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Original question:

Voltage Input - Voltage/Current Output Transmitter Design Reference

XTR or Other Precision Amplifier for 0-1v output

Bhautik Ramoliya98
Intellectual 290 points
Other Parts Discussed in Thread: XTR300, OPA325, LM7705, OPA2325

Hello there,
          I am still not able to select the best product for my application.

         Requirements :-

        Input Side :-

  •  I have one pressure sensor which gives pressure value's output in a form of voltage. In range of the 0.5v DC (Minimum) to 4.5v DC(Maximum). So you can assume that,This pressure sensor works as kind of DAC which gives different voltage on change of the baro pressure.
  •  Power supply can be from 0 to 5v or 0 to 12v DC. (we do not have a negative supply ex -5v to +5v or -12v to +12v).

        Output Side :-

  •  In output we required voltage output in range of 0v to 1v DC. If at input side (voltage from sensor) is 0.5v DC then at output it should give 0v DC , And if input side is at 4.5v DC then at output it requires 1v DC. So we can say that we need to map the 0.5v DC to 4.5v DC input signal to 0v to 1v DC output signal. (Ex , If input signal is 2.5v(which is 50% of the input range) then at output it should be 0.5v(50% of the output range).

       I have checked the XTR300 but it seems it won't work with my power supply requirements.
       Can you please suggest other alternatives and precision amplifier to check with and if i can do simulation with them then it will be very fine.


Thanks,
Bhautik

 

  • 0
    Guru 140200 points

    Hi Bhautik,

    how close must the output go down to 0V? You know, that no OPAmp can fully go down to 0V, when being supplied with 0V...5V or 0V...12V.

    How much supply current may the circuit consume? What precision do you need? How precise is your pressure sensor?

    Kai

  • 0 in reply to kai klaas69
    Intellectual 290 points

    Hello Kai,

          Thanks for your reply.

          Output should be very near to 0 ,Ex 0.01 or more resolution will be good. If 0.05 can also be acceptable.

          For the current there is no issue as system will provide 5/12v with capability of the 1A/2A.And system is not battery operated so no issue of the current consumption at all.

         And sensor is very precise, for example change it changes it's value in 2 decimals. Ex 0.556 to 0.565 . We can say it changes it's value with fraction of 100. (0.01v).


         Input(v)     :-         Output(v)

         0.50      :-              0.01
         2.50      :-              0.50        
         4.50      :-              1.00        

         Let me know if i need to be more clear or some other details are required.


    Thanks,
    Bhautik

         

  • 0 in reply to Bhautik Ramoliya98
    TI__Genius 10850 points

    Hi Bhautik,

    A simple solution to this would be to use a precision amplifier (like the OPA325) and configure it as a difference amplifier with an attenuating gain of 0.222V/V.

    • When Vin is 0.5V, Vout is 0.1mV.
    • When Vin is 4.5V, Vout is 1V. 

    Is this acceptable? I have attached my TINA Spice file below and a simulation image.

    1261.OPA325 - diff amp.TSC

    -Tamara

  • 0 in reply to Tamara M Alani
    Intellectual 290 points

    Hello Tamara,

                It seems that when Vin is 0.5v,Vout is 0.1v instead of 0.1mv which you stated. And Vout = 100mv is not acceptable when Vin = 0.5v.

               Could you please check it and confirm that it is 0.1v or 0.1mv when Vin = 0.5. ?

    Thanks,
    Bhautik

  • 0 in reply to Bhautik Ramoliya98
    Guru 140200 points

    Hi Bauthik,

    you could add the low-noise negative bias generator LM7705 to provide an auxiliary voltage of -0.232V. This would allow the OPAmp to fully go down to 0V at the output. So, you could modify Tamara's circuit in such a way:

    Here, the 5V supply is used to generate another auxiliary voltage of -0.125V, needed to carry out the substraction. This can be done, if your 5V supply voltage is sufficiently precise and stable. (Strictly speaking R6 should be 1.01k and not 1k). But, of course, you can use any other precise voltage you already have in your circuit. The LM7705, on the other hand, should not be used as a reference voltage source, because it might not be precise enough. But there's nothing better than the LM7705 to generate this negative auxiliary voltage to power the OPAmp.

    Instead of two OPA325 you can use one OPA2325, or course.

    You can add a small ceramic cap of some dozens of pF in parallel to R3 and R7 each. This will improve the stability of the circuit by adding some phase margin at high frequencies. Provide proper power supply decoupling as well. And if you want to connect a load with a nonvanishing capacitance to the output, mount an "isolation" resistor of 47...100R directly at the output.

    Kai