Keithley 2790, Reference Manual

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Model 2790 SourceMeter
®
Switch System Reference Manual 7-37
Monitor scan
For this example, channel 101 of the Model 7702 is used to monitor temperature. When
the temperature reading reaches 30°C, it will start the scan. For this 4-channel scan, chan-
nel 101 measures temperature, while channels 102, 103, and 104 measure DCV.
This example uses the channel average feature to measure temperature. With channel aver-
age enabled, two temperature measurements will be taken; one at channel 101 and another
at its paired channel (121). The two measured readings are then averaged to yield a single
reading. It is this averaged temperature reading that will start the scan when it reaches
30°C. Refer to
Section 5 for details on channel average.
Two 5000
Ω
thermistors are used to measure temperature. The thermistors are connected
directly to the screw terminals of the switching module.
Operation
A simplified model of monitor scan operation is shown in Figure 7-6 , while the procedure
steps and programming commands are listed in Table 7-3 .
In Figure 7-6 , notice that there are two modes of operation. While in the monitor mode,
the Model 2790 continuously performs temperature measurements. Keep in mind that
channel average is being used. Therefore each temperature reading is the average of two
temperature measurements (one on channel 101 and one on channel 121). As long as
the average temperature reading remains below 30°C, the instrument will remain in the
monitor mode.
When the temperature reading reaches 30°C, the Model 2790 switches over to the scan
mode. Figuratively speaking, it is as if a “finger” presses the SCAN key when the monitor
detects that the average temperature is at or above 30°C.
The instrument is configured to scan four channels. The monitor TEMP channel reading and
three DCV channel readings are stored in the buffer. After the fourth channel is measured,
operation returns to the monitor mode to again measure temperature. Note that if the average
temperature is still at or above 30°C, the “finger” will again press SCAN to start the scan.