MCP3912 datasheet(26/82 Pages) MICROCHIP

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MCP3912

DS20005348A-page 26

 2014 Microchip Technology Inc.

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 MCP3912 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:

Measure one ADC input with no perturbation on

the other ADC (ADC inputs shorted).

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 3

Step 2 for CH0 Crosstalk Measurement:

• CH0+ = CH0-=AGND

• CHn+ - CHn- = 1.2VP-P @ 50/60 Hz (full-scale

sine wave), n comprised between 1 and 3

The crosstalk for Channel 0 is then calculated with the

formula in Equation 4-10.

EQUATION 4-10:

The crosstalk depends slightly on the position of the

channels in the MCP3912 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 influ-

ence 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:

Where: VOUT is the equivalent input voltage that the

output code translates to, with the ADC transfer

function.

In the MCP3912 specification for DC PSRR, AVDD 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 AVDD = DVDD.

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

(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.

CTalk dB



CH0Power

CHnPower

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





log

PSRR dB



V

OUT

AV

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





log

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