I want to build an analog front-end with a 16 channel multiplexer. On each multiplexer input, there will be a voltage divider made from resistors. On the multiplexer output, there will be an op amp buffer and later on an ADC. The signals are low frequency <100Hz.
simulate this circuit – Schematic created using CircuitLab
Now what I'm concerned about is the offset introduced by the multiplexer due to leakage current. So for selecting the right part, I will need to make some calculations about the worst-case offset. I already have the correct formulas by hand. However, what confuses me is the specifications in the datasheet:
Here's an example of the MUX506IPWR multiplexer from Texas Instruments:
The datasheet specifies three different leakage currents. I am interested in the leakage that occurs for a particular channel during measurement (when the switch is on). So initially, I though that I just have to consider the Output on-leakage \$I_{D(ON)}\$.
Then I noticed that they use the term "switch".
Since a 16 channel multiplexer contains 16 switches (only one can be active at a time, and all switch outputs are connected) do I also need to consider the Output off-leakage \$I_{D(OFF)}\$ in my calculations? e.g.:
$$total\ leakage\ current = 15 \cdot I_{D(OFF)} + I_{D(ON)}$$
or is this somehow already contained in the specifications of \$I_{D(ON)}\$?
Best Answer
The bad news is that this leakage is per switch, so you do have to add all of the component currents.
The good news is that those leakage currents, at low ambient temperatures at least, are dominated by what their production test gear can measure quickly, rather than realistic leakage currents.
In practice, at 25°C, you can assume the leakage currents are typically several orders of magnitude below those worst case figures.
The difference between that 25°C typical, and over temperature worst case, is whether you are making a one-off hobby device to work in a home ambient, or 1000's off professional equipment to work over a full environmental temperature range.