AN1300

AN1300
Designing with the MCP3901 Dual Channel
Analog-to-Digital Converters
Author:
Craig L. King and Vincent Quiquempoix
Microchip Technology Inc.
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 (AVDD)
and digital (DVDD) 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.
ID
IA
0.1 μF
AVDD
0.1 μF
VA
DVDD
VD
MCP3901
MCU
AGND DGND
IA
ID
“Star” Point
D-=
A-=
FIGURE 1:
All Analog And Digital Return
Paths Need to Stay Separate with Proper Bypass
Capacitors.
© 2009 Microchip Technology Inc.
DS01300A-page 1
AN1300
Figure 2 shows a more detailed example with
connection directly to a high voltage line ( e.g. a two
wire 120V or 220V system). A current sensing shunt is
used for the current measurement on the high side
(line side), and this also supplies the ground for the
system. This is necessary as the shunt is connected
directly to the channel input pins of the MCP3901. To
reduce sensitivity to external influences such as EMI,
these two wires should form a twisted pair, as noted in
Figure 2.
The power supply and MCU are separated on the right
hand side of the PCB, surrounded by the digital ground
plane. The MCP3901 is kept on the left hand side
surrounded by the analog ground plane. There are two
separate power supplies going to the digital section of
the system and the analog section, including the
MCP3901. You can see with this placement there are
two separate current supply paths and current return
paths, IA and ID.
Analog Ground Plane
IA
Digital Ground Plane
MCU
MCP3901
IA
ID
ID
VD VA
POWER SUPPLY
CIRCUITRY
Twisted
Pair
LINE
“Star” Point
SHUNT
NEUTRAL
FIGURE 2:
MCP3901 Design with
Proper Analog and Digital Grounding and Power
Supply Layout.
The ferrite bead between the digital and analog ground
planes helps keep high-frequency noise from entering
the device. They are also typically placed on the shunt
inputs and into the power supply circuit for additional
protection. For energy metering or power monitoring
applications like this, the “MCP3905A Energy Meter
Reference Design”, contains all of these approaches
and includes a complete schematic for shunt based
designs like this connected directly to the line [2].
CORRECT USE OF THE MCP3901
DITHERING BLOCK
The MCP3901 device includes a dithering algorithm
that reduces distortion and improves spurious free
dynamic range (SFDR) while maintaining a large
signal-to-noise ratio (SNR). Understanding the
principles behind dithering, the reasons for its
implementation, and the resulting effect on the ADC
performance allows you to determine which situations
require the dithering block to be active. The firmware
supplied with this application note includes the
following commands to activate this dithering block
which toggle control bits <7:6> in the CONFIG register:
DitherCH0(ON);
DitherCH1(ON);
Noise Reduction and Idle Tones
All analog-to-digital converters create noise during the
conversion process. Some of this noise is quantization
noise, due to the conversion process itself. Some of
this noise is repeated or correlated noise, i.e. distortion
created by the converter itself. With delta-sigma
analog-to-digital converters, there is a subset of this
distortion referred to as idle tones. These idle tones are
a by-product of the combined quantization effect and
high-frequency bit-stream output of the delta-sigma
modulator. These tones limit the performance of the
spurious free dynamic range of the device, increase the
harmonic distortion, and overall reduce the signal-tonoise and distortion level of the device. These idle
tones are signal dependent, their amplitude and
frequency depend on the input signal.
Dithering Principles and MCP3901
Implementation
By adding a noise like signal to the analog-to-digital
conversion, and then substracting this noise digitally,
correlated noise introduced by the ADC is reduced and
the overall performance of the device is increased. A
simplified block diagram of this concept is shown in
Figure 3:
Output
Input
+
+
ADC
+
Noise
FIGURE 3:
Concept.
DS01300A-page 2
Simplified Dithering
© 2009 Microchip Technology Inc.
AN1300
For the MCP3901 Sigma-Delta A/D converter, the
dithering concept is implemented by adding the noise
through a perturbation of the quantizer, creating
pseudo-random level shifts in the quantized output.
The noise is removed through the inherent noise
shaping of the feedback loop. The dither signal is
processed by the loop and high-pass filtered, and noise
shaped exactly like the quantization noise. This rejects
the perturbation while at the same time “scrambles” or
“de-correlates” the conversion process. The original
input signal of course is not effected. The MCP3901
implementation is shown in Figure 4.
