ADE7756ANZ Analog Devices Inc, ADE7756ANZ Datasheet - Page 17

• Manufacturer Part Number: ADE7756ANZ
• Manufacturer: Analog Devices Inc

Specifications of ADE7756ANZ

• Lead Free Status / RoHS Status: Compliant

Antialias Filter
Figure 21 also shows an analog low-pass filter (RC) on the input
to the modulator. This filter is present to prevent aliasing. Alias-
ing is an artifact of all sampled systems.
Figure 16 illustrates the effect, frequency components (arrows
shown in black) above half the sampling frequency (also known
as the Nyquist frequency, i.e., 447 kHz) is imaged or folded
back down below 447 kHz (arrows shown in grey). This will
happen with all ADCs no matter what the architecture is. In the
example shown it can be seen that only frequencies near the
sampling frequency, i.e., 894 kHz, will move into the band of
interest for metering, i.e., 40 Hz–2 kHz. This fact will allow us
to use a very simple LPF (Low-Pass Filter) to attenuate these
high frequencies (near 900 kHz) and so prevent distortion in the
band of interest. A simple RC filter (single-pole) with a corner
frequency of 10 kHz will produce an attenuation of approxi-
mately 40 dBs at 894 kHz—see Figure 15. This is sufficient to
eliminate the effects of aliasing.
ADC Transfer Function
Below is an expression that relates the output of the LPF in the
sigma-delta ADC to the analog input signal level. Both ADCs in
the ADE7756 are designed to produce the same output code for
the same input signal level.
Therefore, with a full-scale signal on the input of 1 V, and an
internal reference of 2.4 V, the ADC output code is nominally
165,151 or 2851Fh. The maximum code from the ADC is
± 262,144, which is equivalent to an input signal level of ± 1.6 V.
However, for specified performance it is not recommended that
the full-scale input signal level of ± 1 V be exceeded.
0
2
FREQUENCIES
Code ADC
IMAGE
(
ALIASING EFFECTS
FREQUENCY – kHz
)
447
=
1 512
.
×
V
V
REF
IN
×
262 144
,
894
FREQUENCY
SAMPLING
Reference Circuit
Shown in Figure 17 is a simplified version of the reference output
circuitry. The nominal reference voltage at the REF
is 2.42 V. This is the reference voltage used for the ADCs in the
ADE7756. However, Channel 1 has three input range selections
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 1/2 and 1/4 of the nominal value by using an
internal resistor divider as shown in Figure 17.
The REF
e.g., an external 2.5 V reference. Note that the nominal refer-
ence value supplied to the ADCs is now 2.5 V, not 2.42 V. This
has the effect of increasing the nominal analog input signal
range by 2.5/2.42 × 100% = 3%, or from 1 V to 1.03 V.
The voltage of ADE7756 reference drifts slightly with temperature
—see ADE7756 Specifications for the temperature coefficient
specification (in ppm/°C). The value of the temperature drift varies
from part to part. On A-grade parts, the maximum temperature
drift is not guaranteed. Since the reference is used for the ADCs
in both Channel 1 and 2, any x% drift in the reference will result
in 2x% deviation of the meter accuracy. The reference drift
resulting from temperature changes is usually very small and it is
typically much smaller than the drift of other components on a
meter. However, if guaranteed temperature performance is
needed, one needs to use an external voltage reference or to use
B-grade parts. Alternatively, the meter can be calibrated at
multiple temperatures. Real-time compensation can be easily
achieved using the on the on-chip temperature sensor.
PTAT
IN/OUT
60 A
2.5V
pin can be overdriven by an external source,
LOAD = 10 A
MAXIMUM
1.7k
12.5k
12.5k
12.5k
12.5k
REF
2.42V
IN/OUT
OUTPUT IMPEDANCE
4k
REFERENCE INPUT TO ADC
CHANNEL 1 (RANGE SELECT)
2.42V, 1.21V, 0.6V
ADE7756
IN/OUT
pin