General; Accuracy - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION
This technique, although apparently complex, is the only way of removing the effect
of crosstalk which is not generally in the same phase as the required signal.
Auto-Offset
In an Auto-Offset operation the X offset and Y offset functions are turned on and are
automatically set to the values required to give zero values at both the X and the Y
outputs. Any small residual values can normally be removed by calling Auto-Offset
for a second time after a suitable delay to allow the outputs to settle.
The primary use of the Auto-Offset is to cancel out zero errors which are usually
caused by unwanted coupling or crosstalk between the signal channel and the
reference channel, either in the external connections or possibly under some
conditions in the instrument itself. Note that if a zero error is present, the Auto-Offset
function should be executed before any execution of Auto-Phase.
Auto-Measure
This function only operates when the reference frequency is greater than 1 Hz. It
performs the following operations:
The AC GAIN value is adjusted to maximize the input to the main ADC without
causing overload, and the time-constant is set to an appropriate value for the present
reference frequency. An auto-sensitivity operation is then performed, followed by an
auto-phase.
The Auto-Measure function is intended to give a quick setting of the instrument
which will be approximately correct in typical simple measurement situations. For
optimum results in any given situation, it may be convenient to start with Auto-
Measure and to make subsequent modifications to individual controls.
NOTE: The Auto-Measure function affects the setting of the AC Gain and AC
Gain Automatic controls during execution. Consequently, it may not operate
correctly if the AC Gain Automatic control is turned off. In this case, better results
will be obtained by performing Auto-Sensitivity followed by Auto-Phase functions.
Auto-Default
With an instrument of the design of the model 7280, where there are many controls
of which only a few are regularly adjusted, it is very easy to overlook the setting of
one of them. Consequently an Auto-Default function is provided, which sets all the
controls to a defined state. This is most often used as a rescue operation to bring the
instrument into a known condition when it is giving unexpected results. A listing of
the settings which are invoked by the use of this function can be found in appendix E.
This completes the description of the main functional blocks of the instrument.

3.4 General

3.4.01 Accuracy

When the demodulator is operating under correct conditions, the absolute gain
accuracy of the instrument is limited by the analog components in the signal channel,
and the absolute phase accuracy is limited by the analog components in both the
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