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    current excitation for sensor

    Hi, I have the below circuit which provides the 0.5mA constant current supply for sensor.

    I'm designing a precision measurement and would like to know.

    Does the output of the sensor gets affected with C9 value. Please comment.

    Does this method of current excitation is correct? or Please suggest.

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    Re: current excitation for sensor

    Quote Originally Posted by hemnath View Post

    Does the output of the sensor gets affected with C9 value. Please comment.
    If you are trying to make dynamic measurements, then C9 will have a role to play.



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    Re: current excitation for sensor

    I will measure pressure changes using pressure sensor. which is a dynamic measurement right?

    How to select the capacitor value?



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    Re: current excitation for sensor

    The bridge resistance stays effectively constant during pressure variations. C9 has no effect on signals. It's probably intended as noise filter, I doubt that it has much effect at all.

    To predict if the circuit works correctly you need to know the bridge resistance.



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    Re: current excitation for sensor

    The bridge resistance as mentioned in datasheet.

    Min. Max. Unit
    2600 5500 ohm



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    Re: current excitation for sensor

    That method of current excitation is quite stable.
    It depends mostly on the accuracy/stability of the shunt reference ZD1.

    The only thing that could improve it slightly is to use a P-MOSFET in place of the PNP bipolar transistor to eliminate the small error due to the transistor base current.
    That would require a logic-level type MOSFET [Vgs(th)≦2V max] to operate from a 3.3V supply.
    Zapper
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    Re: current excitation for sensor

    5500 ohm multiply 0.5 mA = 2.75 V, not feasible with the shown circuit, no current regulation achieved. Even minimal value of 2600 ohm is marginal.


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    Re: current excitation for sensor

    Quote Originally Posted by crutschow View Post
    That method of current excitation is quite stable.
    It depends mostly on the accuracy/stability of the shunt reference ZD1.

    The only thing that could improve it slightly is to use a P-MOSFET in place of the PNP bipolar transistor to eliminate the small error due to the transistor base current.
    That would require a logic-level type MOSFET [Vgs(th)≦2V max] to operate from a 3.3V supply.
    Thanks for suggesting the mosfet. I shall replace the transistor with P-Mosfet and check the results.

    ZD1 is LM385Z which is a adjustable shunt reference. Is this IC is sufficient or do you suggest?

    - - - Updated - - -

    Quote Originally Posted by FvM View Post
    5500 ohm multiply 0.5 mA = 2.75 V, not feasible with the shown circuit, no current regulation achieved. Even minimal value of 2600 ohm is marginal.
    which means if i increase the supply voltage to +9V. Will it solve?



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    Re: current excitation for sensor

    Hi,

    ZD1 is LM385Z which is a adjustable shunt reference. Is this IC is sufficient or do you suggest?
    Already answered in post#6.

    You did not tell us your requirements for accuracy and precision, thus we can't verify if the LM385Z is suitable.
    * read the datasheet for the voltage drift of the LM385Z
    * the top right resistor tolerance causes additional errors
    * your ADC and it's tolerance causes additiinal errors.

    Mind: in this (ratiometric) case you may simplify the circuit by connecting the pressure sensor to ADC_ref (even if it is VCC). It may result in more accurate results, since it cancels out both reference tolerance and drifts.

    Klaus
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    Re: current excitation for sensor

    Top right resistor selected: PCF0805-13-2K4-B-T1. , SMD Chip Resistor, Thin Film, 2.4 kohm, 100 V, 0805 [2012 Metric], 100 mW, ą 0.1%, PCF Series
    ADC: LTC2410, 24 bit ADC

    How can I connect the sensor with ADC_REF? Please guide me.



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    Re: current excitation for sensor

    Also watch out for the DAC reference being lower than the voltage you are measuring. The reference is only 377.5mV but the output of the sensor could be several times bigger.

    Brian.
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    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: current excitation for sensor

    Hi,

    How can I connect the sensor with ADC_REF? Please guide me.
    Where do you see a difficulty?
    --> Similar to the circuit shown at the first page of the LT2410 datasheet. (If you didnīt read it: i strongly recommend to read it)

    * the sensor has two supply pins (and two output pins)
    * just leave the one supply pin at GND
    * and connect the other pin with the output of the AD4520
    * omit all the constant current circuitry

    *****
    Also watch out for the DAC reference being lower than the voltage you are measuring. The reference is only 377.5mV but the output of the sensor could be several times bigger.
    I assume you mean "ADC".
    --> The absolute input voltage of each input pin is determined by GND and VCC
    --> the decodable "differential" input voltage is determined by the reference voltage. Usually bridge differential output voltage is small.

    *****
    Btw: I see no benefit in using the voltage divider at ADC_REF+. With 2.048V you get a voltage resolution of about 120nV. This is 1/8000 of a millivolt! Your overall precision wonīt come close to this, I assume.
    The voltage divider adds error. As well as in-accuracy as drift and noise.


    Klaus
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    Re: current excitation for sensor

    Thanks for the valuable input. I don't have problem with this circuit. I just want to confirm the current excitation is correct with the experts in the forum.

    I can able to get 21 noise free bits and displayed on the graphic display.

    The problem starts here, I am displaying the big characters, small characters, bargraph, battery indication, temperature, etc .. While displaying all this, i am getting, 161762 based on my input signal which is fine. But when i just measure only my ADC signal(disabled all other functions), I get 161800. Why this is increasing. I have tried to minimize the noise by properly routing the PCB grounds. Can you guys guess why this could happen?

