ADE7754 datasheet(11/44 Pages) AD

11 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 11 –

ADE7754 ANALOG TO DIGITAL CONVERSION

The analog-to-digital conversion in the ADE7754 is carried

out using second order sigma-delta ADCs. The block dia-

gram in Figure 5 shows a first order (for simplicity)

sigma-delta ADC. The converter is made up of two parts,

first the sigma-delta modulator and secondly the digital low

pass filter.

VREF

....10100101......

Digital Low Pass Filter

MCLK/12

INTEGRATOR

1-Bit DAC

LATCHED

COMPARATOR

Analog Low Pass Filter

Figure 5 - First order Sigma-Delta (

Σ−∆) ADC

A sigma-delta modulator converts the input signal into a

continuous serial stream of 1's and 0's at a rate determined by

the sampling clock. In the ADE7754 the sampling clock is

equal to CLKIN/12. The 1-bit DAC in the feedback loop is

driven by the serial data stream. The DAC output is sub-

tracted from the input signal. If the loop gain is high enough

the average value of the DAC output (and therefore the bit

stream) will approach 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, will a meaningful result be

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 which are proportional to the input

signal level. The sigma-delta converter uses two techniques

to achieve high resolution from what is essentially a 1-bit

conversion technique. The first is oversampling. By over

sampling we mean that the signal is sampled at a rate

(frequency) which is many times higher than the bandwidth

of interest. For example the sampling rate in the ADE7754

is CLKIN/12 (833kHz) and the band of interest is 40Hz to

2kHz. Oversampling has the effect of spreading the quanti-

zation 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 6.

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 6dB (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 sigma-delta 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 6.

Frequency (Hz)

417kHz

833kHz

2kHz

Sampling

Frequency

Shaped

Noise

Antialias filter (RC)

Digital filter

Noise

Signal

Frequency (Hz)

417kHz

833kHz

2kHz

Noise

Signal

High resolution

output from Digital

LPF

Figure 6– Noise reduction due to Oversampling & Noise

shaping in the analog modulator

Antialias Filter

Figure 5 also shows an analog low pass filter (RC) on the

input to the modulator. This filter is present to prevent

aliasing. Aliasing is an artifact of all sampled systems.

Basically it means that frequency components in the input

signal to the ADC which are higher than half the sampling

rate of the ADC will appear in the sampled signal at a

frequency below half the sampling rate. Figure 7 illustrates

the effect, frequency components (arrows shown in black)

above half the sampling frequency (also know as the Nyquist

frequency), i.e., 417kHz get imaged or folded back down

below 417kHz (arrows shown in grey). This will happen with

all ADCs no matter what the architecture. In the example

shown it can be seen that only frequencies near the sampling

frequency, i.e., 833kHz, will move into the band of interest

for metering, i.e, 40Hz - 2kHz. This fact allows us to use a

very simple LPF (Low Pass Filter) to attenuate these high

frequencies (near 900kHz) and so prevent distortion in the

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

corner frequency of 10kHz produces an attenuation of ap-

proximately 40dBs at 833kHz—see Figure 7. This is sufficient

to eliminate the effects of aliasing.

Aliasing Effects

Image

frequencies

2kHz

417kHz

833kHz

Sampling Frequency

Frequency (Hz)

Figure 7– ADC and signal processing

in current channel or voltage channel

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