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VXI Technology, Inc.
16 SVM2608 Introduction
sets. (See 3Microprocessor Commands for more details on available commands.) The command is
sent to the microprocessor via the Command register. Since there are four independent channels
on the board, each of them can take a different command and each of them has its own command
register. The result of the microprocessor calculation is returned in the Result register for the
corresponding channel.
The data stored in the channel memory is raw data. When the microprocessor performs a
resistance calculation, it uses calibrated data, meaning that the microprocessor takes the
calibrations for the Local Current Sources (see above) values into consideration. The raw data the
user downloads from the board represents calibrated voltage measurements. The result calculated
by the microprocessor and placed in the Result register when a Resistance Measurement
command is issued is based on calibrated Current measurements. While the user can perform
calibrated voltage measurements by simply reading the raw data, the calculations for resistance
cannot be accurately performed by the user as they do not have the calibrated current values (the
exact values injected in the resistance under test by the board’s current source). Although it is
possible for the user to read the calibration values (see the
Calibration Commands section) and
use the raw data to perform all the calibrated measurements on their own, the manufacturer
encourages the use of the microprocessor’s capabilities to perform all calibrated resistance
calculations.
Option -01
With the addition of the SVM2608-01 option, two additional channels are available with a
sample rate of 20 MHz and 12-bits of resolution. This option may be purchased at the same time
as the SVM2608 or is factory upgradeable.
The high-speed channels available on Option -01 function independently. The front end of each
channel has both a variable gain amplifier and an attenuator, similar to the low-speed channels. A
5 MHz low-pass filter (LPF) is available on these channels, as opposed to the 20 kHz LPF found
on the low-speed channels. The ADC converter is a 12 bits converter capable of taking as many
as 20 MSamples/s on a scale of -2 V to +2 V. To compensate for offset and gain variation in the
ADCs, each channel has two 12 bits DACs that are used to calibrate the offset and the gain on
each ADC channel. These calibrations are performed at the factory using precision voltage
reference sources. A block diagram for Option -01 is provided on the following page.