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LMP91000: Not working with City sensors

Part Number: LMP91000

I've been using the sensors GS+4NH3-100 ( from DD Scientific ) and 4NH3-100 ( from Honeywell )
with the LMP91000 AFE, with success.
But when I put the NH3 3E 100 from City in the equipment, I'm getting no output voltage variation, even applying gas with 56 ppm concentration and holding the same configurations as the other sensors.

City sensor data sheet : www.citytech.com/.../nh33e100.pdf

That's not the behavior I expect, since the specs and sensibility of all them are very close.
The configuration of the LMP91K, which is working well with DDScientific and Honeywell sensors, is as follows :

LMP91K configuration : 3-Lead Amperometric Cell

TIACN :
TIA gain :120 kΩ
RLoad = 10 Ω

REFCN:
External reference source voltage selection
20% of Vref internal ZERO selection
Negative bias polarity
0% of Vref BIAS selection

MODCN:
Shorting FET feature disabled
Temperature Measurement - TIA ON

Has anybody faced a problem like this with City sensors ?

  • Hi Daniel,

    I guess your gain is much too high. Start with TIA_GAIN = 2.75k and increase carefully.

    Kai
  • Hi Kai ( no pun intended )
    Thak you for your prompt answer.
    But I was thinking the other way around.
    Since the sensibilities of all sensor are the very close ( about 100 nA/ppm ) and the ones
    provided by DDScientific and Honeywell worked fine with 120 kΩ as the TIA gain, I supposed that
    the same should occur with the City sensor. Besides that, my problem is not TIA saturation,
    but no signal at the TIA output. Since this type of sensors sinks current from de TIA, it should have
    a positive voltage signal excursion when exposed to the gas, but TIA's output simply does not change.
    What's wrong with my thinking?
    Daniel
  • Hi Daniel,

    are you sure that the recommended load resistor is 10R?

    Kai
  • Hi Daniel,

    I apologise for the delay, but a TI representative will respond to you on Tuesday CST. Monday is a holiday for us.

    Thanks for your understanding,
    Ren
  • kai klaas69 said:
    Hi Daniel,

    are you sure that the recommended load resistor is 10R?

    Kai

    Hi Kai.

    No. I'm not sure about load resistor, because the datasheet says nothing about that.

    I'm using 10 ohms as a first try.

    Cheers

  • Hi Ren.
    No problem. I'll look forward to it.
    Thanks a lot.
  • Hi Daniel,

    datasheet of "NH3 3E 500 SE" recommends 100R...

    Kai
  • Hi Daniel,

    From the Webench Designer tool located on the right hand side of the LMP91000 website, you can select this device.

     The suggested values and register writes are given.

    From the previous discussions here, it looks as though you have tried some of these things, but I couldn't tell if you had tried to set the OP_MODE(0x12) to 3-lead amperometric (011). By default (000), the sensor is in a deep-sleep mode, so this may explain why there is no output available. Please also see that a 350k value is used for the TIA.

    Best Regards,

    Dan

  • Hi Kai :

    I didn't find this resistor value in the City data sheet, and this is the reason why I started with 10R.
    But after your advice, I've tried with all values allowed in the LMP91K ( including 100R ) without success also.

  • Hi, Dan.

    Thank you very much for your support.

    Regarding the values provided by Workbench Designer, the only differences among these values and the ones I've used are :

    TIA Gain : Me : 120 k, WBD : 350 k ( Ok, I´ll try 350 k, but I think the sensor should present some output with 120 k @ 50% full-scale gas applied,

    which does not occur ).

    Bias sign : Me, Negative, WBD : Positive ( I think this does not matter, once Bias is 0% VREF )

    OP Mode : Me : Temperature measurement (TIA ON), WBD : 3-lead amperometric cell

    I used Temperature measurement (TIA ON) because I wished to get both measurements from the LMP91K : gas concentration and temperature,

    but 

    BTW, what exactly does the OP Mode setting ?

    Nevertheless, I'll try  3 lead amperometric cell.

     

  • Hi Daniel,

    I found this here:

    sensoric_appnote6.pdf

    sensoric_appnote7.pdf

    Seems that the recommended Rload is 0R and that the switch between RE and WE of LMP91000 must be closed.

    Kai

  • Hi Daniel,

    This portion of the data sheet explains the various modes (OP Mode) of operation for the LMP91000.

    If you are still not getting an output after setting the the device to the 3-lead amperometric mode, I would try  the temperature measurement mode with TIA off (MODECN = 0x06) to see if you can get just the temperature reading at Vout.

    Best Regards,

    Dan