ADA4638-1ACPZ-R7 datasheet(21/25 Pages) AD

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ADA4638-1

Data Sheet

Rev. 0 | Page 20 of 24

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

electrical power steering. In such application, it is desirable to

use a shunt with very low resistance to minimize series voltage

drop. This 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 provides a solution. The ADA4638-1, with its

input common-mode voltage that includes the lower supply rail,

can be used for implementing low-side current shunt sensors.

Figure 67 shows a low-side current sensing circuit using the

ADA4638-1. The ADA4638-1 is configured as a difference

amplifier with a gain of 1000. Although the ADA4638-1 has

high common-mode rejection, the CMR of the system is limited

by the external resistors. Therefore, the key to high CMR for the

system are resistors that are well matched from both the

resistive ratio and relative drift, where R1/R2 = R3/R4. The

resistors are important in determining the performance over

manufacturing tolerances, time, and temperature.

100kΩ

VSY

VOUT*

*VOUT = AMPLIFIER GAIN × VOLTAGE ACROSS RS

= 1000 × RS × I

= 100 × I

0.1Ω

100Ω

ADA4638-1

100kΩ

100Ω

Figure 67. Low-Side Current Sensing Circuit

PRINTED CIRCUIT BOARD LAYOUT

The ADA4638-1 is a high precision device with ultralow offset

voltage and offset voltage drift. Therefore, care must be taken in

the design of the printed circuit board (PCB) layout to achieve

optimum performance of the ADA4638-1 at board level.

To avoid leakage currents, keep the surface of the board clean

and free of moisture. Coating the board surface creates a barrier

to moisture accumulation and reduces parasitic resistance on

the board.

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 trace and solder-to-component lead.

Figure 68 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

ADA4638-1.

SOLDER

COMPONENT

LEAD

COPPER

TRACE

VSC1

VTS1

TA1

SURFACE-MOUNT

COMPONENT

PC BOARD

TA2

VSC2

VTS2

IF TA1 ≠ TA2, THEN

VTS1 + VSC1 ≠ VTS2 + VSC2

Figure 68. Mismatch in Seebeck Voltages Causes Seebeck Voltage Error

To minimize these thermocouple effects, orient resistors so that

heat sources warm both ends equally. Where possible, the input

signal paths should contain matching numbers and types of

components to match the number and type of thermocouple

junctions. For example, dummy components, such as zero value

resistors, can be used to match the thermoelectric error source

(real resistors in the opposite input path). Place matching compo-

nents in close proximity and orient them in the same manner to

ensure equal Seebeck voltages, thus cancelling thermal errors.

Additionally, use leads that are of equal length to keep thermal

conduction in equilibrium. Keep heat sources on the PCB as far

away from amplifier input circuitry as is practical.

It is highly recommended to use a ground plane. A ground plane

helps distribute heat throughout the board, maintains a constant

temperature across the board, and reduces EMI noise pickup.

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