Datasheet

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MCP3901

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

following conditions:

•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 V

P-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 V

OUT

is the equivalent input voltage that the

output code translates to with the ADC transfer

function. In the MCP3901 specification, AV

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 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:

EQUATION 4-12:

Where V

= (CHn+ + CHn-)/2 is the common-mode

input voltage and V

OUT

is the equivalent input voltage,

the output code is translated to the ADC transfer

function. In the MCP3901 specification, VCM varies

from -1V to +1V, and for the 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 (also called Soft Reset mode) can

only be entered through setting the RESET<1:0> bits

high 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 the setting of bits in the

Configuration register.

CTalk dB() 10

CH0Power

CH1Power

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

⎝⎠

⎛⎞

log=

PSRR dB() 20

OUT

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

⎝⎠

⎛⎞

log=

CMRR dB() 20

OUT

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

⎝⎠

⎛⎞

log=