Auxiliary Analog Inputs And Outputs (Adcs And Dacs) - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION
3.3.17 Auxiliary Analog Inputs and Outputs (ADCs and
DACs)
3-14
The noise buffer length n can be set to 1, 1000, 2000, 3000 or 4000. Since new input
values to the buffer are supplied at a 1 kHz rate, these correspond to averaging times
of zero, 1 second, 2 seconds, 3 seconds and 4 seconds respectively, and so the control
on the Configuration Menu that adjusts this buffer length is labeled Noise Buffer
Length and can be set to Off, 1 s, 2 s, 3 s or 4 s. Setting a shorter time means that the
system responds more quickly to changes in the mean X-output level, but the noise
reading itself exhibits more fluctuation. Conversely, the fluctuation can be reduced
by setting a longer time, but at the expense of increased settling time following
changes in the mean X-output level.
If a noise output (N calibrated in volts or amps per root hertz, or as a percentage of
full scale) is selected as one of the outputs on the Main Display or for conversion to
an analog signal for output to the CH1/CH2 outputs, and the time constant is not
within the permitted range then a warning message is displayed on the screen.
Similarly, if a noise output value is read via the computer interfaces while the time
constant or slope are outside the permitted range, or if the synchronous time constant
control is enabled, then the response will be -1. Since noise readings can only be zero
or positive, this negative number clearly indicates that the reading is invalid and
should be ignored.
In order to make noise measurements easier, the instrument includes a Noise
Measurement Mode, activated by a control on the Configuration menu or by a
computer command. When this is turned on, the Main Display outputs are set to the
four types most commonly required, and the filter time constant, slope and
synchronous time constant setting are forced to values within the permitted ranges.
When turned off, these restrictions are removed.
When making noise measurements the user is strongly advised to use an oscilloscope
to monitor the signal at the SIG MON output on the rear panel as this is the best way
of ensuring that a random process is being measured rather than line pick-up or other
non-random signals.
Any two of the outputs, including X channel and Y channel signals, vector
magnitude, and phase angle, and even noise may be represented in analog form by
being routed via two further 16-bit DACs to the unit's CH 1 and CH 2 output
connectors.
The model 7280 incorporates four auxiliary ADC inputs of conventional sampled
design offering a resolution of 1 mV in ±10.000 V. These converters may be used at
slow sample rates for digitizing slowly changing or DC signals which are associated
with an experiment, such as those generated by temperature and pressure transducers,
so that they can be incorporated into ratio calculations or transferred to a controlling
computer. They may also be used in conjunction with the instrument's curve buffer to
form a transient recorder operating at sample rates of up to 40 kHz.
Two auxiliary DAC outputs are also provided which offer the same resolution as the
ADCs, namely 1 mV in ±10.000 V.