Measurement Computing USB-2527 - Manual (Page 29)

USB-2527 User's Guide Functional Details
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 Make sure the USB-2527 is mounted on a flat surface.
 Be careful to avoid loading down the digital outputs too heavily (>1 mA). Heavy load down causes
significant heat generation inside the unit and increase the CJC thermistor error.
Shielding
Use shielded TC wire with the shield connected to analog common to further reduce noise.
The USB-2527 has one analog common screw-terminal on TB7 and several analog common pins on the headers
(see "
Connecting the board for I/O operations " starting on page 11). You can connect the shield of a shielded
thermocouple to one of the analog commons. When this connection is made, leave the shield at the other end of
the thermocouple unconnected.
Caution! Connecting the shield to common at both ends results in a ground loop.
Averaging
Certain acquisition programs apply averaging after several samples have been collected. Depending on the
nature of the noise, averaging can reduce noise by the square root of the number of averaged samples.
Although averaging can be effective, it suffers from several drawbacks:
 Noise in measurements only decreases as the square root of the number of measurements—reducing RMS
noise significantly may require many samples. Thus, averaging is suited to low-speed applications that can
provide many samples.
 Only random noise is reduced or eliminated by averaging. Averaging does not reduce or eliminate periodic
signals.
Analog output
The USB-2527 has four 16-bit, 1 MHz analog output channels.
The channels have an output range of -10V to +10V. Each D/A output can continuously output a waveform at
up to 1 MHz. In addition, a program can asynchronously output a value to any of the D/A channels for non-
waveform applications, assuming that the D/A is not already being used in the waveform output mode.
When used to generate waveforms, you can clock the D/As in several different modes.

Internal output scan clock : The on-board programmable clock can generate updates ranging from 1 Hz to
1 MHz.

External output scan clock (XDPCR) : A user-supplied external clock.

Internal input scan pacer clock : The internal ADC pacer clock can pace both the D/A and the analog
input.

External input scan pacer clock (XAPCR) : The external ADC pacer clock can pace both the D/A and the
analog input.
Example: Analog channel scanning of voltage inputs and streaming analog outputs
The example shown in Figure 12 adds four DACs and a 16-bit digital pattern output paced by the input scan
clock to the example presented in Figure 8.