ADE7978 datasheet(88/120 Pages) AD

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ADE7978/ADE7933/ADE7932

Data Sheet

Rev. 0 | Page 88 of 120

Bipolar ac voltage is the most stringent environment. The goal

of a 50-year operating lifetime under the bipolar ac condition

determines the maximum working voltage recommended by

Analog Devices. In the case of unipolar ac or dc voltage, the stress

on the insulation is significantly lower. This allows operation at

higher working voltages while still achieving a 50-year service life.

The working voltages listed in Table 12 can be applied while

maintaining the 50-year minimum lifetime, provided that the

voltage conforms to either the unipolar ac or dc voltage cases.

Treat any cross-insulation voltage waveform that does not con-

form to Figure 111 or Figure 112 as a bipolar ac waveform, and

limit its peak voltage to the 50-year lifetime voltage value listed

in Table 12.

The voltage presented in Figure 111 is shown as sinusoidal for

illustration purposes only. It is meant to represent any voltage

waveform varying between 0 V and some limiting value. The

limiting value can be positive or negative, but the voltage cannot

cross 0 V.

LAYOUT GUIDELINES

Figure 34 shows the test circuit of the ADE7978/ADE7933/

ADE7932 chipset. The test circuit contains four ADE7933

devices and one ADE7978, together with the circuitry required

to sense the phase currents and voltages in a 3-phase system.

The chipset is managed by a microcontroller using the SPI

interface. (The microcontroller is not shown in the schematic.)

Figure 34 replicates the schematic of the ADE7978/ADE7933

evaluation board (see the ADE7978 and ADE7933/ADE7932

Evaluation Board section).

Figure 113 and Figure 114 show a recommended layout for a

printed circuit board (PCB) with two layers; in this layout, the

components are placed only on the top side of the board.

Following these layout guidelines helps to create a low noise

design with higher immunity to EMC influences. Note that

Figure 113 and Figure 114 show layout images that were

cropped from images of a board containing other circuitry

besides the ADE7978 and ADE7933 devices.

The layout of a meter using the ADE7932 is very similar to

the one designed for the ADE7933. The only difference is the

absence of the voltage channel V2P and its related circuitry: the

resistor divider and the protection diodes.

The primary supply voltage is supplied at VDD (Pin 19) of the

ADE7933. Two decoupling capacitors are placed between VDD

and GND (Pin 20): a 10 µF capacitor and a 100 nF ceramic

capacitor. The ceramic capacitor must be placed closest to the

ADE7933/ADE7932 because it decouples the high frequency

noise; the 10 µF capacitor must be placed in close proximity to

the ADE7933/ADE7932.

The ADE7933 VDDISO pin (Pin 1) is decoupled from GNDISO

(Pin 2) using two capacitors: a 10 µF capacitor and a 100 nF

ceramic capacitor. Place the ceramic capacitor closest to the

ADE7933/ADE7932; place the 10 µF capacitor in close prox-

imity to the ADE7933/ADE7932.

The ADE7933 LDO and REF pins (Pin 8 and Pin 9) are each

decoupled from GNDISO (Pin 10) using two capacitors: a 4.7 µF

capacitor and a 100 nF ceramic capacitor. Place the ceramic

capacitor closest to the ADE7933/ADE7932; place the 4.7 µF

capacitor in close proximity to the ADE7933/ADE7932.

Note that the ADE7933/ADE7932 isolated ground point is one

of the shunt poles. This point is directly connected to Pin 10

(GNDISO). It is not necessary to connect the shunt ground pole

to Pin 2; the two GNDISO pins (Pin 2 and Pin 10) are internally

connected to each other.

Note that the top layer components placed on the isolated

secondary side of the ADE7933 are surrounded by a ground plane

connected to GNDISO. The bottom layer extends the ground of

the primary side below the ADE7933 and the related circuitry,

creating a stitching capacitor. This capacitance has an important

role in reducing the emissions generated by the dc-to-dc converter

of the ADE7933/ADE7932. Note that a distance of at least 8 mm

is maintained on both sides between the input pins on the board

and the ground planes.

ADE7978 Datasheet(PDF) 88 Page - Analog Devices

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