ADE7761A

Rev. 0 | Page 13 of 24

ANALOG-TO-DIGITAL CONVERSION

The analog-to-digital conversion in the ADE7761A is carried

out using second-order, Σ-Δ ADCs. Figure 19 shows a first-

order, Σ-Δ ADC (for simplicity). The converter is made up of

two parts: the Σ-Δ modulator and the digital low-pass filter.

....10100101....

1-BIT DAC

LATCHED

COMPAR-

ATOR

INTEGRATOR

MCLK

C

R

ANALOG

LOW-PASS FILTER

DIGITAL

LOW-PASS FILTER

1

24

Figure 19. First-Order, Σ-Δ ADC

A Σ-Δ modulator converts the input signal into a continuous

serial stream of 1s and 0s at a rate determined by the sampling

clock. In the ADE7761A, the sampling clock is equal to CLKIN.

The 1-bit DAC in the feedback loop is driven by the serial data

stream. The DAC output is subtracted from the input signal. If

the loop gain is high enough, the average value of the DAC

output (and, therefore, the bit stream) approaches that of the

input signal level. For any given input value in a single sampling

interval, the data from the 1-bit ADC is virtually meaningless.

Only when a large number of samples are averaged is a meaningful

result obtained. This averaging is carried out in the second part

of the ADC, the digital low-pass filter. By averaging a large

number of bits from the modulator, the low-pass filter can

produce 24-bit data-words that are proportional to the input

signal level.

The Σ-Δ converter uses two techniques to achieve high

resolution from what is essentially a 1-bit conversion technique.

The first is oversampling, which means that the signal is sampled at

a rate (frequency) that is many times higher than the bandwidth

of interest. For example, the sampling rate in the ADE7761A is

CLKIN (450 kHz) and the band of interest is 40 Hz to 1 kHz.

Oversampling has the effect of spreading the quantization noise

(noise due to sampling) over a wider bandwidth. With the noise

spread more thinly over a wider bandwidth, the quantization

noise in the band of interest is lowered (see Figure 20).

However, oversampling alone is not an efficient enough method

to improve the signal-to-noise ratio (SNR) in the band of interest.

For example, an oversampling ratio of 4 is required just to

increase the SNR by only 6 dB (1 bit). To keep the oversampling

ratio at a reasonable level, it is possible to shape the quantization

noise so that the majority of the noise lies at the higher frequencies.

This is what happens in the Σ-Δ modulator; the noise is shaped

by the integrator, which has a high-pass type response for the

quantization noise. The result is that most of the noise is at the

higher frequencies, where it can be removed by the digital low-

pass filter. This noise shaping is also shown in Figure 20.

SHAPED NOISE

HIGH RESOLUTION

OUTPUT FROM

DIGITAL LFP

NOISE

SIGNAL

NOISE

SIGNAL

0

1

225

450

FREQUENCY (kHz)

0

1

225

450

FREQUENCY (kHz)

DIGITAL FILTER

ANTIALIAS FILTER (RC)

SAMPLING FREQUENCY

Figure 20. Noise Reduction due to Oversampling and

Noise Shaping in the Analog Modulator

Antialias Filter

Figure 20 also shows an analog low-pass filter (RC) on input to

the modulator. This filter is present to prevent aliasing. Aliasing

is an artifact of all sampled systems, which 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

frequency below half the sampling rate. Figure 21 illustrates

the effect.

In Figure 21, frequency components (arrows shown in black)

above half the sampling frequency (also known as the Nyquist

frequency), that is, 225 kHz, are imaged or folded back down

below 225 kHz (arrows shown in gray). This happens with all

ADCs no matter what the architecture. In the example shown,

only frequencies near the sampling frequency (450 kHz) move

into the band of interest for metering (40 Hz to 1 kHz). This

fact allows the use of a very simple low-pass filter to attenuate

these frequencies (near 250 kHz) and thereby prevent distortion

in the band of interest. A simple RC filter (single pole) with a

corner frequency of 10 kHz produces an attenuation of

approximately 33 dB at 450 kHz (see Figure 21). This is

sufficient to eliminate the effects of aliasing.

0

1

225

450

FREQUENCY (kHz)

IMAGE

FREQUENCIES

SAMPLING

FREQUENCY

ANTIALIASING EFFECTS

Figure 21. ADC and Signal Processing in Current Channel or Voltage Channel