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
We are useing the EVAL-AD717x-2SDZ (Rev-E) Evaluation Kit with a AD7172-2.
We changed the supply configuration to split supply (unregulated) by doing
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 =
Filter register: 0x0015 // output data rate: 2.5 SPS
ADC mode: 0x0000
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?