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CN0300: Maximum Loop Voltage (AIN8 connected to INT_Vref)

CN0300: What is the maximum loop voltage?

 

The maximum input voltage that the ADP1720 can handle is 28V. This covers most
4-20mA networks but some networks supply voltages can be up to 50V.
CN0300 shows that AIN8 and AIN9 are used to sense the voltage at loop.
Something that’s not clear is how the voltage at these pins is not negative
seeing as they are both labelled as ADC inputs.

The DAC NPN mode is selected by setting DACCON[8]=1. When this bit is set, then
AIN8 becomes an output pin and is connected to the internal 1.2V reference
source. This is not clear on the current datasheet or user guide – this will be
corrected.

AIN9 is used to sense the LOOP-voltage. To ensure that this voltage stays
positive and thus stays within the valid ADC input voltage range, the DAC
output voltage range used is ~0.4V to ~1V (600mV range). The ~400mV DAC offset
ensures that the ADC inputs always stay positive.
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