The sampling rate of the NI C Series Delta-Sigma (DSA) modules depend on a number of factors. One of these factors is the Timing Mode that is set for the device. Each channel will have to wait for a hardware specific conversion time, and the Timing Mode will affect that amount of time.
Refer to the table below to see which modules use this setting. It lists the number of milliseconds per channel required to convert between inputs. For example, an NI 9217 using four channels on High Resolution
will require a total of 800 ms to process one reading for each channel. The NI 9219 module has two sets of time listed. The first and second times in each cell are for when no channels or at least one channel are in Thermocouple
Conversion Time (ms) per channel
|Best 60 Hz Rejection|| || || || || || || ||27.5/30|
|Best 50 Hz Rejection|| || || || || || || ||32.5/35|
Each option listed in the table (e.g. High Speed) is actually a user-programmable setting that can be defined in code. This property is called the ADC Timing Mode. In this property, you can choose from High Speed, Best 60 Hz Rejection, Best 50 Hz Rejection, and High Resolution modes.
is generated when the module is requested to perform at a sampling rate that it is not currently configured to support. For example, requesting a 5 Hz sample rate when the device is configured in high resolution mode and the max sample rate is 2 Hz.
Let's assume you are measuring a single thermocouple channel on your NI-9219 module, and you have configured the DAQmx Timing VI to take measurements at a rate of 50 Hz (the maximum allowed according to the specifications). By default, you will receive data points at a rate of 50 Hz, though many of the data points will be repeated. In fact, the data will actually change at a rate of around 2 Hz. This is due to the default ADC Timing Mode being set to High Resolution
for thermocouple measurements; setting the ADC Timing Mode to High Speed
will allow you to obtain changing thermocouple data at a rate up to 50 Hz. Since temperature is typically a slow varying phenomenon it is usually not necessary to measure more than a couple of times a second. For this reason, thermocouples are normally measured at a slow rate with a low-pass filter to ensure accuracy.