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Original question:

LMH32404: LMH32404 differential output matching to 100ohm

LMH32404: LMH32404 series resistor, when channels are multiplexed to one ADC

Ari Ijas
Prodigy 30 points

Part Number: LMH32404
Other Parts Discussed in Thread: ADS117L11, ADS117L14

Hello, 

we are planning to use LMH32404 as multiplexer mode for 4 photodiodes and have one channel ADC. Datasheet figure 7-3 has a principle of combining outputs, how should we choose Riso resistor value? LMH32404 and ADC place very close to each others on board, so is there need for serial resistors at all?

Initial plan is to use ADS117L11 as ADC, but would you have any other recommendation for ADC component? Sample rate is maximum 1 MSPS, maybe lower in practise. CPU has SPI interface, which has maximum 30MHz SPI master read frequency. The application is not lidar, if you wonder low ADC sample rate.

  • 0
    TI__Mastermind 31875 points

    Hello Ari,

        You are right that if physically close to each other then there is no need for impedance matching. However, we will still recommend a small isolation resistor due to the internal impedance of the ADC. Most likely it wouldn't affect the amplifier, but it is better to place a small 5-10 ohm resistor to isolate it from the internal output impedance of the amplifier.

      I am moving this thread to our ADC team to answer the second part of your question. 

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    TI__Guru 50130 points

    Hello Ari,

    We recommend an RC filter between the amplifier and the ADC inputs; this helps maintain performance of the ADC.  A differential filter using 22Ohm series resistors and 2.2nF differential capacitor works well for most applications.  Since the LMH32404 already has internal 10Ohm resistors, the external resistors can be the difference, or 12Ohm in this case.

    Also, if you multiplex to a single channel, your data rate will be much lower due to the settling time of the internal digital filter.  Using the low-latency SYNC filter, the minimum settling time (latency) will be with OSR setting of 12, 4.38us.  This will limit the effective data rate to 1/4.38u, or 228kSPS.

    I would suggest using the 4-channel version, the ADS117L14, which will maintain full data rate per channel.  This will also allow use of the wideband filter, which has a much flatter frequency response, although for short pulse widths, the SINC filter may be the better option if this is the type of waveform you need to capture.

    The ADS117L14 requires a frame-sync interface; you had mentioned SPI only.  If this is a hard requirement, then you can use the single-channel ADS117L11 and configure as a parallel DOUT to support the maximum data rate of the ADC (this will require 4 SPI ports on your MCU).

    Regards,
    Keith Nicholas
    Precision ADC Applications