CapLog Sampling System Philosophy

Determinism

1. A basic channel is always sampled at a predictable rate without jitter. A corollary is that channels are sampled in the same known order and hence always maintain their phase relationship.

2. A number of basic channels may be grouped and sampled sequentially, with the group being treated as a whole and sampled at a predictable rate.

3. High speed and low speed channels may be mixed and remain jitter free.

4. Precision low-speed channels are achieved by over sampling of the raw signal channel and one or more correcting channels (such as input offset, excitation current etc). Multiple such channels can be interlaced to create the effect of simultaneous sampling. If this interlaced sampling is done over one or more mains cycles (50/60Hz) maximum hum rejection is also achieved.

Overloading Gracefully

As the Cap is given more to do, it reaches a limit when no additional load can be tolerated without reduced performance. A design goal was to retain predictability for as great a load as possible by relaxing such things as sample rates, filtering and resolution. Most of these characteristics tend to exceed real world requirements so in principal, relaxing them is not a big an issue up to a certain point, provided the user is aware of the compromises that have been made.

For example, if we would like to measure 20 thermocouples 50 times a second. By default a thermocouple channel would prefer to take at least 100 sets of evenly space samples every 20ms, but this allows only 10 thermocouples to be read every 20ms. Reducing the number of sample sets to 50 per 20ms will allow the full 20 thermocouples to be read but at the cost of noise rejection decreasing “a little”. This is a fuzzy process but valid provided the user is aware of how “a little” impacts the application.