**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

- With the DAC at midscale, the Excitation Buffer

**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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