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LMH32401: Continuous waves Detection

Part Number: LMH32401
Other Parts Discussed in Thread: OPA858

Hi team,

My customer have some questions about LMH32401:

1.LMH32401 is mainly used in lidar ranging. At present, most lidars are TOF, and the waves are pulsed lasers, but there are also FMCW (frequency modulated continuous wave) lidar, used continuous sine wave. So, can LMH32401 be used to detect continuous waves?

2.Based on the reference design of LMH32401 shown below, if a sinusoidal continuous wave is applied to the APD/PIN in the red box, can it normally respond to a sinusoidal signal?

Looking forward to your reply.

Thanks and best regards!


  • Hi Colin,

    Can you confirm the detector topology? Many of these FMCW circuits use a balanced photodiode pair configuration. The LMH32401 was designed to operate with current feedback being sourced from the input pin, hence the negative VBIAS in the above schematic. I believe there is a significant bandwidth reduction when sinking current, and since the beat frequencies in FMCW can be in excess of 250MHz, this is probably not  a good fit. I have reached out to the designer to confirm this behavior. If I find out anything to the contrary I will let you know.

    In the case of the single APD detector, I don't see any issue outside of the normal considerations. The FMCW technology combines the transmit chirp/sweep and the the receive chirp at the detector, so as long as the resulting beat frequency is within the bandwidth of the LMH32401 at the desired gain configuration, you should be fine. This is highly system dependent though, as it is directly related to the chirp bandwidth and sample time of the FMCW scan.

    Best regards,
    Sam Burnett

  • Hi Sam,

    Thanks for your information!

    1.The topology is as below, for D1 and D2, the flow of photocurrent to TIA is different. For D1, the current flows to TIA, and there is a 100mA charge pump inside the LMH32401. If the two currents are added, whether it will block and fail to respond?

    2.Difference frequency of their FMCW is about 150MHz,  could you please recommend a device that meets their design?



  • Hi Colin,

    Typically, the reference signal and receive signal are detected by D1 and D2, and the difference current is the input to the TIA. For this single-ended application with a balanced photodiode pair, I would recommend our OPA85X family of amplifiers. The OPA858 is a good place to start, just be sure to confirm whether the bandwidth will accommodate the gain requirement for the signal.

    Best regards,
    Sam Burnett