Four
Channel Constant Current (ICP)
Description
The 6290 is a four-channel signal conditioner for ICP or other
types of transducers requiring constant current excitation.
Multiple input stages are used to optimize performance across
an extended Gain-Bandwidth beyond the range of simplier
designs. A programmable constant-current source provides
power to 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 5mV (peak) into a ±10V output
voltage.
Design Features
The 6290 provides a programmable gain range of 1 to 2000 with
a full 400 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.
Input Stage
The input signal is first AC-coupled to block the DC excitation
current, and then is fed to one of two front end amplifier stages.
For gains of 1 to 200, a unity gain input buffer is used. For
gains from 10 to 2000, a x10 low noise pre-amp may be switched
in prior to a variable-gain amplifier used to boost the signal
to a level suitable for A/D conversion. 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.
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 then uses stored offset and gain calibration
factors to correct the digitized data values and generate an
error corrected digitally filtered output. 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 6290 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 of ±0.05% of full-scale.
|
6290
Specifications
Schematic |
