LMH5401EVM: Reflections at output of EVM

Nicholas Vrahoretis

Part Number: LMH5401EVM
Other Parts Discussed in Thread: LMH5401, , TIDA-00826

Hello,

We're working on utilizing the LMH5401 as a wideband single-ended to differential converter for interfacing the output of a photodiode (Thorlabs DET08CFC) to an ADC. the photodiode stated that matching the termination to 50 ohms will maximize performance at high speeds. I modified the eval board to use the resistor values specified for single ended input with 2V/V gain, with RG1 = 90.9R, RT = 76.8R, RG2 = 121R, RF = 175R. I am routing each of the outputs into separate inputs in a high speed oscilloscope, with 50 ohm internal termination, and using a math function to observe the differential output. It would appear that there are reflections along some path in the signal chain, scope pictures attached. is there something I am missing for properly modifying the external resistor values?

over 2 years ago

0 Hruday Kumar Mudireddy over 2 years ago

TI__Expert 4160 points

Hi Nicholas,

Resistor Values looks perfect for gain = 2V/V. Can you help with below questions:

Have you terminated the other end of DET08CFC with 50 ohms?
Are you probing the output puts using an active probe or using sma to bnc connects to Oscilloscope? I recommend using a differential probe/ check the cables they may have asymmetrical responses.

Regards,
Hruday

0 Nicholas Vrahoretis over 2 years ago in reply to Hruday Kumar Mudireddy

Prodigy 40 points

1. There is no other termination for the photodiode, the output is directly fed to the LMH5401EVM board. should there be another 50 ohm termination?

2. the differential outputs of the EVM are sent via SMA cables directly to two separate oscilloscope channels, which have 50 ohm input impedance. we don't yet have active probes fast enough for this device and though this would be sufficient as the scope has 100GHz bandwidth.

0 Nicholas Vrahoretis over 2 years ago in reply to Nicholas Vrahoretis

Prodigy 40 points

I'm also curious if the voltage rails should be set differently to optimize voltage range. right now I have +/- 2.5V rails set for the EVM, but i've seen reference designs (TIDA-00826) that use +3.75V and -1.25V rails for the LMH5401.

0 Hruday Kumar Mudireddy over 2 years ago in reply to Nicholas Vrahoretis

TI__Expert 4160 points

DET08CFC datasheet says to terminate 50 ohms. Can you try that?

I recommend using a transimpedance amplifier stage before feeding to LMH5401 to buffer the photodiode. You can find the suitable TIA's in previous thread

Regards,
Hruday

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LMH5401EVM: Reflections at output of EVM