Datasheet

ADA4522-1/ADA4522-2/ADA4522-4 Data Sheet
Rev. F | Page 28 of 33
OUT+
SENSE–
SENSE+
OUT–
V
EXC
1/2
V+
R1 11.3k
C1 3.3µF
1/2
ADA4522-2
ADA4522-2
R2 11.3k
C2 3.3µF
R
G
60.4
R3
1k
R4
1k
C4
1µF
C5
10µF
C3
1µF
AIN(+)
REF(–)
REF(+)
DOUT/
RDY
DIN
AIN(–)
GND
V
DD
+5V
AD7791
100pF
1µF
100pF
SCLK
CS
LOAD
CELL
13168-079
Figure 83. Precision Weigh Scale Measurement System
PRECISION LOW-SIDE CURRENT SHUNT SENSOR
Many applications require the sensing of signals near the
positive or negative rails. Current shunt sensors are one such
application and are mostly used for feedback control systems.
They are also used in a variety of other applications, including
power metering, battery fuel gauging, and feedback controls in
industrial applications. In such applications, it is desirable to
use a shunt with very low resistance to minimize series voltage
drop. This configuration not only minimizes wasted power, but
also allows the measurement of high currents while saving power.
A typical shunt may be 100 m. At a measured current of 1 A,
the voltage produced from the shunt is 100 mV, and the ampli-
fier error sources are not critical. However, at low measured current
in the 1 mA range, the 100 V generated across the shunt demands
a very low offset voltage and drift amplifier to maintain absolute
accuracy. The unique attributes of a zero drift amplifier provide
a solution. Figure 84 shows a low-side current sensing circuit
using the ADA4522-1/ADA4522-2/ADA4522-4. The ADA4522-1/
ADA4522-2/ADA4522-4 are configured as difference amplifiers
with a gain of 1000. Although the ADA4522-1/ADA4522-2/
ADA4522-4 have high CMRR, the CMRR of the system is limited
by the external resistors. Therefore, as mentioned in the Single-
Supply Instrumentation Amplifier section, the key to high CMRR
for the system is resistors that are well matched from both the
resistive ratio and relative drift, where R1/R2 = R3/R4.
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.
R2
100k
V
SY
V
SY
V
OUT
*
*V
OUT
= AMPLIFIER GAIN × VOLTAGE ACROSS R
S
= 1000 × R
S
× I
= 100 × I
R
L
R
S
0.1
R1
100
I
ADA4522-1/
ADA4522-2/
ADA4522-4
R4
100k
R3
100
I
13168-080
Figure 84. Low-Side Current Sensing Circuit
PRINTED CIRCUIT BOARD LAYOUT
The ADA4522-1/ADA4522-2/ADA4522-4 are high precision
devices with ultralow offset voltage and noise. Therefore, take
care in the design of the PCB layout to achieve optimum
performance of the ADA4522-1/ADA4522-2/ADA4522-4 at the
board level.
To avoid leakage currents, keep the surface of the board clean
and free of moisture.
Properly bypassing the power supplies and keeping the supply
traces short minimizes power supply disturbances caused by
output current variation. Connect bypass capacitors as close
as possible to the device supply pins. Stray capacitances are a
concern at the outputs and the inputs of the amplifier. It is
recommended that signal traces be kept at a distance of at
least 5 mm from supply lines to minimize coupling.
A potential source of offset error is the Seebeck voltage on the
circuit board. The Seebeck voltage occurs at the junction of two
dissimilar metals and is a function of the temperature of the
junction. The most common metallic junctions on a circuit board
are solder to board traces and solder to component leads. Figure 85
shows a cross section of a surface-mount component soldered
to a PCB. A variation in temperature across the board (where T
A1
≠
T
A2
) causes a mismatch in the Seebeck voltages at the solder joints,
thereby resulting in thermal voltage errors that degrade the
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