National Instruments SC-2040 - Manual (Page 36)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-3 SC-2040 User Manual
Analog Circuitry
The analog input circuitry consists of eight channels with DIP-switch-programmable
instrumentation amplifiers followed by buffered track-and-hold amplifiers. In addition, you can
include the voltage-regulation circuitry and input protection in the analog section. Each block is
described in the following paragraphs.
Input Protection
The first block an incoming analog signal encounters is the input protection. Each input terminal
is protected against input voltages up to
±
15 V powered off and
±
30 V powered on. The input
protection consists of a 1 k
Ω
resistor in series with each input line followed by low-leakage
diodes to the supply rails (
±
15 V).
Amplification
Next in the signal path are the instrumentation amplifiers, which fulfill two purposes on the
SC-2040 board. First, the instrumentation amplifiers convert differential input signals into
single-ended signals referred to the SC-2040 analog ground for input common-mode signal
rejection. With this conversion, the SC-2040 can extract the analog input signals from common-
mode noise voltages before the DAQ board samples and converts the signals. Second, the
instrumentation amplifiers amplify input signals, resulting in an increase in measurement
resolution and accuracy. Furthermore, the amplifiers exhibit low bias currents and good
bandwidth, even at high gains.
You can select gains for each channel independently with separate DIP switches. Gains are 1,
10, 100, 200, and 500, although gains of 300, 600, 700, and 800 are available with reduced
accuracy. See Appendix A, Specifications, for details on the performance of the instrumentation
amplifiers.
Track-and-Hold (T/H) Circuitry
The track-and-hold amplifiers operate as simple buffers when in track mode, but freeze their
outputs when placed into hold mode. Because all of the track-and-hold amplifiers in each
SC-2040 board enter hold mode at the same time, they implement simultaneous sampling of all
channels. Simultaneous sampling is useful for preserving phase relationships between channels.
The track-and-hold amplifiers are subject to various imperfections, which are listed in
Appendix A, Specifications. The most relevant of these performance specifications are the track-
mode acquisition time and the hold-mode settling time, as these parameters affect how the DAQ
board acquires the data, most noticeably through the sampling rate.
The hold-mode settling time refers to how long it takes the T/H amplifiers to settle a stable value.
Thus, the hold-mode settling time affects how long the DAQ board must wait before attempting
to acquire data. The track-mode acquisition time refers to how long it takes the T/H amplifiers to