ADE7758 Analog Devices, ADE7758 Datasheet - Page 19

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

+500mV
THEORY OF OPERATION
ANTIALIASING FILTER
The need for this filter is that it prevents aliasing. Aliasing is an
artifact of all sampled systems. Input signals with frequency
components higher than half the ADC sampling rate distort the
sampled signal at a frequency below half the sampling rate. This
will happen with all ADCs, regardless of the architecture. The
combination of the high sampling rate ∑-∆ ADC used in the
ADE7758 with the relatively low bandwidth of the energy meter
allows a very simple low-pass filter (LPF) to be used as an
antialiasing filter. A simple RC filter (single pole) with a corner
frequency of 10 kHz produces an attenuation of approximately
40 dB at 833 kHz. This is usually sufficient to eliminate the
effects of aliasing.
ANALOG INPUTS
The ADE7758 has a total of six analog inputs divided into two
channels: current and voltage. The current channel consists of
three pairs of fully differential voltage inputs: IAP and IAN, IBP
and IBN, and ICP and ICN. These fully differential voltage
input pairs have a maximum differential signal of ±0.5 V. The
current channel has a programmable gain amplifier (PGA) with
possible gain selection of 1, 2, or 4. In addition to the PGA, the
current channels also have a full-scale input range selection for
the ADC. The ADC analog input range selection is also made
using the gain register (see Figure 38). As mentioned previously,
the maximum differential input voltage is ±0.5 V. However, by
using Bit 3 and Bit 4 in the gain register, the maximum ADC
input voltage can be set to ±0.5 V, ±0.25 V, or ±0.125 V on the
current channels. This is achieved by adjusting the ADC
reference (see the Reference Circuit section).
Figure 36 shows the maximum signal levels on the current
channel inputs. The maximum common-mode signal is
±25 mV as shown in Figure 36.
–500mV
The voltage channel has three single-ended voltage inputs:
VAP, VBP, and VCP. These single-ended voltage inputs have a
maximum input voltage of ±0.5 V with respect to VN. Both the
current and voltage channel have a PGA with possible gain
selections of 1, 2, or 4. The same gain is applied to all the inputs
of each channel.
V
V
CM
1
+ V
Figure 36. Maximum Signal Levels, Current Channels, Gain = 1
2
V
COMMON-MODE
1
+ V
DIFFERENTIAL INPUT
± 25mV MAX
2
= 500mV MAX PEAK
V
CM
V
V
1
2
IAN, IBN,
OR ICN
IAP, IBP,
OR ICP
Rev. A | Page 19 of 68
Figure 37 shows the maximum signal levels on the voltage
channel inputs. The maximum common-mode signal is
±25 mV as shown in Figure 36.
The gain selections are made by writing to the gain register.
Bit 0 to Bit 1 select the gain for the PGA in the fully differential
current channel. The gain selection for the PGA in the single-
ended voltage channel is made via Bit 5 to Bit 6. Figure 38
shows how a gain selection for the current channel is made
using the gain register.
Figure 39 shows how the gain settings in PGA 1 (current
channel) and PGA 2 (voltage channel) are selected by various
bits in the gain register.
INTEGRATOR ENABLE
0 = DISABLE
1 = ENABLE
Bit 7 of the gain register is used to enable the digital integrator
in the current signal path. Setting this bit will activate the digital
integrator (see the di/dt Current Sensor and Digital Integrator
section).
PGA 2 GAIN SELECT
00 =
01 =
10 =
+500mV
–500mV
×
×
×
1
2
4
V
Figure 37. Maximum Signal Levels, Voltage Channels, Gain = 1
CM
*REGISTER CONTENTS SHOW POWER-ON DEFAULTS
V2
IAN, IBN, ICN
IAP, IBP, ICP
CURRENT AND VOLTAGE CHANNEL PGA CONTROL
Figure 39. ADE7758 Analog Gain Register
V
IN
Figure 38. PGA in Current Channel
7
0
SINGLE-ENDED INPUT
6
0
± 500mV MAX PEAK
GAIN[7:0]
COMMON-MODE
5
0
GAIN REGISTER*
± 25mV MAX
CURRENT INPUT FULL-SCALE SELECT
00 = 0.5V
01 = 0.25V
10 = 0.125V
4
0
K
×
V
IN
3
0
RESERVED
AGND
2
0
GAIN (K)
SELECTION
V2
V
CM
1
0
VAP, VBP,
OR VCP
V
0
0
PGA 1 GAIN SELECT
00 =
01 =
10 =
N
ADE7758
ADDRESS: 0x23
×
×
×
1
2
4