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LMP91000: Gas sensing from es4-ag1-200

Raviteja Konathala
Prodigy 20 points
Part Number: LMP91000

Tool/software:

  • Hi we are using es4-ag1-200 sensor for our gas sensing application and using lmp91000sde controller for sensing can u help me out register configuration in lmp91000 for gain, bias voltage and rload..., all configuration parameters. We need sincere support for development
  • 0
    Prodigy 20 points
    • Iam using 2.5v ref and mcp3425a0 adc
  • 0
    TI__Mastermind 43795 points

    The LMP91000 has a fixed slave address (now called target address) of 0b1001000 or 0x48. This will be used in all I2C transactions with the LMP91000.

    You should start with a write transaction to the lock register 0x1 to disable the lock. 

    S 0x48+W A 0x01 A 0x00 A P

    You don't have an external resistor at C1/2 so you will need to configure TIA_GAIN to a non-zero value in the TIACN register 0x10. The datasheet for your sensor recommends a 100ohm load resistor, which is the lower 2 bits 0b11. Here I am showing TIA_GAIN=0b001 and RLOAD=0b11 is 0x07.

    S 0x48+W A 0x10 A 0x07 A P

    You have provided an external VREF, which is not required. If you would like to use this external VREF, you must set the REF_SOURCE bit to 1 in the REFCN register 0x11. The default INT_Z of 50% 0b01 should be adequate. The datasheet for your sensor calls for a 0mV bias voltage, so we can leave BIAS_SIGN and BIAS at their default values of 0b0 and 0b0000. This results in REFCN register value 0xA0.

    S 0x48+W A 0x11 A 0xA0 A P

    The FET_SHORT feature performs a similar function to Q1 in your schematic. You may want to remove Q1 or consult with your sensor vendor regarding it. I will assume the FET_SHORT feature should remain disabled as default. You will need to set OP_MODE to 0b011 for operation. This gives us a MODECN register (0x12) value of 0x03.

    S 0x48+W A 0x12 A 0x03 A P

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    Hi i tried using above mentioned, in first startup it was sensing the range, but after that it become 0mv in vout and saturate their and never start sense again

  • 0 in reply to Raviteja Konathala
    TI__Mastermind 43795 points

    You'll have to be more specific about the behavior observed to get further support. What voltage did you see at startup? Was this before or after sending the configuration over I2C? Can you show the I2C transactions that you performed on a logic analyzer or oscilloscope? 

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points
    1. At start vout is 1.48v and with gas it went up to 4.5v, after removal of gas it came down to 56mv and later  not detect any and no vout. It was after i2c data transfer 
  • 0 in reply to Raviteja Konathala
    TI__Mastermind 43795 points

    Are you observing the voltage using an instrument at the VOUT or using the ADC in your circuit? If you are only using the ADC, please confirm the voltage by another means. 

    Your ADC is on the same bus as the LMP9. Please share the bus transactions from a logic analyzer so we can be sure of what is being sent.

    If you power cycle the system and send configuration again, does the functionality return? If it does not, you have permanently altered something: possibly the cell itself.

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    I was measuring at vout pin which is connected to mcp3425a0, after power cycle also same 56mv. If we alter the sensor it was also once, and later power cycle no voltage at vout

  • 0 in reply to Raviteja Konathala
    Prodigy 20 points
    1. I am getting 1.227v at out vout after configure 0x10, 0x07; 0x11, 0xA0). How can we take min and max ranges for mcp3425a0 my connection as v+ connected to vout of lmp91000 and v- connected to 2.5v vref, what's the maximum voltage i can expect at vout pin of lmp9100
  • 0 in reply to Raviteja Konathala
    Prodigy 20 points

    If we continuously kept under gas concentration vout increasing upto some 1.9 or 2v and later vout comes to 36 mv and no changes in value

  • 0 in reply to Raviteja Konathala
    TI__Mastermind 43795 points

    If you are not sending the other commands I suggested, you are not actually turning the amplifier on and should expect no output. Sometimes a high impedance can appear to be an unstable voltage due to leakages.

    The output at VOUT is limited by your 5V VDD and GND, minus some overhead that isn't specified. Your 2.5V VREF input to the TIA should cause zero-input from the cell to create 2.5V output at VOUT. Currents from the cell would then cause the 2.5V output to vary up or down based on polarity of the current and TIA GAIN selected. 

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 20 points

    Hey in gas detection it was going upto 5.24v my vdd after removal of gas slowly vout get reducing at last to 60mv and later no changes in value even in gas. Without sensor my vout was 0.454v. Please help me out

  • 0 in reply to Raviteja Konathala
    TI__Expert 7170 points

    Hi Raviteja,

    Ren is currently out of office and will be back on 12/2. He will be able to respond to your question then.

    Best regards,

    Simon Rojas