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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