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LMH6629: Pattern layout of TIA

Kuramochi Tadahiko
Guru 28715 points
Part Number: LMH6629
Other Parts Discussed in Thread: TINA-TI, OPA857, TIDA-00725, TIDA-00978

Hi,

My customer is evaluating the LM6629 as Trans-Impedance Amp.

However, they could not get enough gain@120MHz on their board.

Though they're aimming 2000V/V, actual gain value is 680V/V. 

They suspect that the cause is the pattern layout, because it can seem to get the gain 2000V/Von TINA-TI.

Could you support their following request if you have evaluated the LM6629 as TIA?

1. How is the GND pattern of TIA routing?

2. Should we countermeasure anything to reduce the input capacitance of TIA?

3. Could you tell the APD's name if you have evaluated the LM6629 with the APD?

    And how do you measure frequency characteristic of APD?

APD: Avalanche Photo Diode

Additionally, I'm requesting their schematic and layout to my customer.

Please let me know if you need more information.

Best Regards,

Kuramochi

  • 0
    TI__Genius 12631 points
    Hello,
    If you are not getting the desired gain at 120 MHz then it probably means that the amplifier closed -loop bandwidth is < 120 MHz. How is the customer measuring the performance at 120 MHz...with a sine wave input?

    Like you mentioned we need to start with the schematic of the circuit.

    Also, what is the capacitance of the photodiode. Has this been incorporated into the TINA simulation?

    In high speed applications it is recommended to remove the GND plane on the feedback path between the amplifiers output and inverting input.

    I would recommend using the OPA857 for this application. It can provide 125 MHz of bandwidth at 5 kOhm gain. We have a few reference designs where we have used the OPA857 along with a photodiode.

    TIDA-00725

    TIDA-00978

    You can purchase the TIDA-00978 EVM and install the APD on there and make some quick measurements.

    I haven't actually measured an APDs response. Usually they are specified at a GHz which is a lot faster than the TIA so shouldn't be a problem.

    -Samir