43
Signametrics
ggered to take
from the computer, whereby the Scanner selects channels from its Scan List table, and the DMM is tri
measurements following each channel selection.
Figure 4-19. Trigger interfacing to an SMX2040 class of DMMs.
ith
e S X20
oth, the A-
C-Bus for 4-Wire measurements (assuming an SM4040 or SM4042 scanner). It is important to consider
stem settling time when making measurements. Time delays exist in any measurement system. These delays are
ontributed by various sources. These include the scanner’s relay actuation times, the DMM input settling and
nce, when making high value
apacitance charge
the source current of the SMX2044 DMM using the 33M
Ω
umber of
rate).
4.15.3 Interface Commands and Timing
The sequence requiring the SMX2040 DMM to make triggered measurements generated by the SMX4032 starts
with the preparation of the SMX4032. Set the SMX4032 desired configuration, with Trigger Output enabled and
positive polarity. Each channel selection will generate a positive pulse with duration equal to the actuation time.
This could be generated by one of the scanning. The SMX2040 must be set up for triggered readings by using the
DMMSetTrigRead()
command. In the following VisualBasic® example, the SMX2040 sends readings during the
scan. Since it’s on board FIFO is limited to 5 readings, and the DMM must continue to send all readings during the
scan, it is important to have a tight loop that reads the measurements fast enough so that no overrun error occurs.
Refer to Figure 4-13 for proper trigger connection.
SCANTriggerOutState(nScan, Enabled, PosEdge)
// Set trigger output to Positive edge.
nReadings = 100
// Total number of measurements to take
DMMSetTrigRead(nDmm, 4, nReadings, NegEdge)
// Total of 100 readings and 4 settling readings
SCANAutoScan(nScan, nSteps)
//
Start
auto
scan
For I = 0 to nReadings -1
// read values as they come
while(DMMReadMeasurement(nDmm, reading) = NO
// wait for each reading and store it
Next
SCANOpenAllChannels(nScan)
// Good idea to open all channels when done
4.15.2 Multiplexing w
th
M
40 DMMs
For two wire measurements, the SMX2040 DMM must be connected to the A-Bus or the scanner, or to b
Bus and
sy
c
wiring capacitance. The latter will varies with the type of measurement. For insta
Ohms measurements the DMM current source level could contribute significant delay due to the c
time. For example, with 1,000pf cable capacitance,
range, is 0.1
µ
A which translates to 33ms (dt = C*dV/I). It is also recommended to set the appropriate n
settling measurements for the DMM (a minimum of 4 is recommended regardless of measurement
Figure 4-20. Triggered reading process and timing of SM4042 Scanner and an SMX2044 DMMs.
Unlike
the previous example,
DMMSetBuffTrigRead()
is not time critical since the DMM saves all
measurements to it’s on-board buffer, which is read after the scan is complete. However, this function
is limited to a maximum of 64 readings per scan.
SCANTriggerOutState(nScan, Enabled, PosEdge)
‘ Set trigger output to Positive edge.
nReadings = 50
‘ Total number of measurements to take
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