Reg: LMP91000

Dear Team,

With respect to LMP91000 amplifier,I am bit unclear with the following lines in the datasheet.

"The LMP91000 supports gas sensitivities over a range of 0.5 nA/ppm to 9500 nA/ppm. It also allows for an easy conversion of current ranges from 5µA to 750µA full scale." 

- Does LMP91000 support 0.5mA/ppm to 9500 nA/ppm or 5uA to 750uA. 

- Can you please let me know what should be supply voltage given to the LMP91000 so that our instrument can measure in nA. We are using sensor's like Citytech's 4CF

Thanks,

Vijayaraj.S

  • Hi Vijayaraj,

    The LMP91000 is supported in the Precision Data Converter forum.  I'll move your post over there and one of our LMP91000 experts will get back to you after the holiday break.

  • In reply to Tom Hendrick:

    Hello Vijayaraj,

    The ranges that you mention are attempting to convey the same thing in two different ways. The LMP91000 supports both ranges. To help understand it's good to start with the governing equation:

    Gas sensitivity (nA/ppm) X  Gas concentration (ppm) = Sensor Output Current (nA).

    The sensor output current, 5μA to 750μA full scale, that the LMP91000 supports is the limiting factor and from there you can calculate that our chip supports certain sensors with a gas sensitivity range of 0.5nA/ppm to 9500 nA/ppm. Of course this sensitivity range (nA/ppm) depends on the individual sensors full scale range (ppm). A list of supporting sensors can be found on our handy WEBENCH design tool at:

    http://webench.ti.com/webench5/sensorafe/?part=LMP91000#

    To your second question the supply voltage is flexible. 4CF Carbon Monoxide sensors have a sensitivity up to 85 nA/ppm with full scale of 500 ppm.

    85 nA/ppm X 500 ppm = 42.5uA.

    We recommend TIA GAIN = 35 kohm = > 42.5uA X 35 kohm = 1.4875V

    So the supply can be anywhere over the operating range of the LMP91000: 2.75V to 5.25V and the output would not rail. Just ensure that the internal zero is set to 20% Vref and Vref is set to be equal or less than VDD.

  • In reply to Jason Seitz:

    Hello Jason,

    Thank you for the detailed explanation. Can you please help me out on the following queries on the LMP9100 architecture

    Internal Zero:

    (a) Can you elaborate on the lines in the datasheet under Internal Zero"This provides both sufficient headroom for the counter electrode of the sensor to swing, in case of sudden changes in the gas concentration, and best use of the ADC’s full scale input range."

    (b) Can you let me know When at what conditions Internal Zero should be set at 67% or 50%.

    (c) Internal Zero @ 20%.: Does this mean at zero Input the output voltage will be at 1V (In case VREF/VDD = 5V*20%= 1V) ?. If so possible ADC output could be between the range 1V and 5V.?

    TIA(Trans Impedance Amplifier)

    (a) As per the following lines in the datasheet "The potentiostat will compare the reference voltage to the desired bias potential and adjust the voltage at the counter electrode to maintain the proper working-to-reference voltage",

    As far as to my understanding ,If i am not wrong does the line "The potentiostat will compare the reference voltage to the desired bias potential" should be like "The potentiostat will compare the reference electrode to the desired bias potential" so that as per the block diagram Reference electrode of the sensor is compared with the bias potential.

    Thanks,

    Vijayaraj.S

  • In reply to Vijayaraj.S:

    Vijayaraj,

    Internal Zero

    a) Because the LMP91000 is intended for low power single supply systems it is important to configure the device in a way that the op amps don't saturate ("rail").

    Depending on type of sensor you interface to the device, the counter electrode will be +/- 100's mV from the set Internal Zero point. FYI - Internal Zero can be thought of as ground in the typical dual supply potentiostat circuit. For your 4CF CO sensor it will be about -300 to -400mV from Internal Zero. So you want to configure the Internal Zero so that control amp does not saturate. Also internal zero will affect the TIA as well. I'll get into that on the next point.

    b) The TIA will produce a relative negative voltage when current goes into the TIA. Current goes into the TIA when the LMP91000 is connected to a electrohchemical cell which undergoes reduction reactions (i.e. NO2). In this case in order to give you the most "headroom" you would set Internal Zero high to 67%. On the other hand, in your case, measuring CO, which is an oxidizing reaction, current will be traveling into the WE, out way from the TIA. The TIA will produce a relative positive voltage and you would set Internal zero low to 20%. 50% is just another option depending on your swing needs.

    The goal is, based on your application (type of gas, sensor sensitivity, gas concentration, etc.) how to set the Internal Zero and TIA gain to utilize the most codes of external A/D full scale range therby getting the most accuracy out of your system.

    c) Yes at zero input current this would be the case.

    TIA

    a) Your understanding of the potentiostat description is correct.

    A good reference on this topic:

    http://www.alphasense.com/environmental-sensors/pdf/AAN_105-03.pdf