Datasheet

 2012-2013 Microchip Technology Inc. DS20002286B-page 27
MCP3911
4.15 Dithering
To suppress or attenuate the idle tones present in any
Delta-Sigma ADCs, dithering can be applied to the
ADC. Dithering is the process of adding an error to the
ADC feedback loop to “decorrelate” the outputs and
“break” the idle tones behavior. Usually, a random or
pseudo-random generator adds an analog or digital
error to the feedback loop of the Delta-Sigma ADC to
ensure that no tonal behavior can happen at its outputs.
This error is filtered by the feedback loop and typically
has a zero average value, so that the converter static
transfer function is not disturbed by the dithering
process. However, the dithering process slightly
increases the noise floor (it adds noise to the part) while
reducing its tonal behavior and thus improving SFDR
and THD (see Figure 2-14 and Figure 2-18). The
dithering process scrambles the idle tones into
baseband white noise and ensures that dynamic specs
(SNR, SINAD, THD, SFDR) are less signal dependent.
The MCP3911 incorporates a proprietary dithering
algorithm on both ADCs to remove idle tones and
improve THD, which is crucial for power metering
applications.
4.16 Crosstalk
The crosstalk is defined as the perturbation caused by
one ADC channel on the other ADC channel. It is a
measurement of the isolation between the two ADCs
present in the chip.
This measurement is a two-step procedure:
1. Measure one ADC input with no perturbation on
the other ADC (ADC inputs shorted).
2. Measure the same ADC input with a
perturbation sine wave signal on the other ADC
at a certain predefined frequency.
The crosstalk is then the ratio between the output
power of the ADC when the perturbation is present and
when it is not divided by the power of the perturbation
signal.
A lower crosstalk value implies more independence
and isolation between the two channels.
The measurement of this signal is performed under the
default conditions at MCLK = 4 MHz:
•GAIN = 1
• PRESCALE = 1
• OSR = 256
•MCLK = 4MHz
Step 1
• CH0+=CH0- = A
GND
• CH1+=CH1- = A
GND
Step 2
• CH0+=CH0- = A
GND
• CH1+ - CH1- = 1.2V
P-P
at 50/60 Hz (Full-scale
sine wave)
The crosstalk is then calculated with the following
formula:
EQUATION 4-9:
4.17 PSRR
This is the ratio between a change in the power supply
voltage and the ADC output codes. It measures the
influence of the power supply voltage on the ADC
outputs.
The PSRR specification can be DC (the power supply
is taking multiple DC values) or AC (the power supply
is a sinewave at a certain frequency with a certain
common mode). In AC, the amplitude of the sinewave
is representing the change in the power supply. It is
defined in Equation 4-10:
EQUATION 4-10:
Where V
OUT
is the equivalent input voltage that the
output code translates to the ADC transfer function. In
the MCP3911 specification, AV
DD
varies from 2.7V to
3.6V. For AC PSRR, a 50/60 Hz sinewave is chosen,
centered around 3.3V with a maximum 300 mV
amplitude. The PSRR specification is measured with
AV
DD
= DV
DD
.
4.18 CMRR
This is the ratio between a change in the
common-mode input voltage and the ADC output
codes. It measures the influence of the common-mode
input voltage on the ADC outputs.
The CMRR specification can be DC (the
common-mode input voltage is taking multiple DC
values) or AC (the common-mode input voltage is a
sinewave at a certain frequency with a certain common
mode). In AC, the amplitude of the sinewave is
representing the change in the power supply. It is
defined in Equation 4-11:
EQUATION 4-11:
Where V
CM
= (CHn+ + CHn-)/2 is the common-mode
input voltage and V
OUT
is the equivalent input voltage
that the output code translates to using the ADC
transfer function. In the MCP3911 specification, VCM
varies from -1V to +1V.
CTalk dB() 10
Δ CH0Power
Δ CH1Power
---------------------------------


log=
PSRR dB() 20
Δ V
OUT
Δ AV
DD
-------------------


log=
CMRR dB() 20
Δ V
OUT
Δ V
CM
-----------------


log=