It should be noted that the dithering is dynamic, and will
be removed for inputs nearing full scale. This will keep
the dithering algorithm from bringing the modulator into
an unstable region with signals near full scale; the
MCP3901 dithering algorithm is reduced to zero
additive dithering noise for large inputs. This ensures
proper stability of the loop.
Figure 5 shows the effect of dithering in the frequency
domain. Here we compare side by side two MCP3901
FFTs of a near full scale input signal (-0.5dB), with and
without dithering. The reduction of correlated noise or
spurs can be seen in the FFT with the dithering block
active.
Dither Noise
Loop
Filter
Differential
Voltage Input
Second
Order
Integrator
Quantizer Output
Bitstream
5-level
Flash ADC
DAC
MCP3901 Sigma-Delta Modulator
MCP3901 Dithering
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
-200
fIN = -0.5dBFS @ 60 Hz
fD = 3.9 ksps
OSR = 256
16384 points
Dithering OFF
0
500
1000
1500
Frequency (Hz)
Figure 5a: Without Dithering
2000
Amplitude (dB)
Amplitude (dB)
FIGURE 4:
Implementation.
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
-200
fIN = -0.5dBFS @ 60 Hz
fD = 3.9 ksps
16384 Point FFT
OSR = 256
Dithering ON
0
500
1000
1500
2000
Frequency (Hz)
Figure 5b: Dithering block active
FIGURE 5:
FFT Analysis Showing Effect of Dithering on Correlated Signals Or Spurs In The
Frequency Domain.
© 2009 Microchip Technology Inc.
DS01300A-page 3
AN1300
100
80
60
40
20
0
-20
-40
-60
-80
-100
-0.5
The repeatable bow that is present in the non-linearity
plot on the left is just another form of correlated noise
that can be reduced through the use of dithering. The
improvement of INL performance is illustrated in
Figure 6 (around 8X improvement when the
oversampling ratio (OSR) is 256).
OSR = 256
Dithering OFF
SCK = 8 MHz
Channel 0
Channel 1
-0.25
INL (ppm)
INL (ppm)
A direct effect of this reduction in correlated noise is an
improvement in the non-linearity error of the device.
Figure 6 shows a comparison of two integral nonlinearity plots, measured with and without the dithering
block active. INL is the equivalent of a distortion
measurement, but for a DC input signal. Since idle
tones occur largely for a DC input signal, they have
negative on the INL performance of the ADC.
0
0.25
0.5
50
40
30
20
10
0
-10
-20
-30
-40
-50
-0.5
OSR = 256
Dithering ON
SCK = 8 MHz
Channel 0
Channel 1
-0.25
Input Voltage (V)
Effect of Dithering on Non-Linearity.
THD is improved by about 20 dB, and the INL improves
by a factor of almost 8x.
Figure 7 shows that for the faster sampling speeds,
where OSR = 32 or 64, the converter does not have
enough time to recover from the uncorrelated noise
introduced by the dithering block. While at the same
time, for all sampling speeds, even at OSR = 32,
correlated noise is removed and the total harmonic
distortion is improved by around 20 dB.
Dithering Limits and Converter Speed
15
14
13
Dithering OFF
12
11
Dithering ON
10
9
8
32
64
128
256
Oversampling Ratio (OSR)
FIGURE 7:
DS01300A-page 4
Total Harmonic Distortion (dBc)
16
Effective Number of Bits
SINAD (dB)
The converter must have enough time to recover from
the noise introduced by dithering effect, else the
dithering will have a negative effect on the total signal
to noise and distortion ratio (SINAD). Dithering will
cause all distortion figures to be improved, while the
noise floor will be increased slightly. This trade-off is
shown in the following figure.
100
95
90
85
80
75
70
65
60
55
50
0.5
Figure 6b: Dithering block active
Figure 6a: Without Dithering
FIGURE 6:
0
0.25
Input Voltage (V)
0
-20
-40
-60
Dithering OFF
-80
-100
Dithering ON
-120
32
64
128
Oversampling Ratio (OSR)
256
Converter Speed and Dithering.
© 2009 Microchip Technology Inc.
AN1300
Summary - When to Use Dithering?
