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LMP91000EVM set up issues

Other Parts Discussed in Thread: LMP91000, LMP91000EVM

Hello TI family. 

I recently purchased a LMP91000 evaluation board, I am planning on using it for an electrochemical measurement on an immunosensor I built and tested already using a potentiostat, more specifically, I want to measure the electrical response of a sample by utilizing the three electrode set up that the board has. This is my first time using this type of technology and I have no idea how to make it work, the data sheet is not very helpful, can somebody please give me some direction ??

  • Hi Andre,

    Welcome to the e2e Community!  The LMP91000 is supported in the Precision Data Converter forum.  I'll move your post there and we'll see if we can get you some assistance from the team that supports that device.

  • Hello Andres,

    if you provide me with more details about the sensor (such as min and max current it sources/sinks, biases/unbiased sensor, ...) I can assist you in the configuration of the LMP91000.

    regards,

    Domenico

  • Thank you so much Domenico,

    The cyclic voltammetry that I am willing to apply to my electrode has a range from -0.5v to 0.5v. The resulting currents are from -0.3 mA to 0.3 mA. If you need more specifications let me know. 

    Once again thank you very much!!

  • Hi Andres,

    I've read something about the cycling voltammetery. If I've well understood hou have to apply a voltage between RE and WE pin (in your case from -500mV up to 500mv) and then you have to read the current.

    If this is the case the LMP91000 can do this job, becasue it is able to generate a bias between RE and WE pin. As written in the DS, the best way to have a stable bias is connecting an external Voltage reference at VREF pin of the LMP91000. The EVM of the LMP91000 is provided with a stable voltage reference, according to the eval board yuo have bought, the Voltage reference could be 4.096V or 2.5V.

    The differential voltage between RE and WE pin can be programmed by fixed steps (see the DS). In case your EVM is LMP91000EVM( 551600776-001 RevA) the voltage reference is 2.5V, so you can generate the following bias:

    1%VREF -> 1% 2.5V = +/-25mV,

    2%VREF -> +/-50mV

    4%VREF -> +/-100mV

    6%VREF -> +/-150mV

    ...2% step

    20%VREF -> +/-500mV

    So in order to have a cyclic voltammeter, you could start wiht  20% VREF (negative bias) reach 0 bias, then change the sign and reach 20%bias and viceversa.

    If you have the complete eval kit LMP91000EVM + SPIO4 board + GUI software you can easily perform this test programming the EVM and acquiring the data.

    regards,

    Domenico.

  • Hello Domenico

    As you suggested I have purchased the SPIO4 board, so I can make sure that the system will output the same results as my lab equipment. Now, where is it that I can obtain the software to set the values  of the evaluation board??

    Is there any guideline you know about to perform cyclic volttametry?

    Thank you so much for your help. 

  • Mr Domenico, 

    I was able to connect all the system already, I was trying to find a cyclic volttametry option within the software but I could not find it. Also, once I have the system tested I  am going to implement it Stand alone so I will bother you with questions about that also. Please let me know if you know how yo configure and obtain the CV.

    Thank you so much for your help. 

  • Hi Andres,

    we do not support this kind of measurement in an automatic way. You can program the bias from the block diagram page and measure the output in the measurement folder. Performing this procedure iteratively you have the voltammetry measurement.

    regards,

    Domenico.

  • Hello Domenico, 

    First of all, I was wondering if I could contact you directly so we could set the board together in just one attempt, I am sure that with your expertise on this matter it will only  take us a few minutes.  I am able to see what you are suggesting me to do, but when I measure the voltage between the CE and WE electrodes I am not getting what I am looking for. 

    As I mentioned before, I need a cyclic voltammety that tracks the current of the three electrode system when a range of -0.6 to -0.6 volts is being applied, I understand that the LMP91000 will not plot the CV directly for me, I could just get the current and voltage values from the experiments and plot them against each other.  I am still a bit lost in how to get the experimental values. I will really appreciate your help as soon as you can, I am running against time and this issue is taking a lot of time already. 

    regards

    Andres Felipe Diaz Cruz

  • Hi Felipe,

    the bias generated by the LMP91000 is bewtween RE and WE pin, not between CE and RE.

