Circuit Note

CN-0188

Rev. B | Page 3 of 5

PCB Layout Considerations

In any circuit where accuracy is crucial, it is important to

consider the power supply and ground return layout on the

board. The PCB should isolate the digital and analog sections as

much as possible. This PCB was constructed in a four layer

stack up with large area ground plane layers and power plane

polygons. See the MT-031 Tutorial for more discussion on

layout and grounding and the MT-101 Tutorial for information

on decoupling techniques.

The power supply to the AD7171 and ADuM5402 should be

decoupled with 10 µF and 0.1 µF capacitors to properly suppress

noise and reduce ripple. The capacitors should be placed as

close to the device as possible with the 0.1 µF capacitor having a

low ESR value. Ceramic capacitors are advised for all high

frequency decoupling.

Care should be taken in considering the isolation gap between

the primary and secondary sides of the ADuM5402. The

EVAL-CN0188-SDPZ board maximizes this distance by pulling

back any polygons or components on the top layer and aligning

them with the pins on the ADuM5402.

Power supply lines should have as large a trace width as possible

to provide low impedance paths and reduce glitch effects on the

supply line. Clocks and other fast switching digital signals

should be shielded from other parts of the board by digital

ground.

A complete design support package for this circuit note,

including board layouts, can be found at

http://www.analog.com/CN0188-DesignSupport.

COMMON VARIATIONS

There are a number of solutions available for high-side sensing

of positive sources. IC solutions using current sense amplifiers,

difference amplifiers, or a combination of these are available.

“High-Side Current Sensing: Difference Amplifier vs, Current-

Sense Amplifier,” Analog Dialogue, January 2008, describes the

use of current sense and difference amplifiers. The article is

available at www.analog.com/HighSide_CurrentSensing.

The following URLs link to Analog Devices products which are

useful in solving the current sense problem:

Current sense amplifiers: www.analog.com/CurrentSenseAmps

Difference amplifiers: www.analog.com/DifferenceAmps

Instrumentation amplifiers: www.analog.com/InstrumentationAmps

Figure3.AlternateGalvanicallyIsolatedNegativeRailCurrentSenseCircuitPoweredDirectlyfrom−48VSourceandADuM5402Isolator(AllConnectionsandDecouplingNotShown)

SCLK

PDRST

DOUT

VDD

GND

REFIN(+)

AIN+

AIN−

ADR381

VIN

VOUT

GND

LOAD

SOURCE

+

−48V

+2.5V

+

–

AD7171

R2

1kΩ

1%

R4

1kΩ

1%

48.7kΩ, 1%

U1

U1:

ADA4051-1

REFIN(–)

R3*

−

R1*

1.24MΩ

1%

+3.3V

ADuM5402

*R1 ADDS 100mV OFFSET TO U1 OUTPUT

AND ALLOWS MEASUREMENT AT ZERO LOAD CURRENT.

IF R1 IS USED, REDUCE R3 T

O 46.4kΩ TO MAINTAIN 2.5V FULL-SCALE