Part Number: ADS124S08
The electrical characteristics in the datasheet show an "absolute input current" and a "differential input current" for the pins.
1. What is the absolute input current?
2. What is the differential input current?
3. Is the below figure an accurate understanding...?
ie: If AINPx input current was the max (2nA), then the AINNx input current would be 2nA - (max diff input current = 1nA) = 1nA?
There are a number of graphs that exhibit the behavior of current through the input pins starting with Figure 8 on page 17 of the ADS124S08 datasheet. It is better to show the behavior as opposed to just giving a number.
There are a couple of conditions to consider and they relate to temperature, the absolute input voltage and whether the PGA is enabled or disabled. The absolute input current shows the current from conversion at various inputs and leakage current over temperature. You will notice that the highest current occurs at high operating temperatures when the input is connected to 0V. This is where the device leakage with respect to ESD cells and internal device switches is the highest and will dominate. The current is quite small relative to cooler temperature and this current will be primarily from bias currents from input chopping.
The differential current is what differs from AINP to AINN. Here you see more of the impact of the input voltage where the differential current increases the farther apart in voltage for AINP to AINN.
If the differential current is large, then there will be an error with respect to the voltage drops across the resistors creating a difference voltage. In most cases with the ADS124S08 the difference voltage is within the level of noise of the conversion.
The absolute input current directly impacts the overall accuracy of the measurement as the current will be drawn from the current source. However, the absolute input current is quite small unless the absolute input voltage is very close to 0V. For example, if you look at Figure 10 where PGA is enabled you will see that the current is quite small and relatively flat except at near 0V. In a normal RTD application you would not come near 0V so now the absolute input current is in the pA range. This is one reason why the graphs are a much better indicator than giving a min/max number.
Best regards,Bob B
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