ADE7763ARS datasheet(20/56 Pages) AD

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ADE7763

Rev. A | Page 20 of 56

Antialias Filter

Figure 39 also shows an analog low-pass filter (RC) on the input

to the modulator. This filter prevents aliasing, which is an

artifact of all sampled systems. Aliasing means that frequency

components in the input signal to the ADC that are higher than

half the sampling rate of the ADC appear in the sampled signal

at a frequency below half the sampling rate. Figure 41 illustrates

the effect. Frequency components (shown as arrows) above half

the sampling frequency (also known as the Nyquist frequency,

i.e., 447 kHz) are imaged or folded back down below 447 kHz.

This happens with all ADCs, regardless of the architecture. In

the example shown, only frequencies near the sampling

frequency, i.e., 894 kHz, move into the band of interest for

metering, i.e., 40 Hz to 2 kHz. This allows the use of a very

simple LPF (low-pass filter) to attenuate high frequency (near

900 kHz) noise, and it prevents distortion in the band of interest.

For conventional current sensors, a simple RC filter (single-pole

LPF) with a corner frequency of 10 kHz produces an attenuation

of approximately 40 dB at 894 kHz—see Figure 41. The 20 dB

per decade attenuation is usually sufficient to eliminate the effects

of aliasing for conventional current sensors; however, for a di/dt

sensor such as a Rogowski coil, the sensor has a 20 dB per decade

gain. This neutralizes the –20 dB per decade attenuation

produced by one simple LPF. Therefore, when using a di/dt

sensor, care should be taken to offset the 20 dB per decade gain.

One simple approach is to cascade two RC filters to produce the

–40 dB per decade attenuation.

SAMPLING

FREQUENCY

IMAGE

FREQUENCIES

ALIASING EFFECTS

447

894

FREQUENCY (kHz)

Figure 41. ADC and Signal Processing in Channel 1 Outline Dimensions

ADC Transfer Function

The following expression relates the output of the LPF in the

Σ-∆ ADC to the analog input signal level. Both ADCs in the

ADE7763 are designed to produce the same output code for the

same input signal level.

(

)

3.0492

262,144

OUT

Code ADC

=×

(1)

Therefore, with a full-scale signal on the input of 0.5 V and an

internal reference of 2.42 V, the ADC output code is nominally

165,151, or 0x2851F. The maximum code from the ADC is

±262,144; this is equivalent to an input signal level of ±0.794 V.

However, for specified performance, do not exceed the 0.5 V

full-scale input signal level.

Reference Circuit

Figure 42 shows a simplified version of the reference output

circuitry. The nominal reference voltage at the REFIN/OUT pin is

2.42 V. This is the reference voltage used for the ADCs. However,

Channel 1 has three input range options that are selected by

dividing down the reference value used for the ADC in

Channel 1. The reference value used for Channel 1 is divided

down to ½ and ¼ of the nominal value by using an internal

resistor divider, as shown in Figure 42.

µA

PTAT

2.5V

1.7k

Ω

12.5k

Ω

12.5k

Ω

12.5k

Ω

12.5k

Ω

REFIN/OUT

2.42V

MAXIMUM

LOAD = 10

µA

OUTPUT

IMPEDANCE

Ω

REFERENCE INPUT

TO ADC CHANNEL 1

(RANGE SELECT)

2.42V, 1.21V, 0.6V

Figure 42. Reference Circuit Output

The REFIN/OUT pin can be overdriven by an external source such as

a 2.5 V reference. Note that the nominal reference value supplied

to the ADCs is now 2.5 V, not 2.42 V, which increases the

nominal analog input signal range by 2.5/2.42 × 100% = 3% or

from 0.5 V to 0.5165 V.

The voltage of the ADE7763 reference drifts slightly with changes

in temperature—see Table 1 for the temperature coefficient

specification (in ppm/°C). The value of the temperature drift

varies from part to part. Because the reference is used for the

ADCs in both Channels 1 and 2, any x% drift in the reference

results in 2x% deviation in the meter accuracy. The reference

drift that results from a temperature change is usually very

small, typically much smaller than the drift of other components

on a meter. However, if guaranteed temperature performance is

needed, use an external voltage reference. Alternatively, the

meter can be calibrated at multiple temperatures. Real-time

compensation can be achieved easily by using the on-chip

temperature sensor.

CHANNEL 1 ADC

Figure 43 shows the ADC and signal processing chain for

Channel 1. In waveform sampling mode, the ADC outputs a

signed, twos complement, 24-bit data-word at a maximum of

27.9 kSPS (CLKIN/128). With the specified full-scale analog

input signal of 0.5 V (or 0.25 V or 0.125 V—see the Analog

Inputs section), the ADC produces an output code that is

approximately between 0x28 51EC (+2,642,412d) and

0xD7 AE14 (–2,642,412d)—see Figure 43.

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