ADA4522-4ARZ-R7 datasheet(29/33 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

APPLICATIONS INFORMATION

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Rev. G | 29 of 33

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 applica-

tions. 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 mΩ. At a measured current of 1 A, the

voltage produced from the shunt is 100 mV, and the amplifier error

sources are not critical. However, at low measured current in the 1

mA range, the 100 μV 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 Am-

plifier 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.

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 junc-

tion. 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 TA1

≠ TA2) causes a mismatch in the Seebeck voltages at the solder

joints, thereby resulting in thermal voltage errors that degrade

the performance of the ultralow offset voltage of the ADA4522-1/

ADA4522-2/ADA4522-4.

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