Signametrics
40
eadStr()
or
DMMReadNorm()
DMMSetDCVSource()
. Leakage current is read using
DMMRead(), DMMR
functions.
Figure 4-10. Leakage test configuration; reverse diode leakage at 5V.
4.5 Measurement Timing
4.5.1 Aperture
The SM2060 and SM2064 DMM’s have several parameters governing measurement timing, including Aperture,
Read Interval and Overhead time. To maintain low noise and high accuracy, the DMM shuts down all
communications and other operations while converting. All other operations such as data transfers and command
processing are performed while the A/D is not active. The A/D is an integrating type and has a time during which it
integrates (a sort of averaging) the input. This time is the A/D Aperture. It is significant, particularly when it relates
to noise rejections. For instance, in the presence of 60Hz power line environment, there is significant 60Hz and its
harmonics which can contaminate a measurement. Setting the Aperture time to an integer multiple of this frequency
dramatically reduces this interference. Apertures of 16.667ms, 33.33ms, 66.667ms, etc. provide this rejection.
Aperture values are made up of a set consisting of 31 discrete values. It is set using the DMMSetAperture()
command, the SM2064 can set it between 2.5us and 5.066s, and the SM2060 can set it between 625us and 5.066s.
While using the various Trigger modes, the Aperture time must be set to 160ms or a lower value.
4.5.2 Read Interval
The Read Interval parameter is the length of time the DMM makes a measurement, including the transfer of the
measurement results. Both the Aperture and Read Interval can be set within their specified limits. Setting them
allows control over measurement timing. Figure 4-11 depicts the various timing elements associated with each
DMM reading cycle. The actual measurement rate is the reciprocal of the actual Read Interval (RI). The time
intervals indicated “Command Reception and Processing” and the “Process & Transmit Data”, are overhead times.
This means that with the Read Interval set to 0, the DMM sets the Delay to 0, resulting in a minimal Read Interval
consisting of the sum of the Aperture and the two overhead times indicated below. Set the Read Interval value using
the DMMSetReadInterval() functions. Keep in mind that setting it to a value lower than the Minimum Read Interval
indicated in the tables below will result in it being the table value.
