4
Channel Extended Range Instrumentation Amplifier
Description
The 6322 four channel programmable gain instrumentation amplifiers
integrates three distinct input stages (x0.04, x1, x100) to
extend both the range of available gains and overall bandwidth
beyond the limits of simplier designs. Included are user-selectable
AC or DC input coupling, sensor voltage excitation, input offset
compensation and programmable low pass output filtering. An
input attenuator is used for high common mode voltage applications
while dedicated x1 and x50 instrumentation amps are used for
higher gain-bandwidth applications. Including a programmable
excitation voltage source with the ability to adjust for DC
input offsets gives a complete solution for most resistive type
sensors.
Design Features
The 6322 utilizes onboard DSPs (one per channel) to configure
the input circuitry and handle amplifier gain and offset compensation.
A mechanical latching relay is used to select AC or DC coupling
to ensure that the input signal is not affected by the switching
circuitry. High performance front-end analog components are
combined with digital signal processing techniques and an ultra-stable
calibration reference source to maintain system accuracy. 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
Stable Amplifier Circuitry
After AC/DC coupling, the signal is amplified by separate fixed-gain
precision instrumentation amplifiers. Offset adjustment is
done
after pre-amplification to isolate the offset voltage generator
from the sensor. For input voltages above 10V, the signal
is first sent through a divide-by-25 input attenuator, which
allows input signal levels as high as 250V peak to be processed.
The configured differential input signals are then fed to either
a x1 or x100 instrumentation grade pre-amp avoiding the use
of switched resistors in the most noise and temperature sensitive
portion of the circuit. Using a fixed-gain nstrumentation amp
as the first stage provides maximum rejection of common-mode
noise on the input lines while still allowing an acceptable
range of input offset compensation to be applied after pre-amplification
DSP - Programmable Gain
After offset correction, the signal is fed to a variable gain
amplifier controlled by an onboard DSP. A 16-bit high-speed
Sigma-Delta converter is then used to convert the amplified
signal to a digital value. Digitizing the signal for signal
processing allows all gain correction and output filtering to
be done digitally, thereby improving system accuracy. The Digital
Signal Processor uses stored offset and gain 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
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.
DSP – AutoZero/AutoGain
Using 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 to a targeted value upon application of a known calibration
signal.
Voltage Excitation with Remote Sense Capability
The 6322 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 over conductor lines
between amplifier and sensor.
System Calibration
High accuracy is obtained during the conversion process
by implementing a unique end-to-end calibration scheme within
the 6322 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
24-bit A/D
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. |
6322
Specifications
Schematic |
