Four
Channel Constant Current
Description
The 5290 is a four-channel signal conditioner for ICP or other
types of transducers requiring constant current excitation.
A programmable constant-current source provides power to
the integrated circuit in the input sensor, and the return
signal (which appears as a varying voltage on the same line)
is amplified
and processed into a ±10V output signal. Each channel
is configured as an AC-coupled amplifier with programmable
gain
capable of converting sensor signals as low as 50 mV (peak)
into a ±10V output voltage. Designed for use in the
instrumentation systems test environment, it includes many
features that simplify
the test setup and improve the accuracy of the collected data.
Design Features
The 5290 provides a gain range of 1 to 5000 with a full 100
kHz bandwidth. A programmable constant-current source provides
excitation power to sensors with internal integrated amplifiers,
and a programmable calibration voltage source is available for
establishing baseline values before and after a test is run.
Front-panel LEDs indicate signal presence and warn of overload
conditions and module operational problems. Stable low temperature-coefficient
components are used to maintain system accuracy over a wide
temperature range, and all circuitry is housed in a shielded
enclosure for improved reliability and noise reduction
Stable Amplifier Circuitry
The input signal is first AC-coupled to block the DC excitation
current, and then is amplified by a unity-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. 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 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 Digital
Signal Processor uses stored offset and gain calibration factors
to correct the digitized data values and generate a digitally
filtered output. Digital low-pass 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 low-pass 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
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.
System Calibration
High accuracy is obtained during the conversion process by
implementing a unique end-to-end calibration scheme within
the 5290 Converter.
A precision programmable voltage generator is connected to
the input, and two calibration voltages (0V and 80% of full-scale)
are fed in, 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
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.
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5290
Specifications
Schematic
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