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© 2009 Microchip Technology Inc. DS01300A-page 1
AN1300
INTRODUCTION
The central goal of this application note is to supply
support material for a new MCP3901 design. Starting
with PCB layout techniques, getting the best
performance out of this device will be given for typical
applications through proper analog and digital
grounding.
Dithering is the second topic. The MCP3901 contains a
dithering block which can be used to increase the
performance of the A/D conversion under certain
situations. Understanding how dithering effects the
device and ultimately the application is important for
proper system design. Dithering results under different
MCP3901 configurations will be shown with measured
data.
The MCP3901 device contains an internal register set
with multiple configurations for the device. A
configuration approach to be used at power on reset
(POR) as discussed in the data sheet will be given here
in firmware, along with individual routines supplied in C
language for each device setting
[1]
. These firmware
routines act as a package firmware driver for all
MCP3901 designs; a total of 23 commands in the total
driver set are supplied in an accompanying firmware
zip file.
The MCP3901 Evaluation Board for 16-bit MCU’s was
used in the development of this application note, with
ordering number “MCP3901EV-MCU16”, available on
Microchip’s website. Please note that in addition to the
firmware for this application note, there is additional
firmware available on the MCP3901 device evaluation
board web page.
HARDWARE LAYOUT / PCB
GROUNDING
The MCP3901 is a mixed signal IC with both analog
and digital ports. For power, it has both analog (AV
DD
)
and digital (DV
DD
) pins. For grounding, it has both
analog and digital ground pins as well, labeled AGND
and DGND, respectively. A MCP3901 system will also
include a microcontroller or DSP. As the device has
been primarily designed for power and energy
measurement applications, direct connection to a
power line is also a likely hurdle in proper design.
In any system, the analog ICs such as references, or
operational amplifiers are always connected to the
analog ground plane. The MCP3901 should also be
considered as a sensitive analog component, and
connected to the analog ground plane. It is important to
understand that the pins AGND and DGND simply
define where the internal connections are going inside
the IC package. Externally, both of these pins should be
connected to the analog ground plane and kept away
from any digital components, power supply connection.
By this, it is meant that the analog circuitry (including
MCP3901) and digital circuitry (MCU) should have
separate power supplies and return paths to the
external ground reference, as described in Figure 1.
FIGURE 1: All Analog And Digital Return
Paths Need to Stay Separate with Proper Bypass
Capacitors.
Author: Craig L. King and Vincent Quiquempoix
Microchip Technology Inc.
V
A
V
D
MCU
“Star” Point
I
A
I
D
I
D
I
A
AV
DD
DV
DD
AGND
DGND
D
-
=
A
-
=
0.1 μF
MCP3901
0.1 μF
Designing with the MCP3901 Dual Channel
Analog-to-Digital Converters
