MCP3910 datasheet(28/90 Pages) MICROCHIP

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MCP3910

DS20005116D-page 28

 2012-2020 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 pseudorandom 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’s 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 MCP3910

incorporates 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 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:

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 perturbation is and is not present, divided

by the power of the perturbation signal. A lower cross-

talk value implies more independence and isolation

between the two 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

• CH1+ = CH1- = AGND

Step 2 for CH0 Crosstalk Measurement:

• CH0+ = CH0- = AGND

• CH1+ – CH1- = 1.2 VP-P @ 50/60 Hz

(full-scale sine wave)

The crosstalk is then calculated with the formula in

Equation 4-10.

EQUATION 4-10:

The crosstalk slightly depends on the position of the

channels in the MCP3910 device.

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

takes 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 MCP3910 specification, AVDD varies 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 takes 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.

EQUATION 4-12:

Where VCM = (CHn+ + CHn-)/2 is the Common-mode

input voltage and VOUT is the equivalent input voltage

that the output code translates to, with the ADC transfer

function. In the MCP3910 specification, VCM varies

from -1V to +1V.

CTalk dB



CH0Power

CH1Power

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





log

PSRR dB



V

OUT

AV

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





log

CMRR dB



V

OUT

V

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





log

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