How do the input offset specifications relate to output error for current sensors.
Typically in all current sensor datasheets, there are two key offset parameters that are specified.
1. Initial Offset
2. Offset Drift (Over temperature).
These specifications are typically given as RTI or Referred To Input.
How can these two specifications be easily related to output error?
Let's take the AD8210 Current Sensor as an example.
It specifies a +-1mV MAX initial offset at room temperature.
As this is an RTI specification, for it to be relevant to the output we must take into account the gain of the part. In this case the gain is 20V/V.
So, worst case output error due to initial input offset is:
+-1mV x 20V/V = +-20mV.
Therefore, the output of the AD8210 at room temperature can be off +-20mV worst case, from its ideal starting point.
So, if a 2.5V output is expected for the AD8210, in a worst case situation the output could be 2.48V (if inital offset if negative) or 2.52V (if initial offset is positive) at room temperature.
The second offset specification for the AD8210 is the Offset Drift Parameter specified as +-8.0uV/C.
This specification is also RTI (Referred to Input) therefore we must take into account the gain of the AD8210 which is 20V/V.
This specification at the output is therefore:
+-8uV/C x 20V/V = 160uV/C.
Let's assume that we measure the output of the AD8210 at 25C and obtain 2.5V.
If we take another measurement at 125C, assuming no changes to the input voltage, then we would have a worst case output due to offset drift of:
2.5V + (+-160uV/C x 100C) = (2.5V + 16mV) or (2.5V - 16mV).
So a worst case output (assuming a 2.5V initial start point) for the AD8210 at 125C due to offset drift is 2.484V (if drift is negative) or 2.516V (if drift is positive).
The initial offset error and the offset drift error must be added together to obtain the total error at the output due to the input offset specifications at a certain temeperature.
As an example. Assume the AD8210 has an initial offset of +1mV and a offset drift of +8uV/C and we expect a 2.5V output at 25C:
The output of the AD8210 under these conditions would be.
+1mV x 20V/V = 20mV
20mV + 2.5V = 2.52V at 25C.
If we measure the output at 125C (100C change in temperature) with the same input voltage then we would obtain:
2.52V + (+8uV/C x 20V/V x 100C) = 2.536V at 125C
It is also worth noting that a part can have a negative inital offset, and have a positive offset drift and vice versa.
Whether the initial offset of the offset drift is negative or positive varies by part, which is why a max specification is necessary to obtain a worst case calculation.
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