# AFE Transmit Stage

Document created by LiamR on Jan 16, 2015Last modified by carol.nelson on Jan 24, 2018
Version 4Show Document
• 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