VXI TECHNOLOGY SVM2608 - Manual (Page 13)

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SVM2608 Introduction 13
The counts stored in memory are 16 bits SIGNED integers. The most significant bit represents
the SIGN. Thus, the hex number 0x4000 and the hex number of 0xC000 represent the same
signal amplitude but in opposite directions, where 0x4000 represents a positive signal while
0xC000 is a negative signal with the same amplitude.
To translate a raw count value into a voltage, multiply the raw count value by the bit weight. The
following example shows this conversion for a SVM2608 using the 10.0 V range:
A reading of 0x4000 = 16,384 counts
Voltage = Counts * Bit Weight
Voltage = 16384 counts* 305.176 µV/count
≈ 5.0 volts.
A reading of 0xC000 = -16,384 counts
Voltage = -16384 counts * 305.176 µV/count
≈ -5.0 volts.
Similarly, for the 5 volt range:
A reading of 0x4000 = 16,384 counts
Voltage = 16,384 counts * 152.588 µV/count
≈ 2.5 volts.
Data acquisition can be made in two modes: Linear or FIFO. In FIFO mode, data can only be
read from a fixed address (FIFO register), while in Linear mode, data can be read from any
address in the memory space of a channel. Linear mode also offers a two more options for
acquisition: Pre-Trigger and Delayed Trigger.
In FIFO mode, data can be retrieved while the acquisition is still in progress. However, if the
memory is not read and the acquisition continues running, new incoming data will overwrite
older data and the older data will be lost. It is also NOT possible to run a measurement command
in FIFO mode.
Acquiring Data
To acquire data, a channel must first be Armed. When a channel is armed, it starts its local
Sample Clock and waits for a Trigger Event to begin sampling. The channel must remain Armed
for the entire duration of the acquisition process. Clearing the ARM bit will reset the internal
state-machines and stop acquisition. Data capturing starts when a Trigger Event occurs. A trigger
event can be caused by an external trigger source, the signal under test or forced by setting a bit
in a register.
Triggering
An external signal, other than one of the sampled signals, can be used to trigger any or all of the
channels. This external signal is compared to a threshold level set by a local DAC and a high-
speed comparator is used to generate an External Trigger signal.
The signal under test can also be used to trigger an acquisition. The signal is compared to a
threshold level set by a local DAC and a high-speed comparator is used to generate a Channel
Trigger signal. Each channel has its own DAC and its own comparator, thus, each channel can
generate a Trigger signal independent of the other channels. Acquisition on any channel can be
triggered by any other Channel Trigger signal (Channel 0 can be triggered by Channel 0 Trigger,
Channel 1 Trigger, Channel 2 Trigger, or Channel 3 Trigger) even if the other channels are not
armed and are not acquiring any data. Only one channel can be the trigger source at any time and
the trigger sources cannot be AND’ed or OR’ed together.