Configuring the Device
The basic trade-off is noise and distortion. Here we
refer to distortion as both harmonic and non-harmonic
distortion, measured respectively by the THD and
SFDR specifications of the device. As shown in the
figures presented in this application note, all distortion
figures will be improved while the noise floor will slightly
increase. So for your application, the question must be
answered which noise is more important to remove? If
there is correlated, repeated noise (distortion) is the
dominant noise source, dithering should be always
used. In some applications, such as energy metering or
power monitoring, there is plenty of time to post
process or average the signal after the analog-to-digital
conversion, inside the MCU. Averaging here will
reduce the noise floor and remove the uncorrelated
noise. However no amount of averaging will remove
the correlated noise, or the distortion, so for these type
of applications it is also recommended that the
dithering block be turned on.
The MCP3901 has many different configuration bits
and device settings that are controlled through three
internal registers, the CONFIG, STATUS/COM, and
GAIN register. These are all defined in the data sheet
and will not be discussed here. However, the firmware
drivers that have been created for this application note
simplify the control of all of these settings.
FIRMWARE DEVICE DRIVERS
The firmware for this application note was written for
the PIC24JF128GA010 device and is compatible with
the MCP3901 Evaluation Board for 16-bit MCUs. The
firmware was written in C language. As will be
described in this section there are numerous routines
in C available here to both configure the device, and to
be used for data retrieval. Please note that in addition
to the firmware for this application note, there is additional firmware available on the evaluation board web
page.
TABLE 1:
For example, to set the oversampling ratio (OSR) to
128, the following command is issued:
SetOSR(128);
Table 1 shows a complete list of the driver commands
available to configure the device. Please note the read
commands will be covered in a later section of this
application note and are not included in this list.
In addition to these individual bit setting commands,
there are also byte write commands that can be used
to set an entire device, if the time of writing and
configuring the device is an issue. This topic is
described in Configuring the Device Quickly and a
global firmware driver command for a write of this kind
is supplied.
DRIVER COMMANDS - DEVICE CONFIGURATION
Command
SetPRESCALE(1);
SetOSR(32);
Possible Settings
Description
1, 2, 4, 8
Sets the prescaler to 1/N where N is the setting
32, 64, 128, 255 (Note 1)
Sets the devices over-sampling ratio(OSR)
1, 2, 4, 8, 16, 32
Sets channel 0 and channel 1 gain, respectively
NONE, GROUPS, TYPES, ALL
Sets the address loop type
ExtVref(ON);
ON, OFF
External VREF setting
ExtCLK(ON);
ON, OFF
External Clock setting
DRHIZ(ON);
ON, OFF
Setting for the DR pin high impedance
DRLTY(ON);
ON, OFF
Setting for the DR pin latency
BOTH, CH0DR, CH1DR, LAG
Setting for DR when PHASE is adjusted
16, 24
Bit width of channel 0 reads
SetGain(16, 1);
AddressLoop(GROUPS);
DRPin(LAG);
WidthCH0(16);
16, 24
Bit width of channel 1 reads
DitherCH0(ON);
ON, OFF
Dithering block setting, channel 0
DitherCH1(OFF);
ON, OFF
Dithering block setting, channel 1
ShutdownADCs(ON, OFF);
ON, OFF
Shutdown mode for ch0 and ch0, respectively
ResetADCs(ON, OFF);
ON, OFF
Rest mode for CH0 and CH1, respectively
BoostADCs(ON, OFF;
ON, OFF
Current boost setting for CH0,CH1 respectively
ModulatorCH0(OFF);
ON, OFF
Modulator output setting for CH0
ModulatorCH1(OFF);
ON, OFF
Modulator output setting for CH1
WidthCH1(24);
Note 1:
An 8-bit wide unsigned integer is passed to SetOSR routine and '255' represents OSR = 256.
© 2009 Microchip Technology Inc.
DS01300A-page 5
AN1300
that the user can start writing the PHASE register and
finish with the CONFIG2 register in only one
communication (see Figure 8). The RESET<1:0> bits
are in the CONFIG2 register to allow to exit the soft
reset mode and have the whole part configured and
ready to run in only one command.
