National Instruments SC-2040 Analog Circuitry

Chapter 4 Theory of Operation

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.

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 kΩ resistor in series with each input line followed by low-leakage

diodes to the supply rails (±15 V).

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.

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