MCP3919 Datasheet, PDF (58/88 Pages) Microchip Technology – 3V Three-Channel Analog Front End

MCP3919
8.2 Power Supply Design and
Bypassing
The MCP3919 device was designed to measure posi-
tive 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 per-
formance and good flexibility that characterize this
ADC enables them 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 interval.
In any system, the analog ICs (such as references or
operational amplifiers) are always connected to the
analog ground plane. The MCP3919 should also be
considered as a sensitive analog component and con-
nected to the analog ground plane. The ADC features
two pairs of pins: A_GND, AV_DD, D_GND and DV_DD. For
best performance, it is recommended to keep the two
pairs connected to two different networks (Figure 8-2).
This way, the design will feature two ground traces and
two power supplies (Figure 8-3).
This means the analog circuitry (including MCP3919)
and the digital circuitry (MCU) should have separate
power supplies and return paths to the external ground
reference, as described in Figure 8-2. An example of a
typical power supply circuit, with different lines for ana-
log and digital power, is shown in Figure 8-3. A possible
split example is shown in Figure 8-4, where the ground
star connection can be done at the bottom of the device
with the exposed pad. The split here between analog
and digital can be done under the device and AV_DD and
DV_DD can be connected together with lines coming
under the ground plane.
Another possibility, sometimes easier to implement in
terms of PCB layout is to consider the MCP3919 as an
analog component and, therefore, connect both AV_DD
and DV_DD together, and A_GND and D_GND together, with
a star connection. In this scheme, the decoupling
capacitors may be larger due to the ripple on the digital
power supply (caused by the digital filters and the SPI
interface of the MCP3919) now causing glitches on the
analog power supply.
I_D
I_A
0.1 μF 0.1 μF C
AV_DD DV_DD
V_A
V_D
MCU
MCP39XX
A_GND D_GND
I_D
I_A
D_-=
A_-=
“Star” Point
FIGURE 8-2:
All Analog and Digital
Return Paths Need to Stay Separate with Proper
Bypass Capacitors.
FIGURE 8-3:
Power Supply with Separate Lines for Analog and Digital Sections. Note the "Net Tie"
Object NT2 that Represents the Start Ground Connection.
DS20005347A-page 58
 2014 Microchip Technology Inc.