Channel Automated Strain Conditioner
The 6078 is a four channel extended range programmable gain
amplifier that provides signal conditioning for bridge type
transducers. Multiple input stages are used to extend both the
range of available gains and overall bandwidth beyond the limits
of simpler designs. In addition to analog signal amplification,
each independent channel includes bridge completion, input
offset bias compensation, voltage excitation with remote sense capability,
local or remote shunt calibration, digital signal processing
with programmable filtering, and output buffering.
The 6078 utilizes onboard DSPs (one per channel) to configure
the input circuitry and handle amplifier gain and offset compensation.
High performance front end analog components are combined with
digital signal processing techniques and ultra stable calibration
reference sources to maintain system accuracy, bandwidth and
gain tolerances. Front-panel LEDs indicate signal presence and
activity level, and warn of module operational problems. All
circuitry is housed in a shielded enclosure for improved reliability
and noise reduction.
On-board bridge-completion resistors are provided for full,
½, or ¼-bridge completion to accommodate a variety
of sensor configurations. Bridge configuration is completely
programmable, including the selection of 120 or 350 Ohm completion
resistors. By setting the configuration to the Full Bridge
the 6078 can also be used as a standard differential amplifier.
When CAL is activated, a pre-selected shunt cal resistor
is electrically connected to the sensor to simulate a known
amount of strain. Selecting Local shunt cal makes this connection
inside the amplifier for simpler sensor wiring. Selecting Remote
shunt cal makes this connection at the sensor (using separate
shunt cal lines). Using Remote shunt cal also allows a user-supplied
external shunt cal resistor to be used if desired.
DSP - Programmable Gain
The differential bridge output is fed to an instrumentation
grade x1 or x10 pre-amp avoiding the use of switched resistors
in the most noise and temperature-sensitive portion of the circuit.
A programmable offset voltage is injected following this preamp
for input offset compensation and automated bridge balancing.
After offset correction, the signal is fed to a variable gain
amplifier controlled by an onboard DSP prior to digitization
and subsequent processing. The Digital Signal calibration factors
to correct the digitized data values and generate a digitally
filtered output that is ready for conversion back into an error-corrected
analog output voltage.
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. Temperature-stable components and ultra stable references
are used to ensure that system calibration holds over a wide
DSP – AutoBalance/AutoGain
Using a programmable offset voltage allows the DSP
to automatically servo the amplifier output to zero adjusting
for bridge imbalance. In a like manner, the DSP can adjust the
amplifier gain to a targeted value upon application of a shunt
resistance or other calibration source.
Voltage Excitation with Remote Sense Capability
The 6078 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. Remote
sense can also be selected to provide feedback accounting for IR losses
conductor lines between amplifier and sensor.
High accuracy is obtained during the conversion process
by implementing a unique end-to-end calibration scheme within
the 6078 conditioner. A precision programmable voltage
source is connected to the input, and two calibration voltages (0V
and 80% of full-scale input) are fed in, amplified
by the input stages, converted by the A/D, processed by the DSP, converted
back to analog and then measured by a high-accuracy
converter. The input and output voltages are compared with
the resulting gain and offset correction values
saved in the DSP memory for real time data correction.