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®
Model No. CI-7599
750 Interface (USB)
17
Appendix D: Theory of Operation
Flash Memory
The 750 stores its operating system software and waveform patterns in
flash memory. After turning on the power supply, the interface
initializes the operating system without requiring user initiation in
DataStudio. When a new revision of DataStudio is later loaded onto
one’s computer, the interface automatically upgrades the system
software and waveforms in flash memory.
Analog Input Channels
All three analog input channels (A,B, and C) on the 750 interface are
identical. Each has differential inputs; in other words, they are like the
inputs to a voltmeter where neither lead is connected to ground. The
voltage measured is the voltage difference between the inputs.
Each channel has three software accessible gain settings: 1, 10, and
100. When an analog sensor is plugged into the interface, the signal is
applied to a device called a differential amplifier. As the name
suggests, a differential amplifier amplifies the difference between two
signals.
In this case, the output of the amplifer is the difference between the
A+ input and the A- input, multiplied by a fixed gain that is selectable
in DataStudio software at 1, 10 or 100. From this point, each analog
signal (A, B, and C plus other internally-generated signals) is applied
to a device called a multiplexer. The multiplexer, which acts like a
switch, selects which input channel is to undergo analog-to-digital
conversion.
There are other analog channels within the 750 interface. Two
channels are dedicated to voltage and current measurements for the
Function Generator, plus others for internal diagnostic purposes.
Channels to be converted are selected by the 750 interface under the
control of DataStudio. The analog-to-digital converter uses 12-bit
conversion, so the maximum range of each analog input, +/- 10 volts
can be converted to a range of digital numbers from 0 to 4095 (2^12).
When the analog value is in digital form, it can be sent immediately to
the host computer for further processing, or stored temporarily with the
750 interface internal memory.