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

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

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