    Adding more options in the display, affects the ADC value? I have routed the ADC, reference voltage, all other grounds separately and connected at one place.



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    Re: current excitation for sensor

    Hi,


    the error is 38 LSBs with a reading of 160,000 LSBs.
    the total range is +/-8,400,000 LSB
    so we are talking about 2.3ppm FS.

    I see two sources for this:
    * either the excitation voltage at the sensor varies, because of modified current
    * or the reference voltage at the ADC varies.

    (BTW: I think the error could be improved by not using the current source circuit, but with true ratiometric as given in the LT datasheet.)

    If the nominal current through the sensor is about 0.5mV... then the error is in the range of 1.1nA.
    --> I hve no good idea on an improvement here.

    I rather expect the problem at ADC_VREF.
    We are talking here about an error of about 850nV.
    Maybe VCC drops and thus the ouput voltage of the AD4520 drops, too. Sadly there is no PSRR specification. Taking the "ripple rejection" for low frequencies to be 110dB, then this means VCC needs to vary about 250mV. You may do some measurements on VCC - but I think this is not the case.
    I rather think itīs the ADC_VREF- wiring.
    I donīt know about your PCB layout.
    But even if you use a GND plane there may be some 100nV between LT2410_GND and AD4520_GND.

    --> Thus I recommend KELVIN wiring of ADC_REF- to AD4520_GND

    Klaus

    **Added:
    I recommend to make the voltage divider for VREF more low ohmic.
    Now the source resistance is about 20kOhms. This is not suitable for a 24 bit ADC.
    --> 5k / 1k may be more suitable.
    Use a second (third) KELVIN connection from lower_resistor to AD4520_GND.
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    Re: current excitation for sensor

    Sorry Back again.

    Pressure sensor is current excitation type. Thus, I have provided 0.5mA . Initially when powered on, whether it is 0.50 mA or 0.60mA, it is not a problem.

    But the current should not change much when the temperature varies between 15 to 40 degC. I should check for temperature coefficient of the LM385Z, right?

    Resistor 2.4Kohm is PCF0805-13-2K4-B-T1. WELWYN SMD Chip Resistor, Thin Film, 2.4 kohm, 100 V, 0805 [2012 Metric], 100 mW, ą 0.1%, PCF Series ą 5ppm/°C

    Does this resistor will be stable?

    In the datasheet LM385BX, output voltage varies between 1.215 to 1.255V. Does this mean, Every time when I power on, the output voltage varies between this values.

    Does any other IC will provide same output for every power ON?

    Please comment.



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    Re: current excitation for sensor

    Hi,

    whether it is 0.50 mA or 0.60mA, it is not a problem.
    You already know that you don't need absolute accuracy. I agree with this.
    Thus you don't need a 0.1% absolute accuracy resistor (or current source...or reference voltage)

    --> you need to focus on "drift", noise and unlinearities to get good precision.
    What's the device with biggest overall errors (drift, noise)? And what's it's expectable value in %FS?
    --> focus on this device and try to improve it's performance.

    Klaus
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    Re: current excitation for sensor

    What's your actual circuit? Have you increased the supply voltage to achieve reliable operation over the expectable sensor resistance range (see post #7)?

    As KlausST pointed out, your latest question can't be answered about a specification of intended accuracy. Please consider the previous suggestions about ratiometric measurement, derive excitation current and ADC reference from the same voltage.



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    Re: current excitation for sensor

    Please clarify for below term.

    Initial Accuracy from TI notes:
    The initial accuracy of VREF indicates how close to the
    stated nominal voltage the reference voltage is
    guaranteed to be at room temperature under stated
    bias conditions. It is typically specified as a percentage
    and ranges from 0.01% to 1% (100-10,000 ppm). For
    example, a 2.5 V reference with 1% initial accuracy
    should be between 2.475 V and 2.525 V when
    measured at room temperature. The importance
    of initial accuracy depends mainly on whether the
    data conversion system is calibrated. Buried-zener
    references have very loose initial accuracy (5-10%)
    and will require some form of calibration.

    Every time when I power ON, the same output voltage will be obtained? or it will vary within the limits. Because, Once the instrument is powered, calibrated, stored and display the values. Next time when powered, if the output voltage changes, the display value will also get changed.

    Lets say, at room temperature 25degC,
    On 1st time, powered ON, Output voltage is 2.510V,
    2nd time, powered ON, output voltage is 2.485V
    3rd time, powered ON, output voltage is 2.500V
    4th time, powered ON, output voltage is 2.490V

    Is it behave like this with initial accuracy of 1%?



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    Re: current excitation for sensor

    Hi,

    What exactly (of the explanation) you need to be clarified?

    A voltage reference will not start at random voltages (why sould it in your eyes).
    But it will
    * drift with time
    * drift with temperature
    * drift with supply voltage
    * be noisy
    * it will even drift with mechanical stress.
    Thus it won't always start with the same voltage either.

    I tried to explain, that you don't need to look for initial or absolute accuracy, you need to look for precision, which is repeatability.

    Klaus
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    Re: current excitation for sensor

    I still didn't hear a specification for the intended overall accuracy.

    Unless you are using an expensive high performance pressure sensor (multi 100$ range), sensor temperature drift will be probably the largest source of inaccuracy.



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