OPA855: Question About Using OPA855 for Differential TIA Design

문 찬희

Part Number: OPA855
Other Parts Discussed in Thread: LMH32401

Tool/software:

Hello, my name is Chanhee Moon.

To get straight to the point, I’m trying to design a differential TIA using the OPA855.
Using Texas Instruments' TINA simulator, I set up a basic differential amplifier circuit and checked the input and output. However, the expected differential gain(Rf/R) was not achieved. Instead, the gain seems to be fixed by .

To investigate this issue, I reviewed discussions on the TI forum, which led me to wonder if the OPA855 might not be suitable for differential amplifier applications.

Could my assumption be correct?
I’ve attached an image of the circuit I tested for your reference.

Thank you in advance for your kind assistance. I look forward to your response and hope you have a great day!

over 1 year ago

0 문 찬희 over 1 year ago

Prodigy 60 points

Additionally, the current generator is intended to represent the APD

+1 Sima Jalaleddine over 1 year ago in reply to 문 찬희

TI__Mastermind 32085 points

Hello,

We apologize for the delay. I was able to review your question, and in a true transimpedance application, the gain will be fixed by Rf. OPA855 can work as a differential amplifier, but not in the method you have shown for a transimpedance application. If you were to convert the current to a voltage similar to the below figure, then it is possible, but will slow your signal response down due to the RC time constant:

Linked here is a quick three-part blog series on TIAs for your reference: Part 1, Part 2, and Part 3. Also, We have developed a TIA circuit design tool: Analog circuit design studio where you can provide all the necessary circuit information and the tool will come up devices and values of estimated output parameters like: TIA bandwidth, noise, rise time and also best TI device suggestions. Currently we are working on including integrated TIAs into the tool, but please try the tool to get better understanding of TIA circuits and you can play around with the parameters too.

If you would like a true differential measurement of the APD, I would instead suggesting doing it this way:

In the example above, INA was used for high CMRR (especially for small differential voltage with a high common-mode voltage developed between the noninverting and inverting input) and/or some have GND isolation, but any VFB amplifier would work depending on your accuracy and bandwidth needs.

Here is a link to an application report that explains the different methods of differential transimpedance applications.

Another note, AC coupling for TIAs would require some type of bootstrapping or DC servo loop since the typical way by adding the series input capacitors does not allow the photodiodes' DC currents to flow, which will affect the DC biasing of photodiodes. If you are not worried about saturating the device at DC, then you may AC couple at the output of the amplifier. If you are worried about saturating the device due to high DC, then you may 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. Or via our integrated TIAs such as LMH32401 with ambient light cancellation. Or via DC servo loop as suggested earlier:

Feel free to ask about any clarifications or additional questions from the feedback above.

Thank you,

Sima

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OPA855: Question About Using OPA855 for Differential TIA Design