Explanation Of Specifications - VTI Instruments Metek EX1202 User Manual

VTI Instruments Corp.
E S
XPLANATION OF PECIFICATIONS
This section provides explanatory notes for EX1200 DMM specifications that may be
misunderstood.
Memory
This is the internal FIFO/buffer memory of the EX1200 mainframe. This does not include any
buffer memory that the individual plug-in cards may have. Some plug-in cards, such as the
EX1200-7500, EX1200-3608, and EX1200-7416, have onboard buffers that allow them to
perform high-speed operations.
Saved States
The DMM can store its configurations inside a non-volatile memory. Recalling these "saved
states" directly from the memory saves programming effort and will speeds up the scanning
process.
Measurement Integration Time
Unlike data acquisition instruments that measure instantaneous values, the EX1200 DMM uses an
integrating ADC for measurements which allows it to take multiple samples and average them by
integrating them over a fixed period of time. This time is referred to as the aperture time and is
usually measured in power line cycles (PLC) counts. One PLC is equal to the time period of the
mains power voltage waveform (i.e., for a 60 Hz mains power, 1 PLC = 16.67 ms, whereas for a
50 Hz mains power, it is 20 ms). If the integration time is set to a multiple of PLCs, these errors
(and their harmonics) will average out to approximately zero. This is, however, only applicable for
dc measurements.
True RMS
The internal DMM of EX126x mainframes measure the root mean square (RMS) value of the
applied waveform unlike the "average measurement" instruments. As a result, the values returned
by the EX1200 DMM are closer to the readings of an analog instrument. A complex algorithm
calculates the true RMS value of the applied waveforms (refer to the Crest Factor section for
errors related to measuring non-sinusoidal waveforms). The inputs are always ac coupled, thus
removing all dc components.
AC Signal Filters
The EX1200 DMM has three different ac filters to allow for a balance between low frequency
accuracy and faster settling times for ac voltage and current measurements. Selection of the
appropriate filter improves the accuracy of reading as well as the scanning speed. Remember that
the minimum frequency input of the Low filter is 3 Hz. This means that the dc component of non-
symmetrical waveform inputs (such as pulse trains) are rejected before true RMS measurements
are made.
Power Supply
Each EX1200 mainframe has an inherent limitation on the maximum power it can supply to the
plug-in cards/modules. Please refer to the power consumption specifications of the plug-in cards
before configuring the cards. This data can be found in the EX1200 Switch Cards User's Manual.
Exceeding these power limits may damage the power supply of mainframe.
28 EX1200 Series Introduction