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AD7124-4: IOUT and voltage sensing on the same pin

Question asked by Bernardo.Ramos on Nov 7, 2017
Latest reply on Nov 9, 2017 by JellenieR

I'm really enjoying the AD7124-4BRUZ! So well developed. The pins are well positioned and this influenced my choice.
I was able to design the PCB without a single via near the ADC chip! The entire lower plane was dedicated to ground.


First I was planing to use the AD7793 but then I realized that I would need an external multiplexer for the IOUT. Then
I knew an ADC from TI that supports multiplex on IOUT but I didn't like it. It has no buffers, the pins are in strange order... And after a while I found the AD7124.


So now I need some help with the transition from AD7793.


Part 1:


Can the IOUT be used in the same AIN pin used to sense voltage? (check the attached image)
I plan to activate the excitation current on the AIN0 pin and sense the voltage. Then to do the same on another pin to check another sensor.
I have not seen this in your examples, but I saw it on another site and the burnout current sources are used in this way.
If yes, should I enable the input buffers? And If yes, should I enable the reference buffers too?


On image 2 I show the same thing with filters. Do you know if this will work with them?


Part 2:


I know that I cannot use 5V on AVDD anymore (AD7793 allows it). So I will use 3.3V.


These are the values for the AIN:
At 0°C: 100mV
At 100°C: 140mV


Voltage in the ADC using PGA=16:
At 0°C: 16 x 100mV = 1.600V
At 100°C: 16 x 140mV = 2.240V


I will use a 2.49 kOhms high precision resistor as a reference.


With 1000uA of excitation current the chip can handle up to AVDD - 0.48V (page 8 on the datasheet).
As I have 3.3V on AVDD it can handle up to 2.82V between IOUT and AVSS.


In my case I have, with unbuffered reference:
Vtotal (IOUT to GND) = 140mV + 2.49V = 2.63V
And with buffered reference (100 ohms headroom):
Vtotal (IOUT to GND) = 140mV + 2.49V + 0.1V = 2.73V
Both bellow 2.82V.


So I am expecting that it will work. Am I right or missing something?