Programmable
Signal Conditioning DTX
5000 Series
Description
The DTX-5000 Signal Conditioner Series provides
up to 64 channels of state-of-the-art signal processing
in
a compact (5U) chassis. Using hybrid mixed-signal technology,
high-performance analog circuitry is combined with Digital
Signal Processing to give gain accuracy performance previously
available only in larger customized modules. A simple-to-use
graphical user interface running under the Windows 2000 operating
system allows for ease of setup and either local or remote
operation as a network-based instrument node. The suite of
conditioner modules include a strain amplifier
with
internal bridge completion, a general-purpose instrumentation
amplifier with selectable AC or DC coupling, a linearized
thermocouple amplifier with cold-junction compensation, a
frequency-to-voltage converter with dual inputs for direction
sensing, and an ICP accelerometer amplifier with constant-current
excitation.
Design
Features
Modular design using separate fully shielded four-channel modules
with isolated power supplies reduces noise and allows for interchange
of module types within a single chassis. During system design,
special attention was given to reducing noise and improving
reliability. All signal input and output connections are made
directly from the external connector to the module backplane,
and from there directly to the module. This eliminates long
input cables and additional interface connectors that can pick
up noise or become intermittent. The 6-layer backplane board
includes large areas of ground plane to reduce noise pickup.
Isolated power supplies are included on each module to separate
the signal grounds from the digital ground. And all the module
electronics are housed in rugged, shielded enclosures. Good
temperature stability is obtained by doing the majority of signal
processing in the digital realm, where temperature drift has
no effect. In the analog circuitry that does exist, premium
stability components are used to keep gain or offset drift to
a minimum.
COTS Technology
Using commercial-off-the-shelf (COTS) computer hardware (Pentium
processor) and a standard operating system ensures compatibility
with future technology trends and facilitates product maintenance.
A standard 10/100 base-T Ethernet interface is provided for
network-based operation, and PC-compatible keyboard, monitor
and mouse ports are available for standalone operation.
Mixed Signal Technology
The actual circuitry inside each module type varies, but they
all share a common design approach. The input signal is first
amplified to a usable level, and is then converted to a digital
signal using a high-speed 16-bit Analog-to-Digital converter.
This allows all gain correction and filtering to be done digitally,
thereby improving system accuracy and stability. A Digital Signal
Processor (one for each channel) uses stored offset and gain
calibration factors to correct the digitized data values, and
performs the requested low-pass filtering. The DSP also compares
the signal level to user-specified limits, and sets the color
and brightness of the front-panel Signal LED accordingly. After
signal processing has been completed, the digital value is converted
back to an analog voltage by a high-speed 16-bit Digital-to-Analog
Converter. A four-pole low-pass filter removes the digitizing
steps in the reconstructed signal, along with any high-frequency
noise, and an output buffer provides up to 50 mA of signal current
to the output load.
Sensor Power/Excitation
Each module also includes separate programmable Excitation
Voltage generator for each channel (Excitation Current in the
case of
the ICP accelerometer conditioner). A power buffer is used
to provide up to 100 mA of load current, and a sense circuit
monitors
and corrects the excitation voltage (or current) value in real
time to compensate for fluctuating load conditions. Good temperature
stability is obtained by doing the majority of signal processing
in the digital realm, where temperature drift has no effect.
In the analog circuitry that does exist, premium stability
components
are used to keep gain or offset drift to a minimum.
Module Calibration
High accuracy is obtained in the 5000 Signal Conditioners by
implementing a unique end-to-end calibration scheme. A precision
programmable voltage generator is connected to the input of
each channel, and two calibration voltages (0V and approximately
80% of full-scale input) are applied during system calibration.
These voltages are amplified by the input stage, converted
by
the A/D, processed by the DSP, converted back to analog by
the D/A, filtered by the output filter, and then measured
by a high-accuracy
24-bit A/D converter. The input and output voltages are compared,
and system gain and offset correction values are computed
and
saved in the DSP memory. When data is being collected, these
correction factors are applied to each data point in real
time,
resulting in a system accuracy better than ±0.05% of
full-scale.
Front Panel Display
Careful attention was given to providing a meaningful front
panel display of signal level and system status that wasn’t
confusing to the operator. With 64 channels being displayed
simultaneously, it can be easy to miss a channel error in a
field of flickering lights. The 5000 Signal Conditioner uses
just two LEDs per channel to display a variety of status conditions
and signal levels. Various normal functions are indicated by
varying the flashing rate or intensity of green LEDs. But system
errors or signal overloads are immediately flagged by changing
the appropriate LED color to red, causing that channel to instantly
stand out from the rest.
Graphical User Interface
The control software display includes
a summary view of the status of all 64 channels, essentially
a copy of the chassis front panel, a spreadsheet view of the
parameter settings for each module, and a detailed block-diagram
view of the settings for each channel. Two dedicated LEDs per
channel give instant feedback of channel status and signal
level,
warning the user of overload or other potential error conditions.
|
