Can Differential ADC replace instrumentation amplifier ?

Discussion in 'Analog & Mixed-Signal Design' started by sohamkul, Sep 24, 2016.

  1. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Greeting Everyone ,

    I am doing a project of data acquisition system
    Let me explain project in simple terms
    Step 1) Sensor : A strain gauge type PRESSURE sensor (Wheatstone bridge)
    Specs : 2mV/V output
    I am applying +- 5Volts (10 Volts) The output swings from 0mV to 20 mv(fullscale)
    sensor.PNG
    Step 2) Instrumentation amplifier : Output of the sensor is given to an instrumentation amplifier which i made using 3 OP07 opamps
    i have also added a filter at the end of amplifier to reduce noise.
    Specs : the circuit has two ZERO pots (to adjust Zero output) and an gain adjustment pot .Gain adjustment pot is connected at R5
    IMG-20160727-WA0000.jpg

    Step 3) ADC : The output from this circuit is gain adjusted (typically gain = 100) and given to ADC ADS1115
    ADS1115 has capability to take the differential input directly and also has provision for programmable gain Upto 128
    Which looks something like this :
    index.jpg

    so i just interface my controller device with the above setup to get data (Data Acquisition ) and plot graphs etc

    ------Question-------
    Can we Totally bypass instrumentation amplifier and directly (maybe add a filter)connect the output of sensor to ADC ?
    1) adc will do work of converting the differential signal into binary data
    2) We can avoid the instrumentation amplifier circuitry
    3)ADC has programmable gain which we can set to 100

    Any suggestions and reviews are welcome !
    Thank you in advance !! :):):)
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,912
    2,176
    The ADS1x15 converters can't measure a signal outside of their operating voltage range. That means that the input voltages have to be between GND and VCC (and not negative IRT ground, relative difference between the voltages that go negative are OK). If your input signal meets that requirement then it should work.
     
    sohamkul likes this.
  3. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Really ?? i thought it was a long shot ! Anyways what if i gave the input voltage as gnd to +10 volts
    Output will then be surely between gnd to 20 mVolts ! can that work as well ??
     
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I haven't looked at your design, I only spoke about the ADC requirements.
     
  5. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    for ADC
    i am using a raspberry pi so i have to power adc using 3.3 v also i need a resolution of 20000 counts is it possible using 0-5volt supply to the sensor ???
     
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,912
    2,176
    Are you sure you can get x128 gain from that ADC?
    The RPi has a 5 volt pin on the gpio header. So it's possible to run the ADC at 5vdc but you would need a Bi-directional Logic Level Converter to connect it to the 3.3 volt RPi bus.
    https://www.sparkfun.com/products/12009
     
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