No output from board in photodiode mode.

Ali Furkan KAMANLI

Part Number: OPA857EVM-978
Other Parts Discussed in Thread: OPA857

Hello;

I have purchased OPA857 EVM for transimpedance amplifier circuit measurement and avalanche Photodiode pulse measurement for my P.h.D thesis.

First I have connected J4 (And I limit the current but when I connect J4 its trying to draw more current each time) for +VS=3.3v to Test_in input. After I put it in the APD mode. 20K gain. 5 V through the APD Bias Voltage. When I cover the photodiode I have measured the voltage differences R6 AND R7 independently but There is no voltage change in the R8 that I can measure. and the Resistance seem like 0 ohm on that resistor. And also I can not take any voltage change across the transformer. I Have measured almost every part. I can only find 2 different things R8 seems like 0 Ohm and Reference voltage sometimes can be quite faulty when I limit the current(Because I am afraid it can burn the opamp thats why I limit it to 50ma).

My application is I will make some very fast pulses from LED and try to measure it through the EVM but I couldn't do it.

Could you please help me with it3

Thanks in advance.

Best regards.

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Ali,

due to R3=49.9R a current of 66mA will flow when you connect 3.3V. So, limiting the current to under 50mA will not make the board work.

Have a look at figure 16 of EVM manual.

Kai

0 Ali Furkan KAMANLI over 6 years ago in reply to kai klaas69

Intellectual 310 points

Hi Kai;
Later I realize it and 89mA is now my board current. I tried ever configuration but I cannot get any differensial output from the Transformer. I want to measure pulses from led source but there is a problem. I can measure every part and I can see the change on R6 and R7 when I use the led source but I cannot get any output ?

Can you help me about it?
I want to get differantial output.

Thanks in advance.
Best regards

0 kai klaas69 over 6 years ago in reply to Ali Furkan KAMANLI

Guru 140200 points

Hi Ali,

R8 is 56.2R. You should be able to measure this resistance, provided there sits no jumper on J7.

Kai

0 Ali Furkan KAMANLI over 6 years ago in reply to kai klaas69

Intellectual 310 points

Hi Kai;

I cannot measure R8 it seems like 0 ohm everytime I try. İs it possible that the transformer impedance interfering the measurement? Or My evaluation board is faulty? And there is no jumper on J7.

Thanks in advance.

0 kai klaas69 over 6 years ago in reply to Ali Furkan KAMANLI

Guru 140200 points

Hi Ali,

yes, you are right, the transformer could have a low DC resistance short-circuiting R8. But 0R? Unfortunately, the datasheet of transformer does not show any DC resistance.

Kai

0 Ali Furkan KAMANLI over 6 years ago in reply to kai klaas69

Intellectual 310 points

Hi Kai;
Why can not I measure anything from the transformer? İs there any other configuration for Pulse detection?

Thanks in advance

0 kai klaas69 over 6 years ago in reply to Ali Furkan KAMANLI

Guru 140200 points

Hi Ali,

if you have fast pulses, then the transformer output makes sense. If you don't have fast pulses but only very low frequency or even DC signals, then you can connect the scope directly to the output of OPA857.

It depends on the signals you expect to have.

Kai

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OPA857EVM-978: No output from board in photodiode mode.