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    Op-Amp Shunt Resistor

    Hi guys,

    i have 3 circuits for measurement Voltage, Current and Temperature. I am using the YHDC HV25 for voltage, YHDC HAX25 for current and a PT100 for temperature measurement. I am on the prototyping phase and from the Proteus simulation i see good result on the voltage and current measurement but not at the PT100's. The circuit logic is the same. Anyone has any idea of circuit improvement? I am using AD817 as difference op-amp. Can an instrumentation op-amp give better performance?

    Note that i want my outputs to be centered at 2.5V, to drive them to an ADC

    1)Voltage 2)Current 3)Temperature
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    Last edited by VirusX2; 15th March 2019 at 17:32.

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    Re: Op-Amp Shunt Resistor

    The Pt100 current source should refer to a precision reference or the ADC reference (ratiometric measurement) but not the 15V supply. It's usually unwanted to have a trimmer potentiometer because it introduces additional drift. The simple difference amplifier has too low input impedance, use an instrumentation amplifier circuit.


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    Re: Op-Amp Shunt Resistor

    Hi,

    in all cases:
    * you are relatively high ohmic. The resistors generate noise. And additionally this makes the whole system prone against introduced noise. I see no problem in reducing 100k --> 10k
    * there are no filters. Use low pass filters to suppress high frequency noise and to suppress aliasing frequencies in case you use an ADC.
    * not related to performance: There is no ESD protectioin and there are no EMC filters

    ***
    PT100 circuit.
    * All the PT100 nodes are relatively high ohmic. In case this is a PT100 with some cable you need to avoid introduced noise.
    --> show the inner PTC circuit. (is it just one PTC with kelvin connetion.. without any other resistor?)

    If cable: IŽd use a shielded cable

    Klaus

    Added: after I read FvMs post:
    IŽd try to lower the PT100 "source" nodes impedances. This also lowers the sense node impedance.
    But I agree that increasing the difference amplifiier (INA) input impedance will improve accuracy. (But neither noise nor precision)
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    Re: Op-Amp Shunt Resistor

    Quote Originally Posted by FvM View Post
    The Pt100 current source should refer to a precision reference or the ADC reference (ratiometric measurement) but not the 15V supply. It's usually unwanted to have a trimmer potentiometer because it introduces additional drift. The simple difference amplifier has too low input impedance, use an instrumentation amplifier circuit.
    Thanks, for the answer.
    I thought it was the low input impedance of AD817 (300kΩ), so i tested an AD621 with better results for the PT100 measurement. But why on the other two circuits the AD817 gives accurate results and not on the PT100? Cause the transducers have a precision current output?
    Would you suggest me to change the AD817 also for the other two circuits?

    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,

    in all cases:
    * you are relatively high ohmic. The resistors generate noise. And additionally this makes the whole system prone against introduced noise. I see no problem in reducing 100k --> 10k
    * there are no filters. Use low pass filters to suppress high frequency noise and to suppress aliasing frequencies in case you use an ADC.
    * not related to performance: There is no ESD protectioin and there are no EMC filters

    ***
    PT100 circuit.
    * All the PT100 nodes are relatively high ohmic. In case this is a PT100 with some cable you need to avoid introduced noise.
    --> show the inner PTC circuit. (is it just one PTC with kelvin connetion.. without any other resistor?)

    If cable: IŽd use a shielded cable

    Klaus

    Added: after I read FvMs post:
    IŽd try to lower the PT100 "source" nodes impedances. This also lowers the sense node impedance.
    But I agree that increasing the difference amplifiier (INA) input impedance will improve accuracy. (But neither noise nor precision)
    It's a 3-wire PT100 like this:
    https://www.ebay.com/itm/RTD-Pt100-T...C/302454638634

    The aim of the system is to measure transient voltages and currents on electric motor, but the transient can be on an unknow frequency, except of the base 50Hz of the supply, so i decided not to use filter. Is this a big mistake?



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    Re: Op-Amp Shunt Resistor

    Hi,

    If ADC: What sampling rate?

    i decided not to use filter. Is this a big mistake?
    Maybe, maybe not. It depends on your requirements.
    Even transients have a range of frequency. What upper frequency are you interested in?

    Maybe the voltage and current transducers are the frequency limiting devices.
    Then IŽd adjust the filters to them.

    ***
    If you want high speed, then this also calls for lower ohmic resistors at the amplifiers (V and I), else any stray capacitance will have bigger influence on the signal.

