This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMP7721: LMP7721

Part Number: LMP7721
Other Parts Discussed in Thread: LM4140

I am using LMP7721 for designing signal conditioning of pH probe.

I have taken sample of LMP7721 from TI (SMD packaging).

The problem with my LMP7721 is that it works fine for few days(hardly one or two day) and after that it goes into saturation (either positive or negative).

I am using LMP7721 as buffer for signal of pH probe.

I have tried at least 5-6 lmp7721 till now but not a single one is working. every lmp7721 gets into saturation.

What should I do?

Any precaution that need to be taken while handling lmp7721 (smd packging).

Please help me out!!!

  • Hi Saish

    We will need a schematic of your circuit and the specifications of your pH probe to suggest a possible solution.

    Dennis
  •  I

    I am using pH probe of Van London whose specifications are almost same as mentioned TI design of AN-1852 designing with pH electrode.

    It gives 59mV/pH at 25 degree. output impedance is about 25MOhm.

    I have attached circuit schematic  : At 1 terminal of LMP7721 , I connected pH probe which has BNC connector (signal ) & other BNC connector terminal (surrounding shield ) is grounded.

    My this circuit works many times with different   LMP7721 before they get saturated. After some time they eventually goes into saturation.

    when they are working fine shows correct output in mV (around 180mV at pH of 4 which is as expected).

    please suggest me solution

  • Saish

    The first thing I notice is that you show a 10V (+-5V) supply. This device is specified for 5.5V max (+-2.75V).

    Dennis
  • So I can give max +2.75 and -2.75 to LMP7721
    the datasheet has given 1.8V to 5.5V that suggest the differential voltage range(V+ - V-)
    what is the range of max. input voltage can be applied?
    please can you tell me my other mistakes in schematic and precautions to be taken while handling.
  • my pH probe output varies between -450mV to +450mV
  • Saish

    Yes, the power supply must be between 1.8 and 5.5V, so if you use a split supply it needs to be between +-0.9V and +- 2.75V.
    If you were to use a +- 2.5V supply you should be ok with +-450mV inputs.

    Here's a few general things to keep in mind:
    1. The temperature of the solution is important. You don't show a temperature sensor in your circuit. This would mean you would have to hold the solution at a fixed temperature which may be hard to do. Even a variation of a few degrees can cause significant error. The TI Design you referred to (AN-1852) discusses this in detail.
    2. Circuit board layout and cleaning is very important. The inputs are very sensitive to leakage currents...refer to the data sheet Chapter 8, Application and Implementation, pages 20 - 24.
    3. These devices should always be handled with ESD in mind. Either an ESD mat and grounded wrist strap, or an ionizer should be used.

    Dennis
  • Thanks a lot
    In conclusion saturation of IC is due to excess of Vcc
    can you please suggest me a suitable way to obtain +-2.5V supply?
    waiting for your reply
  • Can I use unipolar source to drive LMP7721 ?
    Just for curiosity why LMP7721 works for few days even I have given it 10V supply?
  • Saish

    If you don't have a lab supply that will do +-2.5 V you could possibly use an LDO (Linear Regulator) but the selection of a device and the design would be beyond the scope of this forum...you could start at this link:

    www.ti.com/.../overview.html

    You can use a unipolar source and unipolar supply...that's what is being done in the TI Design AN-1852.

    I suspect that the total power dissipation of the part was low even with the overvoltage condition, and that's why they lasted as long as they did.

    Dennis

    If this answered your question please click the green "This Resolved My Issue" button.
  • By using unipolar supply I can get only unipolar values,right?

    I redesign my schematic as follow:

    please check & tell me will it work?

    For second LMP7721 , it is used in differential configuration to obtained only unipolar values Non-inverting input is 1.024V which is obtained from LM4140

    input to first LMP7721 is pH signal

  • will the above schematic work for pH probe signal conditioning?