I am using CN0209 circuit from the lab in a design for my company.
I tested the design an I saw accuracy error in the Voltage measurement. I want to ask some questions:
Q1: Which is the spected accuracy of the interface? expressed in % of the input full scale for example
Q2: Which configuration of thew AD7193 must to be used for voltage reading? (Buffered, chop, Differential, unipolar etc ) If you provide me the config and mode registers is ok
Q3:Which configuration of thew AD7193 must to be used for the rest of interfaces? If you provide me the config and mode registers is ok
Q4: which is the function of the 1K5 resistor and the 100nF after the ad8275?
I think part of the error in accuracy is due the voltage drop in this resistor using unbbufered configuration in ADC. In unbuffered mode, the input current is 3.5uA/V typical. That means that the error voltage is dependent on the magnitude of the input voltage.
If we short the input and test this will cause an error voltage of about 3.5uA*2.5V*1500ohms, or 13.13 mV.. When the input is near positive full scale, the input to the ADC is close to zero (due to the inversion in how the amplifier is used), so the error voltage caused by the input currents is much less, say 3.5uA/V*0.2V*1500ohms, or 1.05 mV. When the input is close to negative full scale, the input to the ADC will be close to 5V. This will cause an error of 3.5uA/V*5V*1500ohms, or 26mV. And these are with the typical input current. It could be, and will be higher at higher temperature. Referred back to the input of the AD8275, the error can be as much as 130 mV or more between plus full scale input and minus full scale input.
Q5: Can we reduce this error (if my hipotesis is correct) lowering or removing the 1K5 resistor? Can we use buffered configuation or this means to loss input range due the reduction in input voltage in buffered configuration to AVDD-0.250V ans AGND+0.250V?
Good to here that what you are seeing makes sense in relation to the error budget.
Which other configurations and formulas are you interested in?
It is possible to use an additional ADG1414 instead of the ADG442, though in this case you would need to control an additional /SYNC line. This would require an isolated channel to the controller.
Two reasons a different switch were used are that the ADG442 is a quad channel device instead of an octal, it was also easy to control using the digital output pins (P2, P3) of the AD7193 thus avoiding an additional isolation channel.