Configuring the Device Quickly
For designs that require fast device configuration for
perhaps data retrieval after POR, the following
sequence is given in the device data sheet for device
initialization after power-up. It is recommended to enter
into ADC reset mode for both ADCs just after power-up
because the desired MCP3901 register configuration
may not be the default one and in this case, the ADC
would output undesired data. Within the ADC reset
mode (RESET<1:0>=11), the user can configure the
whole part with a single communication. The write
commands increment automatically the address so
Here the following command driver is given:
Write3901Config(CONFIG1,CONFIG2);
Write3901All(PHASE, GAIN, STATUS, CONFIG1,
CONFIG2);
After these two commands are given, the device is
configured and proper ADC reads can occur.
AVDD
CS
SCK
SDI
00011000
CONFIG2 ADDR/W
11XXXXXX
CONFIG2
00001110
PHASE ADDR/W
Optional RESET of both ADCs
xxxxxxxx
xxxxxxxx
xxxxxxxx
PHASE
GAIN
STATUS/COM
xxxxxxxx
xxxxxxxx
CONFIG1
CONFIG2
One command for writing complete configuration
FIGURE 8:
Configuration Sequence Using Write3901Config(); & Write3901All(); Commands at
POR for Fastest Configuration.
Data Retrieval Drivers
The firmware supplied with this application note offers
data retrieval through the following methods:
1.
2.
Read individual channels at 16 or 24 bit widths,
returning the bytes individually.
Read the individual channels and return the
entire 24-bit word as a long data type for use in
a dataset.
The following commands are given for the first method:
ReadCH0_24(u8
ReadCH0_16(u8
ReadCH1_24(u8
ReadCH1_16(u8
*high, u8 *middle,u8 *low);
*high, u8 *middle,u8 *low);
*high, u8 *middle,u8 *low);
*middle,u8 *low);
long CalculateMean(long a[], long samplesize)
float CalculateRMSNoise(long a[], long
samplesize, long mean)
These two functions operate on data array a[], up to
“samplesize”.
The two values are then output on the demo board LCD
screen, and these two values are given simultaneously.
A flow chart of this is shown in Figure 9.
N
DR
External
Interrupt?
Reset
DR
Flag
Y
For applications that are not too time sensitive and just
concerned with a single sample or for both samples
after data ready, these commands may be sufficient.
If larger data sets are desired, this application offer the
ability to collect up to 1024 samples and put them in a
long data array for post-processing through the
following command:
MCP3901 (&ADCData,
DataSet, ArrayLocation
Arraylocation ++;
TotalSamples ++’
function CalculateMean();
Display on LCD
function CalculateNoise();
MCP3901(&ADCData, DataSet, ArrayLocation);
The command passes logical ADCData by reference, a
global array DataSet, and the current location in the
array.
FIGURE 9:
Supplied Data Set
Processing and Noise Analysis Routines.
In addition, the firmware supplied here performs two
functions on the data set, it calculates the mean, and
calculates the noise.
DS01300A-page 6
© 2009 Microchip Technology Inc.
AN1300
REFERENCES
[1]
MCP3901 Data Sheet, “Energy Metering IC with
SPI Interface and Active Power Output”,
Microchip Technology Inc., DS22025, ©2008.
[2]
“MCP3901 ADC Evaluation Board for 16-Bit
MCUs User's Guide”, Microchip Technology
Inc., DS51845. ©2009.
© 2009 Microchip Technology Inc.
DS01300A-page 7
AN1300
NOTES:
DS01300A-page 8
© 2009 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
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Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
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DS01300A-page 9
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Taiwan - Kaohsiung
Tel: 886-7-536-4818
Fax: 886-7-536-4803
China - Xiamen
Tel: 86-592-2388138
Fax: 86-592-2388130
Taiwan - Taipei
Tel: 886-2-2500-6610
Fax: 886-2-2508-0102
China - Xian
Tel: 86-29-8833-7252
Fax: 86-29-8833-7256
Thailand - Bangkok
Tel: 66-2-694-1351
Fax: 66-2-694-1350
Italy - Milan
Tel: 39-0331-742611
Fax: 39-0331-466781
Netherlands - Drunen
Tel: 31-416-690399
Fax: 31-416-690340
Spain - Madrid
Tel: 34-91-708-08-90
Fax: 34-91-708-08-91
UK - Wokingham
Tel: 44-118-921-5869
Fax: 44-118-921-5820
China - Zhuhai
Tel: 86-756-3210040
Fax: 86-756-3210049
03/26/09
DS01300A-page 10
© 2009 Microchip Technology Inc.
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