Clock Source Notes

Table of Calibrated Values

SpikeGLX maintains a table of NI clock sources in the file

The entries in this table have the form:

Name : Set_rate=Measured_rate

Simultaneous Sampling vs Multiplexing

How fast can the device go?

S-series (61xx) and some X-series (63xx) devices support simultaneous sampling, which means that each AI channel has its own amplifier and ADC, so can digitize samples independently and in parallel. These devices can sample at their maximum advertised rate at any channel count.

By contrast, M-series (62xx) and some X-series devices have a single amplifier/ADC pipeline that is shared by all AI channels. The inputs are switched into the digitizer in a serial fashion, one at a time. The maximum rate these multiplexing devices can go is the advertised sample rate R0 divided by the number of channels: R0/nchan.

The shortest time it takes to digitize a sample is the convert period which is 1/R0. However, when multiplexed inputs are switched it takes a small amount of time to allow charge from the previous sample to dissipate and the amplifier to settle to the new nominal charge level. To avoid crosstalk and get better accuracy, NI recommends that an estimated settling time of 1e-5 seconds be added to the convert period. The safe rate is then 1/(1/R0 + 1e-5).

Internal Clock Step-down Factor

What sample rate can I actually program?

Digitizing hardware needs to be driven by a square wave: a clock source that can be generated by another external device or by the internal pulse counter (Ctr0) on-board the NI device. If using an external clock, its rate is what it is. We allow you to directly set that value into the rate box; it is only limited by the device's maximum (up to 1 MHz).

However, when the internal source is selected, we program its rate by defining a task that counts a specified number of whole pulses of the device's master timebase. That is, realizable rates are limited to: timebase/integer. In this case, you must enter the integer and we put the result into the rate box.

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