1.12.10 Source Impedance

This is a very common problem when doing ADC designs. The source impedance is the sources capability to deliver charge to the internal sampling capacitor fast enough. If the internal capacitor is not charged to the same level as the analog signal, the result will be wrong.

When using a direct connection from a sensor IC, the impedance is usually stated in the data sheet of the device and it is easy to adjust the speed for the given impedance. When the circuitry is made up by passive components, calculations have to be done to find the actual source impedance. For example, a resistor ladder dividing a high voltage down to a voltage that can be handled by the microcontroller can have very high impedance as large resistors are used to get the voltage sufficiently down. The solution is to either lower the sample rate to be able to measure the correct signal, or to lower the resistance value. Lowering the resistance would cause more leakage current and this can again be solved by adding switching FET to enable and disable the analogue source. An example of this can be seen in the figure below.

Another thing to be aware of in such an application is the time for the source to be stable, as the decoupling will take some time to charge to the correct level after enabling the analog source.