Measurement Computing USB-2527 - Manual (Page 28)

USB-2527 User's Guide Functional Details
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Figure 11. Analog and digital scanning, once per scan mode example
If the three counter channels are all returning 32-bit values and the digital input channel is returning a 1-bit
value, then 18 samples are returned to the PC every scan period, with each sample being 16-bits. Each 32-bit
counter channel is divided into two 16-bit samples—one for the low word and the other for the high word. If the
maximum scan frequency is 166,666 Hz, then the data bandwidth streaming into the PC is 3 MS/s. Some slower
PCs may have a problem with data bandwidths greater than 6 MS/s.
The USB-2527 has an onboard 1 MS buffer for acquired data.
Thermocouple input
You can configure up to four analog inputs on the USB-2527 to accept a TC input. Built-in cold-junction
sensors are provided for each of the screw-terminal connectors, and any TC type can be attached to any of the
four thermocouple channels.
When measuring TCs, the USB-2527 can operate in an averaging mode, taking multiple readings on each
channel, applying digital filtering and cold-junction compensation, and then converting the readings to
temperature.
As a result, the USB-2527 measures channels with TCs attached at a rate from 50 Hz to 10 kHz, depending on
how much over-sampling is selected.
Additionally, a rejection frequency can be specified in which over sampling occurs during one cycle of either
50 Hz or 60 Hz, providing a high level of 50 Hz or 60 Hz rejection.
Tips for making accurate temperature measurements
 Use as much oversampling as possible.
 Warm up the USB-2527 for 60 minutes—including TC wires—so that it is thermally stabilized. This
warm-up time enables the CJC thermistors to more accurately measure the junction at the terminal block.
 Make sure the surrounding environment is thermally stabilized and ideally around 20 °C to 30 °C. If the
board’s ambient temperature is changing due to a local heating or cooling source, then the TC junction
temperature may be changing and the CJC thermistor will have a larger error.
 Use small-diameter, instrument-grade TC wire. Small diameter TC wire has less effect on the TC junction
at the terminal block because less heat is transferred from the ambient environment to the junction.
 Use shielded TC wire (see "
Shielding " on page 29) with the shield connected to analog common to reduce
noise. The USB-2527 has several analog common pins on both the 68-pin connector and the 40-pin
connectors, and the TB-7 has one analog common screw terminal.
You can also minimize the effect of noise by averaging readings (see " Averaging " on page 29), or
combining both shielding and averaging.
Refer to " 68-pin SCSI connector differential and single-ended pin outs (P5) " on page 12, " 40-pin header
connector pin outs " on page 15, and " Four-channel TC terminal pin out (TB7) " on page 19 for the locations
of these analog common pins.