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3.3.23 Auxiliary Analog Inputs (ADCs)
The model 7270 incorporates four auxiliary ADC inputs 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 fast curve buffer to form a
transient recorder operating at sample rates of up to 200 kHz (one channel) or
40 kHz (two channels).
3.3.24 Main Microprocessor - General
All functions of the instrument are under the control of a microprocessor, which in
addition drives the front-panel display, processes front-panel key operations and
supports the computer interfaces. This processor also supports the instrument's 8-bit
digital (TTL) programmable input/output port, which may be used for controlling
auxiliary apparatus or reading the status of external logic lines.
A particularly useful feature of the design is that only part of the controlling
firmware program code, which the microprocessor runs, is permanently resident in
the instrument. The remainder is held in a flash EEPROM and can be updated via the
USB or RS232 computer interface, using an Update Pack that can be downloaded
from the www.signalrecovery.com website.
3.3.25 Main Microprocessor - Spectral Display
In some cases it can be useful to determine the spectral power distribution of the
input signal. The model 7270 can do this, since when the Spectral Display menu is
selected, the output processor performs a discrete Fourier transform on the digitized
input signal and displays the resulting spectrum.
3.3.26 Main Microprocessor - Auto Functions
The microprocessor also controls the instrument's auto functions. These allow easier,
faster operation in most applications, although direct manual operation or special
purpose control programs may give better results in certain circumstances. During
operation of several of the auto functions, decisions are made on the basis of output
readings taken at a particular moment. Where this is the case, it is important for the
output time constant set by the user to be long enough to reduce the output noise to a
sufficiently low level so that valid decisions can be made and that sufficient time is
allowed for the output to settle.
The following sections contain brief descriptions of the auto functions.
Auto-Sensitivity
This function only operates when the reference frequency is above 1 Hz. A single
Auto-Sensitivity operation consists of decreasing the full-scale sensitivity range if
the magnitude output is greater than 90% of full-scale, or increasing the full-scale
sensitivity range if the magnitude output is less than 30% of full-scale. After the
Auto-Sensitivity function is called, Auto-Sensitivity operations continue to be made
until the required criterion is met.
In the presence of noise, or a time-varying input signal, it may be a long time before
the Auto-Sensitivity sequence comes to an end, and the resulting setting may not be
what is really required.
Chapter 3, TECHNICAL DESCRIPTION
3-15
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