OPA657: Device recommendation with TIA

Hi David,

The LMH34400 does not have output voltage range that can reach -1V, the lowest achievable output voltage with the LMH34400 is close to 0V. 

Also can you clarify a few things so I can make the best suggestion:

• What is the input signal frequency of the amplifier
• What is the capacitance of the photodiode

Thanks,

Nick

Hi Nick Nauman,

Good to see you again.

You left reply for my post.

As adding to information for more details,

TIA and Operational Amplifier need a minimum bandwidth as at least 100MHz.

And regarding 3. power consuption at PD:10uA~40uA, Balanced Photo Detector may produce around 10uA~40uA electrical current

The capacitance of the PD is 2.5pF(max 5 pF @ f=1MHz, Vr=5V), but it is only for single photodiode, but we will use two photodiodes for balanced photo detector.

It doesn't need to be LMH34400 as a TIA. So if there is another way to meet above requirement, Let me know.

For instance : two pieces of OPA847, one is for TIA and the other one is for Operational Amplifier .

For the frequency, I am not sure this could be answer for it, the balanced photo detector receives 1550nm wavelength which means 193.414 THz

Requirements

1. 125kV/A ~ 250kV/A

2. Output voltage range: 0V to -1V or +1V (modified)

3. At least 100MHz

I wonder if there are any problems when my circuit consists of LMH34400 and OPA657 or with similar products such as OPA843.

I attached my circuit diagram on this post.

Please kindly review my inquiry.

Thanks.

Hi Seongjin,

Can you clarify a few more things based on your response:

1. Are you trying to achieve a gain between 125kV/A to 250kV/A or are you trying to scale you output so it is 0V to 1V? If the current of the photodiode is 40uA and there is a gain of 250kV/A, the output voltage would be 10V. However, if the gain was 25kV/A, you would achieve your 1V output that you mentioned.

2. To clarify the reason for having a output voltage that ranges from 0 to +1V or 0 to -1V: is this to differentiate between which photodiode is outputting the larger current?

Thanks,

Nick

Good day Nick Nauman,

Here are my replies for your questions.

1. Achieving gain in range of 125kV/A to 250kV/A is more important. please do not consider output voltage range in 0V~1V or 0V~-1V.

2. Voltage amplitude does't matter now and using balanced detection is able to enhance the Optical Singal to Noise Ratio(OSNR).

    This has advantage of coherent detection.

Thanks

Hi Seongjin,

I am looking into your design to determine the best options. I will get back to you by the end of the day tomorrow.

One more question: what is the purpose of having the second amplifier in your design set to a gain of 10? That would cause the circuit to hit output swing limits.

Thanks,

Nick

Hi Seongjin,

I have attached a TINA schematic of how to implement a balanced photodetector transimpedance amplifier using OPA657. I believe this best meets all the requirements you are looking for. I have the gain set at 125kV/A. However, if you have a current up to 40uA, you will not be able to achieve a full +/-5V output swing but rather about +/- 3.5V depending on the load. Also, if you are inputting current signals that are 100MHz, you will likely need to have two stages and lower the gain on the first stage and have some gain on a second stage. 

Please let me know if you have any questions about this. But I hope this helps meet your application needs.

OPA657_BalancedPhotodetection_E2E.TSC

Thanks,

Nick

Hi Nick Nauman,

Thanks for your kindness.

Unfortunately, OPA657 is not in stock.(SOIC type is only available for 250ea)

So I would like to have plan B.

Can you check OPA847 for the same purpose?

In case that OPA847 doesn't meet the gain set in ragne of 125kV/A ~ 250kV/A, I'd like to use a post-amplifier behind the TIA.

HI Nick Nauman,

It is really helpful to get through my project.

Thanks from the bottom of my heart.

Seongjin