SaM - Remove the Offset in the Measurement System

Memory Card

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Suppose we want to measure a temperature in a small temperature range $T\in \left[T_{MIN}÷T_{MAX}\right]$. We measure it with a linear sensor, suppose an RTD, this is its function:

And for the input to the read-out electronics we use a simple voltage divider like so:

So we will have the output:$V_0=V_{DC}\frac{R_{REF}}{R(T)+R_{REF}}$With a maximum, minum, and the reference value voltage defined as:$\begin{array}{l}{V}_{0_{MIN}}=V_{DC}\frac{R_{REF}}{R_{MAX}+R_{REF}}\\ V_{0_{MAX}}=V_{DC}\frac{R_{REF}}{R_{MIN}+R_{REF}}\\ V_{REF}=V_{DC}\frac{R_{REF}}{R(T_{REF})+R_{REF}}\end{array}$Where: $T_{REF}$ is taken as $\frac{T_{MAX}-T_{MIN}}{2}$.

So the output of this system will have this function:

• $V_{REF}$ can be seen as the offset of the system.
• While $\left[V_{0_{MIN}},V_{0_{MAX}}\right]$ is its dynamic range, the information given by the sensor lies all in this small range that we will call $\Delta V$.

This way we don’t fully utlize the range given by the acquisition system (A/D) in a complete measurement system, that goes for example between $\left[-V_{DC},+V_{DC}\right]$. Ideally I wold select an amplifier that amplify the small range $\Delta V$ into the whole range of the aquisistion sysmtem, so $2V_{DC}$. But in this case we cannot do so becasue we have an offset.

To solve this we need to compensate the offset, so bring the output signal to have a $V_{REF}=0$:
To do so we can use a balanced bridge, instead of a voltage divider, like so:

And even if we use a good linear solution for meauring a resisitance sensor:
We need to consider that the sensor may be distant from the read-out electroncis, and we might need to consider the wires resistances, and this will result into another offset.

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Conclusion on Amplifier’s DC Offsets

• And the part which cannot be corrected are the drifts of the offset, and this can’t be corrected.
• ==And usually you can consider drift as $\frac{1}{10}$ of the value of the offset==, in a span of about, let’s say, $50$ degrees, the whole temperature span, which you can have.