MCP3918 Datasheet, PDF (59/88 Pages) Microchip Technology – 3V Single-Channel Analog Front End

MCP3918
8.0 BASIC APPLICATION
CONFIGURATION
One of the main applications for the MCP3918 is
energy/power measurement in systems where the
ADC sampling the current needs to be isolated from the
rest of the design. Figure 8-1 can be used as a starting
point for MCP3918 applications.
For power measurements, since MCP3918 is a
1-channel ADC, it is recommended to use it for current
samples acquisition and to use the MCU ADC for
voltage samples acquisition.
The isolator used between MCU and ADC needs to be
fast enough to support the high-speed clock between
MCU and ADC and the data coming from ADC to MCU.
This is typically the case in a polyphase shunt-based
power/energy metering or monitoring application. In
this case, each phase needs to be isolated from the
rest of the design and, since the sensor is not providing
this isolation, the isolation needs to be provided at the
output of the analog front-end.
The MCP3918 device is built to work seamlessly with a
large variety of two-channel unidirectional digital
isolators (opto-couplers, capacitive or inductive
integrated digital isolators with or without embedded
power supplies).
A_3.3A
L2
A_GNDA
A_3.3D
U18
MCP1754-3.3V
2 _{VOUT VIN} 3
C57
0.1uF
U7
VBT1-S5-S5
5
C63
+Vout +Vin
4.7uF DC DC
4 _{-Vout -Vin}
5V
2
C22
4.7uF
1
A_GNDD
NT3
GND
A_3.3D
C60
0.1uF
0603
A_GNDD
A_3.3D
U21
3910A_SDO
3910A_CLKIN
8
7
6
5
VDD2VDD1
VIA VOA
VOB VIB
GND2GND1
FOD8012
A_GNDD
3.3D
1
2
3
4
3910A_SDO_MCU/RC3
3910_CLKIN_MCU_A
3.3D
C68
0.1uF
0603
GND
GND
A_3.3D A_GNDD
R80
1k
HIGH
J24
OSR0
321
LOW
CP1
Via_1.6x1
LINE_SHUNT1
A_GNDA
R3
DNP
FB2
LINE_SHUNT2
CP2
Via_1.6x1
R6
FB3
DNP
A_GNDA
A_GNDA
C1
0.1uF
R4
1k
R5
1k
C5
0.1uF
A_GNDA
A_3.3A
A_GNDA
A_3.3D
R100
10
C43 R76
0.1uF 10
A_GNDD
C52
0.1uF
A_GNDA
A_GNDA
A_GNDA
C46
0.1uF
U3
1 _{RESET / OSR0}
2 _DVDD
3 _AVDD
4 _CH0+
5 _CH0-
6 _NC
7 _NC
8 _AGND
9 _RFIN/OUT+
10 _RFIN-
SDI / OSR1
SDO
SCK / MCLK
CS / BOOST
OSC2 / MODE
OSC1/CLKI
DR / GAIN1
MDAT0
MDAT1
DGND
20
19 R86
10
18 R87
10
17
16 R79
1k
15
14
13
12
11
HIGH
J25
OSR1
321
LOW
3910A_SDO
3910A_CLKIN
A_3.3D
BOOST
HIGH 3 2 1 LOW
J26
HIGH
J27
GAIN0
321
LOW
HIGH
J28
GAIN1
321
LOW
MCP3918A1T/ISS
A_GNDA A_GNDA
A_GNDD
FIGURE 8-1:
Phase.
PHASE A
MCP3918 Three-Phase Shunt Energy Meter – Typical Application Schematic for Each
8.1 Power Supply Design and
Bypassing
To power the isolated ADC, an isolated DC/DC
converter that can be embedded with the isolated data
communication channels (as in Figure 8-1) or other
structures that provide isolated power supplies (e.g.,
fly-back converter) can be used.
For single-phase designs where isolation between
ADC and MCU is not required, the SPI connection is
also available. This SPI interface could also be used
with isolators but this would require four isolators
instead of two (for the 2-wire mode) and, therefore, this
configuration is not preferred.
8.2 Power Supply Design and
Bypassing
The MCP3918 device was designed to measure
positive and negative voltages that might be generated
by a current-sensing device. This current-sensing
device, with a common-mode voltage close to 0V, is
referred to as A_GND, which is a shunt or current
transformer (CT) with burden resistors attached to
ground.
The high performance and good flexibility that
characterize this ADC enable it to be used in other
applications, as long as the absolute voltage on each
pin, referred to A_GND, stays in the -1V to +1V range.
 2014 Microchip Technology Inc.
DS20005287A-page 59