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5790A
Operators Manual
4-38. Loading
The 5790A input impedance may affect ac voltage source output levels. Refer to Table 4-
4 for the input impedance on each range. Sources with resistive dividers on the output are
especially sensitive to loading errors. Cables present an additional capacitive load to the
source.
Check the specifications and instruction manual for each type of ac voltage source before
connecting the source to the 5790A. Avoid exceeding the drive capability of the ac
voltage source, no matter what the transfer application is. When in doubt, use an
oscilloscope or spectrum analyzer to make sure the ac source is not loaded to the point
that it is producing a distorted signal.
Before you calibrate an ac voltage source, there is another source of loading error to
consider. What will the ac voltage source be used for after calibration? Many meters have
highly capacitive inputs and present a much heavier load than the transfer standard. One
way to compensate for meter loading is to simulate a meter load during calibration with
the 5790A. To simulate a meter load, connect a load equivalent in capacitance and
resistance to the meter input across the 5790A input.
4-39. Mechanical Contact
Connection contact resistance variation is a potential error source on the ranges above
2.2V. By using high quality cables and connectors, you can minimize this source of error.
Use stainless steel coaxial connectors if possible. They are machined with more precise
threads, which make better electrical contact. The Type "N" connectors on the 5790A are
all stainless steel.
To achieve the highest quality measurements, do not disturb the instrumentation setup
during the course of a transfer. Try not to move, jostle, or vibrate the 5790A, the source,
or any of the input signal wiring from the time you apply the first input voltage until you
take the last reading of the transfer.
Most importantly, always repeat the measurements until you are satisfied
that the results are repeatable relative to the specification being measured.
4-40. Thermal EMFs
Avoid thermal emf errors in the cabling between a dc voltage source and the INPUT 2
binding posts. They contribute to the uncertainty of the dc voltage output as seen by the
5790A. To reduce thermal emf errors, use low thermal emf cables and connectors and
avoid touching any connection during a transfer. All it takes to change the emf error and
adversely affect a transfer is to briefly touch a connector or binding post. It typically
takes five minutes to thermally stabilize a connection after it has been touched.
4-41. EMI
The 5790A inputs, especially the WIDEBAND input, are broadband inputs. This means
you should avoid applying RF signals inadvertently picked up by cables.
To minimize cable pickup, use short coaxial leads when possible, especially at test
voltages below 2 V. Shielded cables and connectors can minimize the contribution of
EMI to transfer uncertainty. A common-mode choke at the input terminals is an effective
EMI suppressor when using low-level inputs. Do not use a common-mode choke for the
ranges above 2.2 V.
Soldering irons, fluorescent lights, anything with a motor, and all similar things can
radiate EMI. Turn off soldering irons, and keep the 5790A setup away from fluorescent
lights.
4-28
Note
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