AMP04
Programming the Gain
The gain of the AMP04 is programmed by the user by selecting
a single external resistor—RGAIN
signal routing practice to minimize stray coupling and ground
loops is recommended. Leakage currents can be minimized by
using high quality socket and circuit board materials, and by
carefully cleaning and coating complete board assemblies.
:
Gain = 100 kΩ/RGAIN
As mentioned above, the high speed transition noise found in
logic circuitry is the sworn enemy of the analog circuit designer.
Great care must be taken to maintain separation between them
to minimize coupling. A major path for these error voltages will
be found in the power supply lines. Low impedance, load re-
lated variations and noise levels that are completely acceptable
in the high thresholds of the digital domain make the digital
supply unusable in nearly all high performance analog applica-
tions. The user is encouraged to maintain separate power and
ground between the analog and digital systems wherever pos-
sible, joining only at the supply itself if necessary, and to ob-
serve careful grounding layout and bypass capacitor scheduling
in sensitive areas.
The output voltage is then defined as the differential input volt-
age times the gain.
VOUT = (VIN+ – VIN–) × Gain
In single supply systems, offsetting the ground is often desired
for several reasons. Ground may be offset from zero to provide
a quieter signal reference point, or to offset “zero” to allow a
unipolar signal range to represent both positive and negative
values.
In noisy environments such as those having digital switching,
switching power supplies or externally generated noise, ground
may not be the ideal place to reference a signal in a high accu-
racy system.
Input Shield Drivers
Often, real world signals such as temperature or pressure may
generate voltages that are represented by changes in polarity. In
a single supply system the signal input cannot be allowed to go
below ground, and therefore the signal must be offset to accom-
modate this change in polarity. On the AMP04, a reference in-
put pin is provided to allow offsetting of the input range.
High impedance sources and long cable runs from remote trans-
ducers in noisy industrial environments commonly experience
significant amounts of noise coupled to the inputs. Both stray
capacitance errors and noise coupling from external sources can
be minimized by running the input signal through shielded
cable. The cable shield is often grounded at the analog input
common, however improved dynamic noise rejection and a re-
duction in effective cable capacitance is achieved by driving the
shield with a buffer amplifier at a potential equal to the voltage
seen at the input. Driven shields are easily realized with the
AMP04. Examination of the simplified schematic shows that the
potentials at the gain set resistor pins of the AMP04 follow the
inputs precisely. As shown in Figure 5, shield drivers are easily
realized by buffering the potential at these pins by a dual, single
supply op amp such as the OP213. Alternatively, applications
with single-ended sources or that use twisted-pair cable could
drive a single shield. To minimize error contributions due to
this additional circuitry, all components and wiring should re-
main in proximity to the AMP04 and careful grounding and by-
passing techniques should be observed.
The gain equation is more accurately represented by including
this reference input.
V
OUT = (VIN+ – VIN–) × Gain + VREF
Grounding
The most common problems encountered in high performance
analog instrumentation and data acquisition system designs are
found in the management of offset errors and ground noise.
Primarily, the designer must consider temperature differentials
and thermocouple effects due to dissimilar metals, IR voltage
drops, and the effects of stray capacitance. The problem is
greatly compounded when high speed digital circuitry, such as
that accompanying data conversion components, is brought
into the proximity of the analog section. Considerable noise and
error contributions such as fast-moving logic signals that easily
propagate into sensitive analog lines, and the unavoidable noise
common to digital supply lines must all be dealt with if the accu-
racy of the carefully designed analog section is to be preserved.
1/2 OP-213
Besides the temperature drift errors encountered in the ampli-
fier, thermal errors due to the supporting discrete components
should be evaluated. The use of high quality, low-TC compo-
nents where appropriate is encouraged. What is more important,
large thermal gradients can create not only unexpected changes
in component values, but also generate significant thermoelec-
tric voltages due to the interface between dissimilar metals such
as lead solder, copper wire, gold socket contacts, Kovar lead
frames, etc. Thermocouple voltages developed at these junc-
tions commonly exceed the TCVOS contribution of the
1
V
2
3
8
OUT
6
1/2 OP-213
Figure 5. Cable Shield Drivers
AMP04. Component layout that takes into account the power
dissipation at critical locations in the circuit and minimizes gra-
dient effects and differential common-mode voltages by taking
advantage of input symmetry will minimize many of these errors.
High accuracy circuitry can experience considerable error con-
tributions due to the coupling of stray voltages into sensitive
areas, including high impedance amplifier inputs which benefit
from such techniques as ground planes, guard rings, and
shields. Careful circuit layout, including good grounding and
REV. A
–7–
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