ade7753 Analog Devices, Inc., ade7753 Datasheet - Page 15

• Manufacturer Part Number: ade7753
• Description: Active And Apparent Energy Metering Ic With Di/dt Sensor Interface
• Manufacturer: Analog Devices, Inc.

frequency below half the sampling rate. Figure 19 illustrates
the effect. Frequency components (arrows shown in black)
above half the sampling frequency (also know as the Nyquist
frequency, i.e., 447kHz) get imaged or folded back down
below 447kHz (arrows shown in grey). This will happen with
all ADCs regardless of the architecture. In the example
shown, only frequencies near the sampling frequency, i.e.,
894kHz, will move into the band of interest for metering, i.e,
40Hz - 2kHz. This allows the usage of very simple LPF (Low
Pass Filter) to attenuate high frequency (near 900kHz) noise
and prevents distortion in the band of interest. For conven-
tional current sensor, a simple RC filter (single pole LPF)
with a corner frequency of 10kHz will produce an attenuation
of approximately 40dBs at 894kHz—see Figure 18. The
20dB per decade attenuation is usually sufficient to eliminate
the effects of aliasing for conventional current sensor.
For di/dt sensor such as Rogowski coil, however, the sensor
has 20dB per decade gain. This will neutralize the -20dB per
decade attenuation produced by the simple LPF. Therefore,
when using a di/dt sensor, care should be taken to offset the
20dB per decade gain coming from the di/dt sensor. One
simple approach is to cascade two RC filters to produce the
-40dB per decade attenuation needed.
ADC transfer function
Below is an expression which relates the output of the LPF
in the sigma-delta 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.
Therefore with a full scale signal on the input of 0.5V and an
internal reference of 2.42V, the ADC output code is nomi-
nally 165,151 or 2851Fh. The maximum code from the
ADC is ±262,144, this is equivalent to an input signal level
of ±0.794V. However for specified performance it is not
recommended that the full-scale input signal level of 0.5V be
exceeded.
ADE7753 Reference circuit
Shown below in Figure 20 is a simplified version of the
reference output circuitry. The nominal reference voltage at
the REF
for the ADCs in the ADE7753. However, Channel 1 has
three input range selections which 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 ½ and
¼ of the nominal value by using an internal resistor divider
as shown in Figure 20.
REV. PrF 10/02
Code
0
Figure 19 —ADC and signal processing in Channel 1
2kHz
(
ADC
frequencies
IN/OUT
image
)
. 3
pin is 2.42V. This is the reference voltage used
Aliasing Effects
0492
Frequency (Hz)
447kHz
V
V
out
in
PRELIMINARY TECHNICAL DATA
262
,
144
Sampling Frequency
894kHz
–15–
PTAT
The REF
e.g., an external 2.5V reference. Note that the nominal
reference value supplied to the ADCs is now 2.5V not 2.42V.
This has the effect of increasing the nominal analog input
signal range by 2.5/2.42 100% = 3% or from 0.5V to
0.5165V.
The voltage of ADE7753 reference drifts slightly with
temperature—see 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 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. Alternatively, the meter can
be calibrated at multiple temperatures. Real time compensa-
tion can be easily achieved using the on the on-chip temperature
sensor.
CHANNEL 1 ADC
Figure 21 shows the ADC and signal processing chain for
Channel 1. In waveform sampling mode the ADC outputs a
signed 2’s Complement 24-bit data word at a maximum of
27.9kSPS (CLKIN/128). With the specified full scale ana-
log input signal of 0.5V (or 0.25V or 0.125V – see Analog
Inputs section) the ADC will produce an output code which is
approximately between 2851ECh (+2,642,412 Decimal)
and D7AE14h (-2,642,412 Decimal). This is illustrated in
Figure 21.
Channel 1 Sampling
The waveform samples may also be routed to the WAVE-
FORM register (MODE[14:13] = 1,0) to be read by the
system master (MCU). In waveform sampling mode the
WSMP bit (bit 3) in the Interrupt Enable register must also
be set to logic one. The Active, Apparent Power and Energy
calculation will remain uninterrupted during waveform sam-
pling.
When in waveform sample mode, one of four output sample
rates may be chosen by using bits 11 and 12 of the Mode
register (WAVSEL1,0). The output sample rate may be
27.9kSPS, 14kSPS, 7kSPS or 3.5kSPS—see Mode Register.
The interrupt request output IRQ signals a new sample
Figure 20 —ADE7753 Reference Circuit Ouput
IN/OUT
60 A
2.5V
pin can be overdriven by an external source,
Maximum
Load = 10 A
1.7k
12.5k
12.5k
12.5k
12.5k
REF
2.42V
IN/OUT
Output
Impedance
6k
Reference input to ADC
Channel 1 (Range Select)
2.42V, 1.21V, 0.6V
ADE7753