    Try to measure between the right pins and then let me know.

    regards,

    Domenico.

  • Hello Domenico and happy new year.

    I have been following your advice regarding the LMP91000 but unfortunately I still can not get it to work as I expected. From your previous messages I understand that the LMP91000EVM does not do a cyclic voltammetry automatically, and that I need to modify the bias voltage in the block diagram in order to obtain different differential voltages between the WE and the RE. I tried doing that and I did not see a change in the voltage between RE and WE, I am not sure if I am missing any step in the set up process.

    Also, in the LMP91000 software there is a section in which I need to choose a sensor, and most of them if not all, are gas sensors. I have a sensor that I built myself which detects Cortisol concentration in human samples by performing cyclic voltammetry on them. So I do not know which sensor to choose from the list. 

    My other concern is that I would not know where to measure the output currents from the cyclic voltammetry once I have the correct voltages. Please let me know if I could contact you by phone or by an email which you probably check more often, this is an important part of my team's project and we need to get it done soon. 

    Thank you for your time Domenico and have a great afternoon. 

    Andres Felipe Diaz Cruz

  • Hi Andres,

    in order to modify the register of the LMP91000 from the block diagram you have to push also the button present in the toolbar which have the tootips "write all registers to board", the icon of this button is a pcb with a green arrow on top.

    Regarding the sensors in table, you are right they are all gas sensors, but there is a button "NEW" to create a custom sensor. In order to create a new sensor you have to know the sensitivity, the full scale range, the zero level, the bias required.

    Once you have put this data the software configures the LMP91000. For sure you can play with the registers in order to change their content.

    The measurement of the current is one of the job of the LMP91000. The current is measure with the TIA, it means that at the output of the LMP91000 you have a voltage which is proportional to the current sourced/sunk by the sensor. The formula is the following: I_sensor=(Vout-Internal_zero)/RTIA.

    P.S. The user interface, is able to show the current when you select the y-axis unit of the graph.

    Please let me know if it helped.

     

    regards,

    Doemenico

     

  • Hello Domenico,

    Thank you for your quick response. I remember the "write all registers to board", I was pushing it every time I made any change in the bias voltage of the circuit and it still did not show any change in the differential voltage between WE and RE, I tried several times, restarted the program and it did not work. Is there any other consideration I should have in mind before doing the measurements? I am not sure if there is any connection I have to change in the LMP91000EVM, according to the data *** I have to short two pins of the J-MENB connector, I tried doing that but it did not seem to work either. Regarding the output current, I am still not able to see any difference in the interface since the WE and RE differential constant is remaining constant.

    Thanks a lot

    Andres Felipe Diaz Cruz

  • Hi Andres,

    first of all check the following on the LMP91000EVM:

    - on the bottom side the p channel JFET transistor should not be present (usually we leave it un-stuffed), if it is present remove it otherwise you can have problem in biasing the sensor.

    - select a sensor from the DB even if it doesn't match your sensor. If you do not select a sensor the graphical user interface will not configure the LMP91000 !

    - Configure the bias through the Graphical user interface without connecting the sensor and measure the voltage between WE and RE. If your sensor has a huge equivalent capacitance between RE and WE it requires a lot of time to be charged.

    regards,

    Domenico

  • Hello Domenico.

    I have checked and done what you suggested and I still can not get the interface to work, when I measure the differential voltage between the WE and RE pins I am not getting a fixed voltage. When I measure the VDD and the Vref I am getting 3.3 and 2.5 volts which indicates that the device is properly connected. I do not really know what else to do, I am installing the application in another computer to see if it works. As I asked you earlier, I do not now if we could get in contact to set the device together via phone or video conference. 

    Thank you for your help sir.

    Andres Felipe Diaz Cruz

  • Hi Andres,

    provide me with your contact (may be uptdating your e2e profile) and I'll setup a call. I'm in the GMT+1 time zone.

