Appendix B Electronic Calibration Of The 3458A (Product Note 3458A-3) - Agilent Technologies 3458A Calibration Manual

Appendix B Electronic Calibration of the
3458A (Product Note 3458A-3)
A voltmeter has four basic functional blocks.
The input signal must first pass through some type of signal
conditioner. For a DC input voltage, the signal conditioner may
consist of an attenuator for the higher voltage ranges and a DC
amplifier for the lower ranges. If the input signal is an AC voltage, an
RMS converter changes the AC signal to an equivalent DC value. By
supplying a DC current, an ohms converter changes resistance to a
DC voltage. In nearly all cases, the input signal conditioner converts
the unknown quantity to a DC voltage that is within the range of the
A-to-D converter.
The job of the A-to-D converter is to take a pre-scaled DC voltage and
convert it to digits. A-to-D converters are single range DC voltage
devices. Some take a 1 V full-scale input while others take a 10 V
full-scale input. For this reason, the signal conditioner must attenuate
higher voltages and amplify lower voltages to give the voltmeter a
selection of ranges.
Let's take an example. Suppose we apply 250 V AC to a voltmeter
with an A-to-D converter that requires a 1 V DC input. The AC signal
is attenuated on the 1000 V AC range and converted to a DC voltage
equal to 0.25 V. The final reading appears as
"250.0 V AC." (In general, AC in the 3458A Multimeter uses
2 V full-scale.)
These first two building blocks govern the voltmeter's basic
characteristics such as its number of digits, its ranges, and its
sensitivity. The A-to-D converter governs a voltmeter's speed,
resolution, and, in some cases, its ability to reject noise.
The logic block manages the flow of information and the correct
sequence of various internal functions. The logic also acts as a
communicator with the outside world. Specifically, the logic manages
the outward flow of digital information and accepts programming
instructions from other devices. The display communicates visually
the result of a measurement. In selecting a voltmeter to fill a specific
application, these building blocks combine to give the instrument its
specific performance characteristics.

Appendix B Electronic Calibration of the 3458A (Product Note 3458A-3)

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