MCP3913 Datasheet, PDF (25/82 Pages) Microchip Technology – 3V Six-Channel Analog Front End

4.15 Dithering
In order 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 in order to “decorrelate” the
outputs and “break” the idle tone’s behavior. Usually a
random or pseudo-random generator adds an analog
or digital error to the feedback loop of the delta-sigma
ADC in order to ensure that no tonal behavior can
happen at its outputs. This error is filtered by the feed-
back loop and typically has a zero average value, so
that the converter static transfer function is not dis-
turbed 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. 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 MCP3913 incor-
porates a proprietary dithering algorithm on all ADCs in
order to remove idle tones and improve THD, which is
crucial for power metering applications.
4.16 Crosstalk
Crosstalk is defined as the perturbation caused on one
ADC channel by all the other ADC channels present in
the chip. It is a measurement of the isolation between
each channel 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 all the other
ADCs at a certain predefined frequency.
Crosstalk is the ratio between the output power of the
ADC when the perturbation is and is not present,
divided by the power of the perturbation signal. A lower
crosstalk value implies more independence and
isolation between the channels.
The measurement of this signal is performed under the
default conditions of MCLK = 4 MHz:
• GAIN = 1
• PRESCALE = 1
• OSR = 256
• MCLK = 4 MHz
Step 1 for CH0 Crosstalk Measurement:
• CH0+ = CH0- = AGND
• CHn+ = CHn- = AGND
n comprised between 1 and 5
Step 2 for CH0 Crosstalk Measurement:
• CH0+ = CH0-=AGND
• CHn+ - CHn- = 1.2V_P-P @ 50/60 Hz (full-scale
sine wave), n comprised between 1 and 5
MCP3913
The crosstalk for Channel 0 is then calculated with the
formula in Equation 4-10.
EQUATION 4-10:
CTalkdB = 10log_^-----CC----HH-----0n---PP----oo---ww-----ee---rr_
The crosstalk depends slightly on the position of the
channels in the MCP3913 device. This dependency is
shown in the Figure 2-32, where the inner channels
show more crosstalk than the outer channels, since
they are located closer to the perturbation sources. The
outer channels have the preferred locations to
minimize crosstalk.
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 sine wave at a certain frequency with a certain
common mode). In AC, the amplitude of the sine wave
represents the change in the power supply. It is defined
in Equation 4-11.
EQUATION 4-11:
PSRRdB
=
20
log


^{-----AV----VO---}_D^-U---_DT^{--}
Where: V_OUT is the equivalent input voltage that the
output code translates to, with the ADC transfer
function.
In the MCP3913 specification for DC PSRR, AV_DD var-
ies from 2.7V to 3.6V, and for AC PSRR, a 50/60 Hz
sine wave is chosen centered around 3.0V, with a
maximum 300 mV amplitude. The PSRR specification
is measured with AV_DD = DV_DD.
4.18 CMRR
CMRR 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
sine wave at a certain frequency with a certain common
mode). In AC, the amplitude of the sine wave
represents the change in the power supply. It is defined
in Equation 4-12.
 2013 Microchip Technology Inc.
DS20005227A-page 25