MCP3903 datasheet(21/54 Pages) MICROCHIP

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DS25048B-page 21

MCP3903

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 six ADCs

present in the chip.

This measurement is a two-step procedure:

Measure one ADC input with no perturbation on

any other ADC (ADC inputs shorted).

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 six channels.

The measurement of this signal is performed under the

following conditions:

•GAIN = 1,

• PRESCALE = 1,

• OSR = 256,

• MCLK = 4 MHz

Step 1

• CH0+=CH0-=AGND

• CHn+=CHn-=AGND, n different than 0

Step 2

• CH0+=CH0-=AGND

• CHn+ - CHn-=1VP-P @ 50/60 Hz (Full-scale sine

wave)

The crosstalk is then calculated with the following

formula:

EQUATION 4-10:

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 4-11:

Where VOUT is the equivalent input voltage that the

output code translates to with the ADC transfer

function. In the MCP3903 specification, AVDD varies

from 4.5V to 5.5V, and for AC PSRR a 50/60 Hz

sinewave is chosen, centered around 5V with a

maximum 500 mV amplitude. The PSRR specification

is measured with

DVDD = 3.3V.

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

(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:

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 MCP3903 specification, VCM varies

from -1V to +1V, and for AC specification a 50/60 Hz

sinewave is chosen centered around 0V with a 500 mV

amplitude.

4.19

ADC Reset Mode

ADC Reset mode (called also soft reset mode) can only

be entered through setting high the RESET<5:0> bits in

the configuration register. This mode is defined as the

condition where the converters are active but their

output is forced to 0.

The registers are not affected in this reset mode and

retain their values.

The ADCs can immediately output meaningful codes

after leaving reset mode (and after the sinc filter settling

time of 3/DRCLK). This mode is both entered and

exited through setting of bits in the configuration

Each converter can be placed in soft reset mode

independently. The configuration registers are not

modified by the soft reset mode.

CTalk dB

()

ΔCH0Power

ΔCHnPower

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

⎝⎠

⎛⎞

log

PSRR dB

()

ΔV

OUT

ΔAV

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

⎝⎠

⎛⎞

log

CMRR dB

()

ΔV

OUT

ΔV

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

⎝⎠

⎛⎞

log

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