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ADE7753
Antialias Filter
Figure 47 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. Aliasing means
that frequency components in the input signal to the ADC,
which are higher than half the sampling rate of the ADC,
appear in the sampled signal at a frequency below half the
sampling rate. Figure 49 illustrates the effect. Frequency
components (arrows shown in black) above half the sampling
frequency (also know as the Nyquist frequency, i.e., 447 kHz)
ADE7753 Reference Circuit
Figure 50 shows a simplified version of the reference output
circuitry. The nominal reference voltage at the REF IN/OUT pin is
2.42 V. This is the reference voltage used for the ADCs in the
ADE7753. 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 ? and ? of the nominal value by using an
internal resistor divider, as shown in Figure 50.
OUTPUT
are imaged or folded back down below 447 kHz. This happens
with all ADCs regardless of the architecture. In the example
MAXIMUM
LOAD = 10 μ A
IMPEDANCE
6k Ω
REF IN/OUT
shown, only frequencies near the sampling frequency, i.e.,
894 kHz, move into the band of interest for metering, i.e., 40 Hz
PTAT
60 μ A
1.7k Ω
2.42V
to 2 kHz. This allows the use of a very simple LPF (low-pass
filter) to attenuate high frequency (near 900 kHz) noise, and
prevents distortion in the band of interest. For conventional
current sensors, a simple RC filter (single-pole LPF) with a
2.5V
12.5k Ω
12.5k Ω
corner frequency of 10 kHz produces an attenuation of
approximately 40 dB at 894 kHz—see Figure 49. The 20 dB per
decade attenuation is usually sufficient to eliminate the effects
12.5k Ω
12.5k Ω
REFERENCE INPUT
TO ADC CHANNEL 1
(RANGE SELECT)
2.42V, 1.21V, 0.6V
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 needed.
ALIASING EFFECTS
02875-0-049
Figure 50. ADE7753 Reference Circuit Output
The REF IN/OUT pin can be overdriven by an external source, for
example, an external 2.5 V reference. Note that the nominal
reference value supplied to the ADCs is now 2.5 V, not 2.42 V,
which has the effect of increasing 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 ADE7753 reference drifts slightly with
IMAGE
FREQUENCIES
SAMPLING
FREQUENCY
temperature—see the ADE7753 Specifications for the temperature
coefficient specification (in ppm/°C). The value of the temperature
drift varies from part to part. Since the reference is used for the
ADCs in both Channels 1 and 2, any x% drift in the reference
results in 2×% deviation of the meter accuracy. The reference
0
2
447
FREQUENCY (kHz)
894
02875-0-048
drift resulting from temperature changes is usually very small
and it is typically much smaller than the drift of other components
Figure 49. ADC and Signal Processing in Channel 1 Outline Dimensions
ADC Transfer Function
The following expression relates the output of the LPF in the
Σ-Δ ADC to the analog input signal level. Both ADCs in the
ADE7753 are designed to produce the same output code for the
same input signal level.
on a meter. However, if guaranteed temperature performance
is needed, one needs to 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
Code ( ADC ) = 3 . 0492 ×
V IN
V OUT
× 262 , 144
(1)
Figure 51 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
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 2851Fh. 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, it is recommended that the
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 0x2851EC (+2,642,412d) and
0xD7AE14 (–2,642,412d)—see Figure 51.
full-scale input signal level of 0.5 V not be exceeded.
Rev. C | Page 23 of 60
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