10 TEGAM WAY • GENEVA, OHIO 44041 440-466-6100 • FAX 440-466-6110 • www.tegam.com
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Section III – Principles of Operation
greater)signal up to 80 dB greater than the signal being measured. Precise and highly-stable offset
current is also applied to the input amplifier via a 16-bit DAC whose output value is set by the front panel
INPUT OFFSET control.
The output of the multi-stage amplifier is applied to the input of a precision 24-bit analog-to-digital
converter. This high-precision, high-stability analog-to-digital converter samples the input signal (including
input noise) many times per second. Various filtering/averaging algorithms are applied to this 24-bit result
based on front panel FILTER settings. The processor then converts the 24-bit word into a bit stream that is
optically coupled to the Front Panel Subsystem. The 24-bit conversion gives a resolution in excess of one
part in 16 million to provide a high degree of overall linearity and resolution (ultimately, after processing,
18 bits—17-bits of amplitude resolution and one sign bit). The 24-bit conversion allows extensive digital
filtering and manipulation to assist in the measurement and noise reduction process.
Once the digitized (and optically isolated) measurement information is received by the Front Panel
Subsystem the output is converted from serial information and is sent to a 20-bit digital-to-analog
converters with updates every 200 mS. The digital-to-analog converter output is buffered and sent to the
4 ½ inch, zero-center, mirror-backed meter movement. This meter provides a high-resolution readout
with both 10 – 0 – 10 and 3 – 0 – 3 scales. A second 20-bit digital-to-analog converter output is sent to a
buffer amplifier. This output is normalized to ±1 V at full scale (corrected for any gain normalizations set
by the operator who can make gain adjustments which set the ISOLATED OUTPUT full scale level at ±0.5
V - ± 1.5 V for a full scale input) and is applied to the ISOLATED OUTPUT rear panel terminals. This output
allows the operator to utilize the AVM-2000 as a high-gain, high-linearity, (chopper-stabilized for ranges of
1mV and below) instrumentation amplifier with isolated output. A front panel knob allows operator
adjustment of the isolated output amplitude.
The Front Panel Subsystem control circuits consist of a microprocessor, LCD, 6 pushbuttons and 3 rotary
encoders. The microprocessor continuously scans the switches and rotary encoders to detect any changes.
It also regularly updates the LCD with the current AVM-2000 status as selected by the user with the
pushbuttons and rotary encoder.
The microprocessor regularly poles the Isolated Analog Subsystem via a high-speed optically isolated
serial communications bus. The Isolated Analog Subsystem responds, supplying the results of the current
analog to digital conversions of the applied input signal. In addition to polling the Isolated Analog
Subsystem, Front Panel Subsystem communications include information such as current RANGE, FILTER,
OFFSET, OPERATE/ZERO, ZIN, and ZERO selections. When operated in the setup mode (special menu on
the LCD selected by depressing the Range Control), the microprocessor communications are modified to
contain special requests such as ZERO, GAIN and setting the 2.667 volt reference.
