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LMP91200: LMP91200

Thoufiq M
Prodigy 30 points
Part Number: LMP91200
Other Parts Discussed in Thread: ADS1114, , ISO1540

Hi,

As per your datasheet,i have doubt in these two pins The pin No11: VOCM and the pin No:12 Vout
The pin 12: Vout - Can i able read the PH vale in this pin If i connect to the analog pin of microcontroller ...? and pin 11:Vocm to Microcontroller GND

  • 0
    TI__Genius 9380 points

    Thoufiq M, 

      Pin 12 Vout is designed to provide an analog voltage out that can be measured with the ADC of a microcontroller. However PIN 11 is the zero reference for the sensor and can't be connected to ground. The intended purpose of this pin is to be used as the reference or zero for a differential ADC. When used with Vout the differential ADC will provide the relative difference between the reference or zero, and the Vout voltage. If you don't have a differential ADC, then you need to measure Vout single ended by connecting the Vout to the ADC and measure the Vout with reference to ground and internally subtract the reference zero Vocm voltage from the Vout elevated voltage. 

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    Hi,

    Answer is very well and thanks sir,

    For testing purpose, we need to connect directly to the microcontroller but we need to have power and data isolation (ADC - Digital o/p i2c) is it possible to get the PH value in the IC of ads1114 

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

      The ADS1114 is an ADC. It measures the voltage coming from Vout of the LMP91200. The sensor provides a current based on the available PH level. The LMP91200 has a transimpedance amplifier that will convert the current into a voltage. The LMP91200 does not provide a PH level but a voltage that reflects the current in the probe. You will have to use PH standards to get different voltages from the LMP91200 that the ADC will convert from analog to digital. In the microcontroller you will have to convert the known PH level voltages into a PH level with respect to the zero. Above zero is positive PH and below zero is negative PH. There is no shortcut to this, you will have to measure multiple standards PH +/- and derive a formula to get the PH value from the voltage. 

    If you have to have Isolation, then look at TI's ISO1540 I2C isolator to isolate the ADC form the controller.

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    Fine,

    1. Last Para:-  The LMP91200 has a transimpedance amplifier that will convert the current into a voltage

    Verification :- The voltage which is in Analog Signal Right..?, We can't isolate the analog signal so I can use ADC ( ADS1114 Digital o/p i2c ) then I can isolated 

    2. What about the Hardware connections between LMP91200 and ADS1114 (Especially for Pin 12 Vout and Pin 11 Vocm )...?

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

      That is true, you can isolate on the digital side with the recommended ISO1540. I am not sure I understand your question in number 2. I would put the LMP91200 and the ADS1114 on the same side of the isolation barrier. The ISO1540 will provide the isolation barrier between the sensor side and the controller side. You will need an isolated power supply for the sensor side to power the IC's.

  • +1 in reply to Gordon Varney
    Prodigy 30 points

    Hi Sir,

    I am having the PH sensor probe in Co-axial type connector but i know  lmp91200 has guard pin (Triaxial connector)  facility so if I left the guard pin connection I didn't get the proper value in vocm and vout .Is there any issue has a raised..?

    PFA of Schematic 

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

    The sensor output of the LMP91200 is very high impedance (2.5Meg Ohms). If you look closely in the data sheet at layout section in the data sheet you will see that it is necessary to add a guard ring around the traces on the PCB going to the sensor. That being said you are using a coax and have the sensor possibly up to 1 meter away. Yes you will see a lot of noise. The fact that you can take a measurement under these conditions is very good.

    Any situation in which the source to be measured has a very high output impedance and is vulnerable to leakage current will require a driven shield. The use of a triaxial cable may be used as the outer shield of the triaxial cable can be held at the same potential as the sensor wire. Remember that any noise on the guard may be amplified in the output.

    What you will need to do is use a driven shield as a guard ring in your triaxial cable to protect low-current circuits against leakage current. A driven shield is often referred to as a driven guard ring. It is used in situations where the tiny leakage of current through the insulating surfaces of a wire or PCB board would otherwise cause error in the measurements or functionality of the device. The basic principle is to protect the sensing wire by surrounding it with a guard conductor that is held at the same voltage as the wire so that no current will flow into or from the wire. This is achieved using guard1 and guard2 of the LMP91200 which matches the guard voltage to the sensing wire voltage. The leakage from the shield to other circuit elements is of little concern as it is being sourced from a buffer which has a low output impedance.

    Cleanliness is very important as well. Make sure to remove all flux from the PCB and wire solder connections at both ends. Any flux or moisture will become a leakage path for current and will cause you errors and noise.

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    The circuit is correct, The problem is in hardware connection and cleanliness. Is it right..?

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

      The image you sent is not a complete circuit. Comparing to the data sheet your pin out is correct.  Yes proper probe connection and cleanliness will be your biggest problems.

