# AD7172-2 Use Case and Configuration

Question asked by dpetraut on Oct 4, 2017
Latest reply on Oct 10, 2017 by JellenieR

Hello,

at first few information about what we intend to do. We want to measure a small current from a Biochemical Cell and convert it to a voltage with a low bias current TIA Stage. Based on the measured Gas (i.e. a reducing Gas Sensor) the TIA output is a positive or negative voltage from -2V5 to 2V5 volt. In order that we can use a single ADC (AD7271-2) we try to use the mentioned one with its inputs configured as pseudo-differential inputs referenced to AIN4 (currently AIN1 with the DevKit) (AGND).

we have the following hardware setup an want to make sure everything is

working correct:

We changed the supply configuration to split supply (unregulated) by doing

the following:

1. Remove R58 to R62

2. Insert R85 for +/-2.5V Operation

3. Set SL2 to B for ext AVDD1

4. Set SL3 to B for ext AVSS

5. Set SL5 to A for ext IOVDD

6. Remove LK 8 to 12

7. Set LK1 to B for for 2.5V AVDD1

8. Set LK2 to B for ext. power supply from J5

Then we connect the power supplies as follwing:

AVSS = -2.5V

AGND = 0V

AVDD = 2.5V

IOVDD = 3.3V

And the input as follows:

AIN0 = variable Voltage from -2.5 to 2.5

AIN1 = AGND

Finally we configure the AD7172-2 per software:

Setup register: 0x1000 //Bipolar coded output (offset binary)

Channel0 register: 0x8001 // Channel 0 enable; AINNEG0 = AIN1; AINPOS =

AIN0

Filter register: 0x0015 // output data rate: 2.5 SPS

We notice the following result:

Setting input voltage on AIN0 to -2.5V gives -1.25V as output

Setting input voltage on AIN0 to 0V gives 0.0V as output

Setting input voltage on AIN0 to +2.5V gives +1.25V as output

For us this makes sense because we use differential mode and  0 + 2,5 / 2 = 1,25, right?

At last we' like to know if this is a working way to get the input offset from a ADC input:

We will use the ADG1419 SPST switch to disconnect the input trace as close as possible from the source and instead connect this with AGND?

Regards

Dirk