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LMP91200: About some design questions...

Peter Tseng
Genius 5555 points
Part Number: LMP91200
Other Parts Discussed in Thread: LMP7721

Dear Sirs,

Here are some questions from my customer. Attached is their schematics.

Please help to input your comments.

Thanks.

1. while the ph probe did not attached, the readout ph value will be 0 or 14 sometimes.

what voltage should be for Vout/Vocm while no ph probe attached?

The AVref=5V in schematics.

2. While measuring ph of water in vats, the Vout/Vocm=0V, that means ph value is 14. If customer take same water of vats and put in smaller container, the ph we get is 7.2

Is there anything wrong with it or affect by something? 

Since there are old/new version of LMP91200.(difference is removed temperature sensor function), does it cause by this change? 

3. Is there any recommended additional schematics for ph probe or LMP91200 to enhance signal or reduce noise?

  • 0
    TI__Expert 5695 points
    Hi Peter,
    Our support team for this products is on vacation due to end of year holidays. We will send a reply in the first week of January 2018.
    Sorry for the inconvenience.

    Regards,
    Jose
  • 0 in reply to Jose Arteaga4
    Intellectual 570 points

    Facing same issue. Looking forward to your reply.

    I am very confused about the statement on the datasheet, 

    Current Sources (iDACs)
    PGA   
    Serial SPI   

    In addition, I thought the reference should be 0.512v rather than 2.048V, does the internal divider make it to 0.512v?

    Haven't decided if I should use LMP7721 or LMP91200 (the specification for LMP91200 is misleading).

    regards

    Shanguo

  • 0 in reply to Shanguo Lu
    TI__Expert 6475 points
    Peter and Shanguo,

    I'm looking into this right now. Thank you for your patience.

    -Kelvin
  • 0
    TI__Expert 6475 points
    Peter,

    1. When the probe is not attached, the INP is floating so your VOUT will drift to either rail. This leads to saturation on your pH measurement. VOCM should be 1/2 of Vref.
    2. "Vout/Vocm=0V, that means ph value is 14". How did you come with this number? Why don't you use the equation 1 on pg15 of the datasheet.
    Vout-Vcm=59.16 mV/pH * PH.
    I'm not familiar about the old part but based on the datasheet the Vout-Vcm should be 7pH*59.16mV/pH = 414mV.
    3. Do you really need R63? I assume this is the bias resistor to match impedance. Wouldn't you need like a 7.5k (10k||30k) instead? If the bias current of the +4 gain stage is low, avoiding the R63 should help with 4kTR noise. What kind of noise are you seeing?

    For additional reference please check out the EVM design.

    -Kelvin
  • 0 in reply to Shanguo Lu
    TI__Expert 6475 points
    Shanguo,

    Which device did you find the specification from? On what page of the datasheet? I don't see it the 2.048V you're referring to.

    -Kelvin