Datasheet

Data Sheet ADA4522-1/ADA4522-2/ADA4522-4
Rev. F | Page 27 of 33
APPLICATIONS INFORMATION
SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER
The extremely low offset voltage and drift, high open-loop gain,
high common-mode rejection, and high power supply rejection of
the ADA4522-1/ADA4522-2/ADA4522-4 make them excellent op
amp choices as discrete, single-supply instrumentation amplifiers.
Figure 82 shows the classic 3-op-amp instrumentation amplifier
using the ADA4522-1/ADA4522-2/ADA4522-4. The key to high
CMRR for the instrumentation amplifier are resistors that are well
matched for both the resistive ratio and relative drift. For true
difference amplification, matching of the resistor ratio is very
important, where R5/R2 = R6/R4. The resistors are important
in determining the performance over manufacturing tolerances,
time, and temperature. Assuming a perfect unity-gain difference
amplifier with infinite common-mode rejection, a 1% tolerance
resistor matching results in only 34 dB of common-mode rejection.
Therefore, at least 0.01% or better resistors are recommended.
V
IN1
V
IN2
A1
A3
A2
R
G1
R
G2
R1
R3
R2
R4
R5
V
OUT
R6
R
G1
=R
G2
, R1 = R3, R2 = R4, R5 = R6
V
OUT
=(V
IN2
–V
IN1
)(1 + R1/R
G1
)(R5/R2)
13168-078
Figure 82. Discrete 3-Op-Amp Instrumentation Amplifier
To build a discrete instrumentation amplifier with external resis-
tors without compromising on noise, pay close attention to the
resistor values chosen. R
G1
and R
G2
each have thermal noise that
is amplified by the total noise gain of the instrumentation amplifier
and, therefore, a sufficiently low value must be chosen to reduce
thermal noise contribution at the output while still providing an
accurate measurement. Table 10 shows the external resistors noise
contribution referred to the output (RTO).
Table 10. Thermal Noise Contribution Example
Resistor
Value
(kΩ)
Resistor Thermal
Noise (nV/√Hz)
Thermal Noise
RTO (nV/√Hz)
R
G1
0.4 2.57 128.30
R
G2
0.4 2.57 128.30
R1 10 12.83 25.66
R2 10 12.83 25.66
R3 10 12.83 25.66
R4 10 12.83 25.66
R5 20 18.14 18.14
R6 20 18.14 18.14
Note that A1 and A2 have a high gain of 1 + R1/R
G1
. Therefore,
use a high precision, low offset voltage and low noise amplifier
for A1 and A2, such as the ADA4522-1/ADA4522-2/ADA4522-4.
Conversely, A3 operates at a much lower gain and has a differ-
ent set of op amp requirements. Its input noise, referred to the
overall instrumentation amplifier input, is divided by the first
stage gain and is not as important. Note that the input offset
voltage and the input voltage noise of the amplifiers are also
amplified by the overall noise gain.
Any unused channel of the ADA4522-1/ADA4522-2/ADA4522-4
must be configured in unity gain with the input common-mode
voltage tied to the midpoint of the power supplies.
Understanding how noise impacts a discrete instrumentation
amplifier or a difference amplifier (the second stage of a 3-op-
amp instrumentation amplifier) is important, because they are
commonly used in many different applications. The Load
Cell/Strain Gage Sensor Signal Conditioning section and the
Precision Low-Side Current Shunt Sensor section show the
ADA4522-1/ADA4522-2/ADA4522-4 used as a discrete
instrumentation or difference amplifier in an application.
LOAD CELL/STRAIN GAGE SENSOR SIGNAL
CONDITIONING USING THE ADA4522-2
The ADA4522-2, with its ultralow offset, drift, and noise, is well
suited to signal condition a low level sensor output with high gain
and accuracy. A weigh scale/load cell is an example of an
application with such requirements. Figure 83 shows a configura-
tion for a single-supply, precision, weigh scale measurement
system. The ADA4522-2 is used at the front end for amplification
of the low level signal from the load cell.
Current flowing through a PCB trace produces an IR voltage
drop; with longer traces, this voltage drop can be several
millivolts or more, introducing a considerable error. A 1 inch
long, 0.005 inch wide trace of 1 oz copper has a resistance of
approximately 100 mΩ at room temperature. With a load
current of 10 mA, the resistance can introduce a 1 mV error.
Therefore, a 6-wire load cell is used in the circuit. The load cell
has two sense pins, in addition to excitation, ground, and two
output connections. The sense pins are connected to the high
side (excitation pin) and low side (ground pin) of the
Wheatstone bridge. The voltage across the bridge can then be
accurately measured regardless of voltage drop due to wire
resistance. The two sense pins are also connected to the analog-
to-digital converter (ADC) reference inputs for a ratiometric
configuration that is immune to low frequency changes in the
power supply excitation voltage.
The ADA4522-2 is configured as the first stage of a 3-op-amp
instrumentation amplifier to amplify the low level amplitude
signal from the load cell by a factor of 1 + 2R1/R
G
. Capacitors
C1 and C2 are placed in the feedback loops of the amplifiers
and interact with R1 and R2 to perform low-pass filtering. This
filtering limits the amount of noise entering the Σ- ADC. In
addition, C3, C4, C5, R3, and R4 provide further common-mode
and differential mode filtering to reduce noise and unwanted
signals.
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