ADE7753ARSZ datasheet(26/60 Pages) AD

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ADE7753

Rev. C | Page 26 of 60

ADC 2

ANALOG

INPUT RANGE

0.5V, 0.25, 0.125,

62.5mV, 31.25mV

REFERENCE

LPF1

ACTIVE AND REACTIVE

ENERGY CALCULATION

VRMS CALCULATION

AND WAVEFORM

SAMPLING

(PEAK/SAG/ZX)

PGA2

×1, ×2, ×4,

×8, ×16

{GAIN [7:5]}

V2P

V2N

2.42V

0x2852

0x2581

0xDAE8

0xD7AE

0x0000

LPF OUTPUT

WORD RANGE

02875-0-054

Figure 55. ADC and Signal Processing in Channel 2

VRMS[23:0]

LPF3

|x|

LPF1

CHANNEL 2

0x17D338

0x00

VRMOS[11:0]

VOLTAGE SIGNAL (V(t))

sgn

22 21 20

02875-0-0055

0x2518

0x0

0xDAE8

Figure 56. Channel 2 RMS Signal Processing

Channel 2 has only one analog input range (0.5 V differential).

Like Channel 1, Channel 2 has a PGA with gain selections of 1,

2, 4, 8, and 16. For energy measurement, the output of the ADC

is passed directly to the multiplier and is not filtered. An HPF is

not required to remove any dc offset since it is only required to

remove the offset from one channel to eliminate errors due to

offsets in the power calculation. When in waveform sampling

mode, one of four output sample rates can be chosen by using

Bits 11 and 12 of the mode register. The available output sample

rates are 27.9 kSPS, 14 kSPS, 7 kSPS, or 3.5 kSPS—see the Mode

signals that a sample is available by going active low. The timing

is the same as that for Channel 1, as shown in

Figure 52

Channel 2 RMS Calculation

Figure 56 shows the details of the signal processing chain for the

rms estimation on Channel 2. This Channel 2 rms estimation is

done in the ADE7753 using the mean absolute value calculation, as

shown in Figure 56. The Channel 2 rms value is processed from

the samples used in the Channel 2 waveform sampling mode.

The rms value is slightly attenuated because of LPF1. Channel 2

rms value is stored in the unsigned 24-bit VRMS register. The

update rate of the Channel 2 rms measurement is CLKIN/4.

With the specified full-scale ac analog input signal of 0.5 V, the

output from the LPF1 swings between 0x2518 and 0xDAE8 at

60 Hz—see the Channel 2 ADC section. The equivalent rms

value of this full-scale ac signal is approximately 1,561,400

(0x17D338) in the VRMS register. The voltage rms measure-

ment provided in the ADE7753 is accurate to within ±0.5% for

signal input between full scale and full scale/20. Table 8 shows

the settling time for the VRMS measurement, which is the time

it takes for the rms register to reflect the value at the input to

the voltage channel. The conversion from the register value to

volts must be done externally in the microprocessor using a

volts/LSB constant. Since the low-pass filtering used for

calculating the rms value is imperfect, there is some ripple noise

from 2ω term present in the rms measurement. To minimize

the noise effect in the reading, synchronize the rms reading

with the zero crossings of the voltage input.

Table 8.

95%

100%

220 ms

670 ms

Channel 2 RMS Offset Compensation

The ADE7753 incorporates a Channel 2 rms offset compensation

used to remove offset in the Channel 2 rms calculation. An

offset could exist in the rms calculation due to input noises and

dc offset in the input samples. The offset calibration allows the

contents of the VRMS register to be maintained at 0 when no

voltage is applied. One LSB of the Channel 2 rms offset is

equivalent to one LSB of the rms register. Assuming that the

maximum value from the Channel 2 rms calculation is

1,561,400d with full-scale ac inputs, then one LSB of the

Channel 2 rms offset represents 0.064% of measurement

error at –60 dB down of full scale.

VRMS = VRMS0 + VRMSOS

(6)

where VRMS0 is the rms measurement without offset correction.

The voltage rms offset compensation should be done by testing

the rms results at two non-zero input levels. One measurement

can be done close to full scale and the other at approximately

full scale/10. The voltage offset compensation can be derived

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