LMH32404RHFEVM: Product selection assistance for multichannel transimpedance amplifier

Hello Archie,

  For first question, yes, the EVM has capability of multiplex mode via adjusting the jumpers manually. 

"The pins M1 through M4 control the multiplexer switches for channels one through four respectively. The multiplexer pins will default to logic low which places the channel in the standby mode if left unconnected. The corresponding multiplexer pin must be connected to a logic high value to enable the channel. The EVM features four easy to use jumpers labeled M1 through M4 that control the multiplexer functions for channels one through four respectively. The EVM ships by default with the multiplexer pins set to logic high, which enables all four channels. Any channel can be easily disabled or enabled by changing its corresponding jumper pin position."

   Therefore, this must be done manually via moving jumpers from high to low on EVM rather than done digitally.

   For second question, how low of a photocurrent do you need to measure? The LMH32404 has an integrated 20kOhm gain, and input referred noise and output noise can be found in datasheet figures on page 13: https://www.ti.com/lit/ds/symlink/lmh32404.pdf

   The EVM as is does not have a place to put a photodiode, but LMH32401 EVM does have an optional EVM configuration for photodiode input: https://www.ti.com/tool/LMH32401RGTEVM 

    Both devices have integrated ambient light cancellation feature and input clamp limit. Which both features will be present in your case with negative reverse bias since both devices are optimized for a sinking current configuration where device is being used with a PD that is configured with its cathode tied to the amplifier input and the anode tied to a negative supply voltage. 

Thank you,

Sima

Dear Sima,

Thank you for your detailed explanation.

I would like to ask one more question regarding the LMH32404EVM modification.
In my case, the photodiodes are integrated on a photonic chip, so it is not possible to solder them on the back side of the EVM as in the LMH32401 configuration.

Would it be possible to remove or bypass specific components on the LMH32404EVM input stage to allow direct connection to real photodiode current outputs (-2V bias) from our chip (e.g., via SMA or wired connection)? If so, could you please specify which components should be removed or shorted?

I am not an electronics engineer and do not have access to one in my team, so I am looking for a plug-and-play solution that is straightforward to use, does not require a custom PCB design, and takes as little space as possible.

Thank you very much for your help and clarification.

Aleksandra

Hello Aleksandra,

   Thank you for the additional explanation! That is much easier to deal with then soldering on to an EVM if you are able to connect it via SMA cable and have the photodiode and reverse biasing done on a different PCB board. However, TIA (transimpedance amplifier) applications are sensitive, and we usually recommend the photodiode to be as close to the inverting input of the amplifier as possible. You might run into instability issues, which can be remedied by keeping the C21, C25, C34, C40 input capacitance on the EVM. And, keeping your cable as short as possible. In that case, you would just need to:

• remove R11, R15, R25, and R32
• replace C17, C23, C33, C39 with a 0 ohm resistor
• replace R9, R13, R24, and R30 with a 0 ohm resistor

   Is the photonic chip have an output current to interface to a TIA, and what is estimated internal capacitance at -2V reverse bias?

Thank you,
Sima

Dear Sima,

the photonic chip is now bonded into a PCB with gold pins, so I can easily connect it with another board. Photodiodes on a chip are characterised with about 20 nA dark current at -2 V and about 0.8 A/W responsivity in C-band. 

In the laboratory setup that I used before, when I wanted to do multichannel measurements, I have used 4 TIAs from Thorlabs (AMP145). In a consequence, the setup was big, messy and sometimes introduced noise to measurements. So having now bonded chip I would like to connect mutlichannel amplifier to further signal processing, keeping compact size of the setup. Could you please advise, it using LMH32404RHFEVM with suggested modification will be suitable for my application? Do you have anything else that you could advise?

Best regards,
Aleksandra

Hello Aleksandra,

  I apologize for missing a reply to this thread. Is the photonic chip still going to be connected to the LMH32404RHFEVM via the SMAs? If so, there shouldn't be an issue using the above suggested modifications to the LMH32404RHFEVM. I asked earlier about the expected input capacitance since this will affect the resulting bandwidth/stability of your application:

Thank you,

Sima