Operator`s manual

Figure 4-9. Leakage test configuration; reverse diode leakage at 5V.
4.5 Anatomy of measurement timing
4.5.1 Aperture
The SMU2060 and SMU2064 DMM’s have several parameters governing measurement timing, including
Aperture (section 2.12), 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. The time during which it integrates (averages) the input is the 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.
There are two DMM functions that set the Aperture. The DMMSetAperture() and DMMSetPLC(). The
DMMSetAperture() sets the SMU2064 Aperture to one of 31 possible values between 2.5us and 5.066s,
and the SMU2060 can be to 26 values between 625us and 5.066s. While using the various Trigger modes,
the Aperture time must be set to 160ms or a lower value. The DMMSetPLC() sets the Aperture to a value
that is the multiple of power line cycles. It specifies the power line to be used, 50Hz, 60Hz or 400Hz, and
the number of cycles to integrate (1 to 50).
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 4-10 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.
43 Signametrics