    Klaus

    Added:
    Do a search for standard RTD circuits.
    In my eyes the best is ratiometric with ADC_VREF as FvM already mentioned. Or dual constant current source (derived from ADC_VREF) with wiring resitance compensation.
    Depends on your need according accuracy and precision.
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    Re: Op-Amp Shunt Resistor

    Quote Originally Posted by KlausST View Post
    Hi,

    If ADC: What sampling rate?


    Maybe, maybe not. It depends on your requirements.
    Even transients have a range of frequency. What upper frequency are you interested in?

    Maybe the voltage and current transducers are the frequency limiting devices.
    Then IŽd adjust the filters to them.

    ***
    If you want high speed, then this also calls for lower ohmic resistors at the amplifiers (V and I), else any stray capacitance will have bigger influence on the signal.

    Klaus

    Added:
    Do a search for standard RTD circuits.
    In my eyes the best is ratiometric with ADC_VREF as FvM already mentioned. Or dual constant current source (derived from ADC_VREF) with wiring resitance compensation.
    Depends on your need according accuracy and precision.
    I want a maximum sampling rate of 5 ksps for voltage and 5 ksps for current. On the PT100 there is no sense of that high speed cause the PT100 itself is has a much slower response.



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    Re: Op-Amp Shunt Resistor

    Hi,

    I want a maximum sampling rate of 5 ksps for voltage and 5 ksps for current.
    OK...
    With 5ksps.. IŽd definitely use an appropriate low pass filter.
    Read about nyquist theorem.

    When I hear "transients" then I think about narrow peaks with a timing of less than 10us, maybe less than 10ns...
    As said: only you know the requirements...

    Hint:
    Just to give you a "picture": think about the transient as a triangle pulse with constant risetime and constant fall time.
    I recomend to use paper and pencil. Draw a chart with X = time, and Y = volts (amperes)
    Draw a little vertical line every 200us (5kHz) just to mark the sampling events.
    If you have use a tranparent foil and put it over the timing chart.
    Now draw your "transient" signal onto the foil.
    Focus on the points where the sampling event meets the transient signal curve. This is what your software will see. Only the points ,not the line inbetween.
    Now slowly shift the transient signal (foil) to the right with respect to the sampling events. See how the poins change.

    If it was my application .. and I just wanted to see the peak of the transients, but with a 5kHz sampling rate, then IŽd use some "peak detector". In simplest case a capacitor charged via a diode.. and slowly discharged by a resistor.

    All this can simply be simulated with excel..

    On the PT100 there is no sense of that high speed cause the PT100 itself is has a much slower response.
    Especially here a low frequency low pass filter gives more stable and more reliable readings.

    Also here applies nyquist.

    Especially with wired sensors there is a good chance that mains frequency is introduced. 50Hz or 60Hz.. it will cause your temperature reading to fluctuate (randomly)...but you know that the temperature will not change that fast. Thus suppress the 50Hz, focus on the expected teperature change rate and get more stable and reliable readings.

    Klaus
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    Re: Op-Amp Shunt Resistor

    It's a 3-wire PT100 like this
    The presented circuit topology (current source + differential amplifier) doesn't work for 3-wire. You need a special circuit that compensates for the measured cable voltage drop. Google for "Pt100 3-wire amplifier". The current source needs to drive a ground referenced load.



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    Re: Op-Amp Shunt Resistor

    Quote Originally Posted by FvM View Post
    The presented circuit topology (current source + differential amplifier) doesn't work for 3-wire. You need a special circuit that compensates for the measured cable voltage drop. Google for "Pt100 3-wire amplifier". The current source needs to drive a ground referenced load.
    I compensate the cable resistance in software calculations. Also can you suggest me a precise voltage divider to use with the 5V reference voltage for the 2.5V centered waveform?
    This to make the hole system ratiometric. A resistor voltage divider can be so precise?



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    Re: Op-Amp Shunt Resistor

    I compensate the cable resistance in software calculations.
    That's not possible with the shown circuit, you need to measure the cable voltage drop separately. If you mean substracting a constant cable resistance, unfortunately the cable resistance is temperature dependent. You may of course decide to use a more simple 2-wire circuit and don't measure or compensate the real cable resistance at all. That's particularly feasible for Pt1000 sensors and cable lengths up to a few meters.

    Ratiometric means to use the same reference for current source and ADC. Voltatge divider is the most simple version, but it's a 2-wire circuit.



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