Encoders produce periodical signals. Therefor, the signal itself doesn't really need to be measured, the number of edges alone gives the information. I suggest you read the following tutorial :
To make encoder measurements, you need a basic electronic component called a counter. Based on its several inputs, a basic counter emits a value that represents the number of edges (low to high transitions in the waveform) counted. Most counters have three relevant inputs – gate, source, and up/down. The counter counts the events registered in the source input, and, depending on the state of the up/down line, it either increments the count or decrements it. For example, if the up/down line is “high” the counter increments the count, and if it is “low,” the counter decrements the count.
>> Encoder Measurements: How-To Guide
In your case, and since you need to measure 16 encoders, I guess you can go for a HSDIO device as the PCIe-6535 I've mentioned earlier.
Have a nice day,
Marc-Junior
