ade7760 Analog Devices, Inc., ade7760 Datasheet - Page 12

• Manufacturer Part Number: ade7760
• Description: Energy Metering Ic With On-chip Fault Detection
• Manufacturer: Analog Devices, Inc.

ADE7760
INTERNAL OSCILLATOR
The nominal internal oscillator frequency is 450 kHz when
used with the recommended R
between RCLKIN and DGND (see Figure 13).
The internal oscillator frequency is inversely proportional to the
value of this resistor. Although the internal oscillator operates
when used with a R
is recommended to choose a value within the range of the
nominal value.
The output frequencies on CF, F1, and F2 are directly
proportional to the internal oscillator frequency; thus, the
resistor R
drift. A low tolerance resistor limits the variation of the internal
oscillator frequency. Small variation of the clock frequency and
consequently of the output frequencies from meter to meter
contributes to a smaller calibration range of the meter. A low
temperature drift resistor directly limits the variation of the
internal clock frequency over temperature. The stability of the
meter to external variation is then better ensured by design.
ANALOG-TO-DIGITAL CONVERSION
The analog-to-digital conversion in the ADE7760 is carried out
using second-order Σ-Δ ADCs. Figure 14 shows a first-order
(for simplicity) Σ-Δ ADC. The converter is made up of two
parts, the Σ-Δ modulator and the digital low-pass filter.
REFERENCE
ADE7760
*RB + VR = RF
2.5V
OSC
Figure 13. ADE7760 Internal Oscillator Connection
must have a low tolerance and low temperature
Figure 12. Typical Connection for Channel 2
RA*
4k
REF
RB*
VR*
OSC
IN/OUT
9
resistor value between 5 kΩ and 12 kΩ, it
AGND
±660mV
OSCILLATOR
R
C
INTERNAL
OSC
F
OSC
14
R
R
R
RCLKIN
F
F
F
resistor value of 6.2 kΩ
C
C
C
F
F
T
V
V
V
V
2N
2N
2P
2P
17
DGND
Rev. 0 | Page 12 of 24
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 ADE7760, 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 resolu-
tion 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 ADE7760 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 15).
However, oversampling alone is not an efficient enough method
to improve the signal-to-noise ratio (SNR) in the band of inter-
est. 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 frequen-
cies. 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 15.
LOW-PASS FILTER
ANALOG
R
C
Figure 14. First-Order Σ-∆ ADC
INTEGRATOR
V
REF
1-BIT DAC
....10100101....
MCLK
LATCHED
COMPAR-
ATOR
LOW-PASS FILTER
1
DIGITAL
24