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Four Channel Multi-Mode Signal
Conditioner 5492
Description
The 5492 is a multi-mode signal conditioner which combines both
constant current and voltage excitation capability in a single
4 channel module. Combining the features of the 53xx series
of instrumentation amplifiers with programmable current sources
for use with IEPE type transducers allows multiple transducer
types to be handled by a single module. This module also includes
Programmable Linearized Thermocouple as found in the 5120
(see specification). The use of programmable excitation voltage
sources with the ability to adjust for DC input offsets gives
a complete solution for most sensor types not requiring bridge
completion. TEDS (Transducer Electronics Data Sheet) capability
further enhances system wide integration with the ability
to automatically read embedded transducer data sheets for system
level calibration and test setup.
Design Features
The DTX-5492 features 4 individual programmable voltage sources which
can be set from 0.01 volts up to 10.00 volts in 10 mV steps
alongside 4 individual programmable current sources, which
can be set from 1 mA to 20 mA in 0.1 amp steps while maintaining
a full 24V compliance.
In addition, TEDS capability is included for ease of setup
and error free calibrations. Programmable bandpass filtering,
automated or manual input
bias balance, AC/DC input coupling, on board NIST traceable
calibration and front panel status monitors are standard
features. As with all the DTX-5000
series
products, input noise is less than 10µV with full signal bandwidth of 100
kHz.
Programmable Excitation
Current
A programmable constant current
source provides excitation power for the sensor. A Digital-to-Analog
Converter creates a programmable control voltage that is
used to control the output of current regulator. The current
being drawn is sensed and
used as a feedback signal to keep the current constant. Each
circuit can provide up to 20 mA of excitation current with
a maximum output voltage of 24V.
Voltage Excitation with
AutoZero/AutoGain Capability
The 5492 provides four independent programmable excitation voltage sources
for use in powering input sensors. Each source is controlled by a
16 bit DAC with an output range from
0 to 10V and 100 mA output buffering. Injecting a programmable offset
voltage allows the DSP to automatically servo the amplifier output
to zero adjust any input offset voltage
imbalance. In a like manner, the DSP can adjust the amplifier gain
for a targeted analog output voltage value upon application of a
known calibration signal.
Stable Amplifier Circuitry
After AC/DC coupling, the signal is amplified by a fixed-gain precision
instrumentation amp. Using this approach ensures high common-mode
rejection to reduce noise pickup on the sensor wires, and avoids the use of
switched gain resistors in the most noise and temperature sensitive
portion of the circuit. Offset adjustment is done after preamplification to isolate the
offset voltage generator from the sensor. A programmable gain amplifier is then
used for additional signal magnification as required by subsequent processing
stages. Precision op-amps and resistors are used throughout all of the
analog circuitry to improve temperature stability.
DSP - Programmable Gain
The variable gain amplifier is controlled by an onboard Digital Signal
Processor (DSP) prior
to digitization and subsequent processing. A 16-bit high speed
Sigma-Delta converter is then used to convert the amplifier input to a
digitized signal for subsequent processing. The DSP uses stored offset and gain
calibration factors to correct the digitized data values
and generate a digitally filtered output. Digital lowpass
filtering is done by the DSP, providing a better response curve and more
flexibility than switched analog filters. The result is an amplified, error-corrected,
and digitally-filtered output that is ready to be converted back to an analog
output voltage.
Output Circuit
The processed digital output is converted back to
an analog voltage by a high-speed 16-bit Digital-to-Analog Converter.
A four-pole lowpass filter/buffer-amp removes the digitizing
steps in the reconstructed signal, along with any high-frequency
noise. As with the input circuit, temperature-stable components
are used to ensure that system calibration holds over a wide
temperature range.
Signal and Status Monitor LEDs
Front-panel LEDs are used to monitor both the signal level and the
operating status of each channel. The DSP compares each digital sample to
the level set by the user, and adjusts the intensity and color of the Signal
LED accordingly. The DSP also monitors the excitation current level
and overall digital operation, and sets the color and flash-rate of the Status
LED as needed to warn the user of a problem.
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