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    Hi Sir,

    After your statement, I have changed the lmp91200 circuit and its design (attached in the comment) from breadboard into dot matrix board and I cleaned the lmp91200 ic and respective track completely but I didn't get the result (ADC value) in PH up and PH Down solution.

    Can you suggest any other alternate solution for LMP91200 IC's

    Note: The PH probe is in working condition.

    PFA of Image - Circuit Design & Board

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

    If you are getting some type of reading just not what you expect then most likely you do not have the LMP91200 configured correctly for your probe. The sensor manufactures have basic settings for their sensors. However these basic settings don't always apply to every design or IC that can be used to measure the sensor. The LMP91200 is a transimpedance amplifier that is very sensitive and low current. Small adjustment to gain and Rload will have big impact to your readings.

    Most designers, begin with the incorrect settings for the LMP91200 board. In the GUI Software find a sensor that is similar or the same as you are using and change to these default values to begin. Only if you are using a standard solution with the sensor can you know exactly what value you should be getting. Adjust the settings until you get the correct voltage feedback for the standard you are using. 

    Try adjusting gain and Rload after you configure the GUI for your probe. This should get you a good reading. You will have to craft a formula to get the real values from the voltages you will be measuring. Use the standards to get known voltage for know standards then you will be able to create the formula. Don't forget to temperature compensate the voltage to get more accurate measurements. 

    FYI: I have never seen anyone make this part work correctly on a bread board. It may function, but not accurately.

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    Thanks Sir FYI - Gordon Varney,

    I have a query about ADC input type. Your suggestions will be helpful.
    1) "Is it necessary to have a differential input capable ADC for Vocm and Vout, or it is fine to use single ended ADC with seperate channels for Vout and Vocm."
    If we use single ended ADC channels for Vout and Vocm, can we deduct Vocm from Vout and use that value.

    2) We see some noises in the signals, min, max values separated by 100-200mv. so we think a single instance of ADC sample would not be sufficient. It may be required to take multiple Samples with pepper intervals then calculate Mean value or RMS and use that.
  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M, 

      If you are using single ended measurement, then you will need to subtract the VOCM from VOUT. Differential measurements will do this automatically for you. Yes you can measure both the VOCM and VOUT, then subtract VOCM to get a more accurate end value.

    The noise could be from the bread board setup. Bread boards do not have a good low impedance ground so you get ground bounce. Add a 100pf capacitor as close to the ADC pin as possible. 

    You may need an RC filter on the VOUT and VOCM to get a cleaner ADC measurement. In any case add the 100pf to the ADC to minimize the inrush current of the ADC sample capacitance.

  • 0 in reply to Gordon Varney
    Prodigy 30 points

    Hi Gordon Varney - Sir,

    I think you already know that The PH value depends upon the temperature of the solution so We have referred to the datasheets (LMP91200 old version) We came to know that the LMP91200 has the features to connect the temperature sensor i.e PT100 (RTD & Thermistor sensor in the pin of 2 - CAL with Reference Resistor and 3 -RTD of LMP91200).

    1. I have a doubt, I think the output of the PT100 will be the serial communication over SPI protocol (In pins 14,15, and 16 of LMP91200) Is it correct..?
    2. Can we get the output of PT100 (Connecting in the pins of 2 - CAL with Reference Resistor and 3 - RTD of LMP91200) in any other pin of LMP91200 except 14th,15th, and 16th pin of LMP91200..?
    3. May I know the reason for hiding those features (Pins of 2,3,14,15 and 16) in the new version of datasheets is there any issue in LMP91200 Ic ..?

    Regards,

    Thoufiq M.

  • 0 in reply to Thoufiq M
    TI__Genius 9380 points

    Thoufiq M,

      I am sure that you are aware that TI uses Core silicon to produce several variations of a similar part. This means that inside of the silicon packaging could be features and options that are not being used. The ability to turn off or not use selected circuits in the IC allow TI to create a part that is tailored specifically to customer needs. The silicone is only a part of the overall cost to develop and IC. The testing and validation can be a much larger part of the cost of the IC.

    What that means is that if a circuit inside of the silicone is deemed not necessary for this specific part number, then we either turn it off and or do not test it. This reduces the cost associated with the IC. Therefore, we can sell this part cheaper than another part that may have the circuit enabled and tested. Most of this is determined by the requests from many customers.

    If the data sheet does not call out the circuit, and by chance you discover that the circuit may exist, then I can definitely say, do not use this feature. It has not been tested, and may not function as you think it should. TI will not support attempting to use a part in a manor that is not covered in the data sheet.

    If an earlier version of the data sheet calls out the circuit, but a new version does not, then it was a choice to remove this feature. The testing and varication of the feature has been removed from the process.  TI will not support this circuit.

    This would be similar to you designing a circuit on a PCB and not populating it on selected versions of the product. If a customer attempted to populate this circuit and tried to use it, you would say that this circuit is not intended for this version and not support it.