Search FAQs on the left to see if your question has been answered. Click on the dropdown to view all of the documents associated with the product. If you can't find your question, click on Ask a Question

AFE Transmit Stage

  • How does the ADuCM350 set a voltage on the sensor ?
  • First step is to consider the DAC and it's transfer function.
    • The VDAC Output Range is 0.2V to 1.0V.
  • This signal is then biased and gain up to produce the output voltage from the Excitation Amplifier, VD, which is the voltage seen by the sensor.
  • VD = ( ( VD-0.6V ) X 2 ) + VBIAS
    • VDAC is referenced to the INAMP reference of 0.6V
    • The Excitation/INAMP Loop has a gain of 2.
    • VBIAS: The Common Mode of the System is setup by the +ve terminal of the TIA.
  • This leads to a transmit channel transfer function that looks like:
  • How does the TIA set the Common Mode of the AFE Measurement Loop ?
  • See diagram below as reference.
  • Assume 2-Wire Measurement with P and N tied internally.
    • In 4-Wire, Raccess would be automatically compensated for on D/P/N and T.
  • TIA Sets Common Mode
    • Positive terminal of the TIA is connected to VBIAS.
    • Therefore the negative terminal of TIA forced to VBIAS. This is a Virtual Ground.
  • T = N = VBIAS
    • T and N are shorted internally
  • Excitation Buffer Forces D so that P – N = 0V
    • But N is forced to VBIAS by TIA
    • So Excitation Buffer drives P to = VBIAS
  • Hence D is forced to have a common mode of VBIAS
    • With the DAC at midscale, the Excitation Buffer
      output sits at VBIAS
  • Example Calculation: Sensor needs to see +450mV on Counter and 0V on Working Electrode. i.e +450mV step across Sensor.
  • Goal is to have P – N = +450mV
    • Assume VBIAS = 1.1V.
    • T = 1.1V = Virtual Ground
    • Given that N = T = VBIAS, this implies that the voltage at the P node needs to be 1.55V
  • Signal swing of Excitation Amplifier output is -800mV to +800mV
  • Dividing this range by the resolution of the DAC
    • 1.6V / 2^12 = 390.725µV
    • Therefore each DAC LSB is equivalent to 390.725µV out of the Excitation Amplifier
    • Midscale of the DAC is equivalent to a 0V difference between P and N
    • Midscale DAC code = 2048 = 0x800
  • 450mV / 390.725µV = 1152 = 0x480
  • 1,1V + 450mV =  0x800 + 0X480 = 0xC80
  • Therefore, the DAC code required for a +450mV step on the sensor is 0xC80
Comments
Anonymous
Related