3.2 - Stream Mode

The highest input data rates are obtained in stream mode.  Stream is a continuous hardware timed input mode where a list of channels is scanned at a specified scan rate.  The scan rate specifies the interval between the beginning of each scan.  The samples within each scan are acquired as fast as possible.

As samples are collected, they are placed in a small FIFO buffer on the U6, until retrieved by the host.  The buffer typically holds 984 samples, but the size ranges from 512 to 984 depending on the number of samples per packet.  Each data packet has various measures to ensure the integrity and completeness of the data received by the host.

The U6 uses a feature called auto-recovery.  If the buffer overflows, the U6 will continue streaming but discard data until the buffer is emptied, and then data will be stored in the buffer again.  The U6 keeps track of how many packets are discarded and reports that value.  Based on the number of packets discarded, the UD driver adds the proper number of dummy samples (-9999.0) such that the correct timing is maintained.

The table below shows various stream performance parameters.  Some systems might require a USB high-high configuration to obtain the maximum speed.  A “USB high-high” configuration means the U6 is connected to a high-speed USB2 hub which is then connected to a high-speed USB2 host.  Even though the U6 is not a high-speed USB device, such a configuration does often provide improved performance.

Stream data rates over USB can also be limited by other factors such as speed of the PC and program design.  One general technique for robust continuous streaming would be increasing the priority of the stream process.

A sample is defined as a single conversion of a single channel, while a scan is defined as a single conversion of all channels being acquired.  That means the maximum scan rate for a stream of five channels is 50k/5 = 10 kscans/second.

Above tables measured with firmware V1.13 and UD driver V3.18.

*Note:  Stream mode literally returns only 16-bits of binary data per sample.  The RMS resolution values exceeding 16.0 reflect the low noise of the 16-bit data.

ENOB stands for effective number of bits.  The first ENOB column is the commonly used “effective” resolution, and can be thought of as the resolution obtained by most readings.  This data is calculated by collecting 128 samples and evaluating the standard deviation (RMS noise).  The second ENOB column is the noise-free resolution, and is the resolution obtained by all readings.  This data is calculated by collecting 128 samples and evaluating the maximum value minus the minimum value (peak-to-peak noise).  Similarly, the Noise Counts column is the peak-to-peak noise based on counts from a 16-bit reading.

Interchannel delay is the time between successive channels within a stream scan.

The first table applies to all gains (ranges) for a single-channel stream, or a multi-channel stream at gain x1 only.  The other 3 tables cover multi-channel streams for gains x10, x100, and x1000.  The reason they are much slower is the amount of settling time required to account for the possibility that the input might swing from volts to microvolts.  The user can override the automatic settling if desired.