ADE7932 datasheet(87/120 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932

Rev. 0 | Page 87 of 120

ADE7978 AND ADE7933/ADE7932 CLOCKS

Provide a digital clock signal at the XTALIN pin to clock the

ADE7978. The ADE7978 is clocked at the frequency provided

at this pin; this frequency is referred to as CLKIN throughout

this data sheet. The ADE7978 is specified for a CLKIN value of

16.384 MHz, but frequencies of 16.384 MHz ± 1% are acceptable.

Alternatively, a 16.384 MHz crystal with a typical drive level of

0.5 mW and equivalent series resistance (ESR) of 20 Ω can be

connected across the XTALIN and XTALOUT pins to provide

a clock source for the ADE7978 (see Figure 109).

ADE7978

XTALIN

XTALOUT

CP1

CP2

Figure 109. ADE7978 Crystal Circuitry

The total capacitance (TC) at the XTALIN and XTALOUT pins

is as follows:

TC = C1 + CP1 = C2 + CP2

where:

C1 and C2 are the ceramic capacitors between the XTALIN and

GND pins and between the XTALOUT and GND pins (C1 = C2).

CP1 and CP2 are the parasitic capacitors of the wires connect-

ing the crystal to the ADE7978 (CP1 = CP2).

The load capacitance (LC) of the crystal is equal to half the total

capacitance TC because it is the capacitance of the series circuit

composed by C1 + CP1 and C2 + CP2.

CP2

CP1

Therefore, the value of Capacitors C1 and C2 as a function

of the load capacitance of the crystal is

C1 = C2 = 2 × LC − CP1 = 2 × LC − CP2

For the ADE7978, the typical total capacitance TC of the

XTALIN and XTALOUT pins is 40 pF (see Table 2).

Select a crystal with a load capacitance of

LC = TC/2 = 20 pF

For example, if the parasitic capacitances of CP1 and CP2 are

equal to 20 pF, select Capacitors C1 and C2 equal to 20 pF.

The ADE7933/ADE7932 clock circuit does not need a crystal

when it is used in conjunction with the ADE7978 because the

ADE7978 generates the 4.096 MHz clock used by the ADE7933/

ADE7932. However, if the ADE7933/ADE7932 are used as stand-

alone chips, a 4.096 MHz crystal with a typical drive level of

0.5 mW and ESR of 20 Ω can be connected across the ADE7933/

ADE7932 XTAL1 and XTAL2 pins to provide a clock source.

The values of the ceramic capacitors (C1 and C2) are calculated

in the same way as for the clock circuitry of the ADE7978.

INSULATION LIFETIME

All insulation structures eventually break down when subjected

to voltage stress over a sufficiently long period. The rate of insu-

lation degradation is dependent on the characteristics of the voltage

waveform applied across the insulation. In addition to the testing

performed by the regulatory agencies, Analog Devices carries

out an extensive set of evaluations to determine the lifetime of

the insulation structure within the ADE7933/ADE7932 devices.

Analog Devices performs accelerated life testing using voltage

levels higher than the rated continuous working voltage. Accel-

eration factors for several operating conditions are determined.

These factors allow calculation of the time to failure at the actual

working voltage.

The values shown in Table 12 summarize the peak voltage for

50 years of service life for a bipolar ac operating condition and

the maximum CSA/VDE approved working voltages. In many

cases, the approved working voltage is higher than the 50-year

service life voltage. Operation at these high working voltages

can lead to shortened insulation life in some cases.

The insulation lifetime of the ADE7933/ADE7932 devices

depends on the voltage waveform type imposed across the

isolation barrier. The iCoupler insulation structure degrades

at different rates depending on whether the waveform is bipolar

ac, unipolar ac, or dc. Figure 110, Figure 111, and Figure 112

illustrate these different isolation voltage waveforms.

RATED PEAK VOLTAGE

Figure 110. Bipolar AC Waveform

RATED PEAK VOLTAGE

Figure 111. Unipolar AC Waveform

RATED PEAK VOLTAGE

Figure 112. DC Waveform

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