Datasheet

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MCP3911

4.15 Dithering

In order to suppress, or attenuate, the idle tones pres-

ent 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 tones 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 filter 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 in order 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 = 4 MHz

Step 1

• CH0+=CH0-=AGND

• CH1+=CH1-=AGND

Step 2

• CH0+=CH0-=AGND

• CH1+ - CH1-=1.2V

P-P

@ 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 as:

EQUATION 4-10:

Where V

OUT

is the equivalent input voltage that the

output code translates to with the ADC transfer

function. In the MCP3911 specification, AV

varies

from 2.7V to 3.6V, and 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

= DV

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 as:

CTalk dB() 10

CH0Power

CH1Power

---------------------------------

⎝⎠

⎛⎞

log=

PSRR dB() 20

OUT

-------------------

⎝⎠

⎛⎞

log=