10 TEGAM WAY • GENEVA, OHIO 44041 440-466-6100 • FAX 440-466-6110 • www.tegam.com
3-1
Section III – Principles of Operation
Understanding the AVM-2000 Measurement Process
Basic operation of the AVM-2000 and an understanding of how it achieves its specified levels of
performance follow. The AVM-2000 architecture consists of three main sections: an Isolated Analog
Subsystem; a Front Panel Subsystem; and a Power Supply Subsystem. The Isolated Analog Subsystem is
optically (and therefore electrically) isolated from the balance of the instrument to ensure minimum noise
introduction into the measurement signals from control/digital signals and to ensure that, when operating
as a floating null meter, the impedance to chassis/earth ground from either of the two input terminals is
extremely high.
The heart of the AVM-2000 is in the Isolated Analog Subsystem. This is where nano-volt level signals are
amplified via low-drift, low-noise, amplifiers to a level where they can drive a wide dynamic range 24-bit
analog-to-digital converter and where initial analog filtering (10 Hz 4-pole Butterworth, low-pass) is
applied. Once the signal is in digital format, it is further filtered, scaled and transferred across the isolating
boundary for meter display and rear panel output as a high-speed serial signal. An ISOLATED OUTPUT is
available so the AVM-2000 may be operated as a high-gain instrumentation amplifier with isolated output.
When used as an instrumentation amplifier, the amplifier’s gain is the inverse of the AVM-2000’s range
control setting. For example, at the 1
μ
V range and with the ISOLATED OUTPUT set to 1 V for full scale
response, the amplifier gain is 1 V / 1
μ
V = 1,000,000.
Input signals are applied to the AVM-2000’s low-thermal EMF binding posts. From here the signals are
routed directly to the programmable input attenuator. Depending on the selected measurement range, the
input signal is applied directly to the input amplifier (100 nV – 1 mV ranges) via a matched set of low-
thermal EMF polarity reversal relays, or is attenuated to be compliant with the input amplifier’s dynamic
range and then applied to the input amplifier (3mV – 1000V). On all ranges, all stages of the input
amplifier allow sufficient head room so peak noise does not cause limiting and very high loop gains ensure
low-distortion. This allows the AVM-2000 to make full use of the digital filtering technology to eliminate
unwanted noise from the measured signals.
Before signals are applied to the input amplifier, they pass through input protection circuitry. This protects
the input amplifier from the direct application of excess voltage (up to 1,100 VDC/peak) on any input
range. Additionally, the AVM-2000 input attenuator can be configured (in the 100-nV to 1-mV full scale
ranges) as a 1 M
Ω
, 10 M
Ω
, 100 M
Ω
or 1 G
Ω
input termination resistance (user selectable). Input
impedances for ranges between 3 mV full scale and 300 V full scale are 10 M
Ω
or 100 M
Ω
. The 1 kV range
input impedance is fixed at 100 M
Ω
.
Signals that are to be applied directly to the amplifier pass through a matched set of polarity reversing
relays. When measuring signals on ranges of 1 mV or less, one half of all measurements are made with
the relays in the NORMAL configuration and one half of all measurements are made with the relays in the
ALTERNATE (polarity reversal) configuration. When the respective signals are digitized, they are
subtracted from each other thus minimizing the impact of noise, thermal drifts and other undesired signals
that tend to be of a single polarity. Further, averaging of these responses also minimizes the impact of
drift.
Amplification of the input signal occurs in a low-noise, low-drift, multi-stage programmable gain amplifier.
The input amplifier housing is shielded to ensure minimal impact from external electro-magnetic signals
and short-term temperature changes in the operating environment. The amplifier components are chosen
to ensure Johnson Noise and other similar noise contributors fall below an equivalent noise resistance of
25
Ω
.
Early stages of the input amplifier are provided with basic filtering that permits the amplifier to maintain
its DC performance characteristics with the simultaneous application of a line-frequency (50 Hz or