     

    regards,

    Domenico

  • Thank you so much Domenico,

    My phone number is +1 786 338 8667. I am in GMT -5 time zone, we could do a meeting on Tuesday, Wednesday or Friday morning, which would be your afternoon, please let me know when it is better for you so I can fix my schedule accordingly.

    Thank you

    Andres Felipe Diaz Cruz

  • Hi Andres,

    let's have a call on Friday 25th at 8.30am your time.

    I'll phone you.

    regards,

    Domenico

  • Thank you Domenico, I look forward your call. 

  • Hello Domenico,

    Earlier this week, I sent you a file with some information regarding the LMP91000, I would appreciate it if you can take a look at it and give me your comments. 

    Thank you for all the assistance

    Andres Felipe Diaz Cruz  

  • hai andres,

    I have also been working with LMP91000 for past few months and am trying amperometry. I have not tried Cyclic voltammetry. But as far as i know, In case of cyclic voltammetry, the voltage between the WE and RE should roll over the range with time (e.g -0.5 - 0.5). But with LMP91000 AFE software, you can only be able to provide the constant working potential between WE and RE. So performing cyclic voltammetry is difficult as far as i know.

  • I have a similar setup as the original poster. I'm attempting to use the LMP91000 with a 3-lead amperometric sensor. As it says on page 2 of the EVM user's guide (http://www.ti.com/lit/ug/snau121a/snau121a.pdf) I have to keep the 2-wire pins unshorted. However, when I do this and apply a direct voltage (say 4.17V at 24% Bias) I should see the correct differential (1V) between my WE and RE, however I don't see any differential there at all.

    What I do measure is as follows: WE = 2.08V, RE = 0V, CE = 5.10V, Differential_RE,WE = 0V, Differential_WE,CE = 3V, Differential_RE,CE = 0V. 

    I have everything set up as follows: VDivider = 50%, VBias = -24%, VRef = 4.17V, FET_SHORT = disabled, OP_MODE = 3-lead amperometric cell, TIA_Gain = 3.5k, Rload = 10 ohms.  I've removed R6 and R7 for stand alone operation.

    The odd part about it is that when I short the 2-wire pins, my differential functions exactly as expected (I see 1V when applying 4.17V at 24% Bias). However, I obviously cannot use this setup with my sensor as it expected the CE and RE not to be shorted.

    Any insight into this issue you could provide would be appreciated.

    Bradley

  • Hi Bradley,

    your setup is fine, you can have the exact bias between RE and WE only when a sensor is connected to the LMP91000 or when you short CE to RE pin.

    The A1 op amp usually called control loop amplifier, is an amplifer in open loop configuration (see the block diagram in the datasheet) it means that the RE pin will show the right  level of voltage only when the loop around the op amp is closed.

    There are 2 ways to close the loop:

    1) connecting a 3-lead gas sensor

    2) shorting CE to RE.

    P.S. when you connect a gas sensor which requires a bias, the bias will appear between WE and RE only after a while becasue the LMP91000 has to charge/discharge the internal capacitance of the gas sensor. The LMP91000  is able to sink or source at max 10mA at CE pin.

    Regards,

    Domenico

  • Domenico,

    Thanks for the quick response.

    I have the loop closed with the 3-lead sensor but am still getting something wrong. I also think the time to charge/discharge isn't the issue, because I start out with a 2 minute deposition of a constant -1V and even at the end of the 2 minutes, I still don't see the -1V between the RE and WE. Beyond the deposition, I'm moving in steps of ~5mV at a time, so I don't think the timing to discharge should be too much of an issue.

    Also tried using a second LMP that I have as a sanity check, but got the same results.

    Running some more tests today and will post any discoveries, but any further insights you could provide, would be great!

    Bradley

  • Hi Andres Felipe Diaz Cruz,

    I would like to know how do you solve the problem with the LMP91000 and SPIO-4 Board. Can you please give me the right direction for LMP91000EVM and SPIO-4 Board? 

    Thank you.

    Sincerely,

    Matthew