MCP3911 datasheet(25/72 Pages) MICROCHIP

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DS22286A-page 25

MCP3911

4.5

OSR - Oversampling Ratio

This is the ratio of the sampling frequency to the output

data rate. OSR= DMCLK/DRCLK. The default OSR is

256, or with MCLK = 4 MHz, PRESCALE = 1, AMCLK

= 4 MHz, fS = 1 MHz, fD = 3.90625 ksps. The following

bits in the CONFIG register are used to change the

oversampling ratio (OSR).

4.6

Offset Error

This is the error induced by the ADC when the inputs

are shorted together (VIN = 0V). The specification

incorporates both PGA and ADC offset contributions.

This error varies with PGA and OSR settings. The

offset is different on each channel and varies from chip

to chip. The offset is specified in µV. The offset error

can be digitally compensated independently on each

channel through the OFFCAL registers with a 24-bit

calibration word.

The offset on the MCP3911 has a low temperature

coefficient, see typical performance curves for more

information, Figure 2-33.

4.7

Gain Error

This is the error induced by the ADC on the slope of the

transfer function. It is the deviation expressed in %

compared to the ideal transfer function defined by

Equation 5-3. The specification incorporates both PGA

and ADC gain error contributions, but not the VREF

contribution (it is measured with an external VREF).

This error varies with PGA and OSR settings. The gain

error can be digitally compensated independently on

each channel through the GAINCAL registers with a

24-bit calibration word.

The gain error on the MCP3911 has a low temperature

coefficient; for more information, see Figure 2-34.

4.8

Integral Non-Linearity Error

Integral non-linearity error is the maximum deviation of

an ADC transition point from the corresponding point of

an ideal transfer function, with the offset and gain

errors removed, or with the end points equal to zero.

It is the maximum remaining error after calibration of

offset and gain errors for a DC input signal.

4.9

Signal-To-Noise Ratio (SNR)

For the MCP3911 ADCs, the signal-to-noise ratio is a

ratio of the output fundamental signal power to the

noise power (not including the harmonics of the signal),

when the input is a sinewave at a predetermined

frequency. It is measured in dB. Usually, only the

maximum signal to noise ratio is specified. The SNR

figure depends mainly on the OSR and DITHER

settings of the device.

EQUATION 4-3:

SIGNAL-TO-NOISE RATIO

4.10

Signal-To-Noise Ratio And

Distortion (SINAD)

The most important figure of merit for the analog

performance of the ADCs present on the MCP3911 is

the

Signal-to-Noise

And

Distortion

(SINAD)

specification.

Signal-to-noise and distortion ratio is similar to signal-

to-noise ratio, with the exception that you must include

the harmonics power in the noise power calculation.

The SINAD specification depends mainly on the OSR

and DITHER settings.

EQUATION 4-4:

SINAD EQUATION

The calculated combination of SNR and THD per the

following formula also yields SINAD:

EQUATION 4-5:

SINAD, THD, AND SNR

RELATIONSHIP

TABLE 4-3:

MCP3911 OVERSAMPLING

RATIO SETTINGS

CONFIG

OVER SAMPLING

RATIO

OSR

OSR<2:0>

000

001

010

128

256 (DEFAULT)

100

512

1024

2048

4096

SNR dB

()

SignalPower

NoisePower

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

⎝⎠

⎛⎞

log

SINAD dB

()

SignalPower

Noise

HarmonicsPower

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

⎝⎠

⎛⎞

log

SINAD dB

()

SNR

-----------

⎝⎠

⎛⎞

THD

–

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

⎝⎠

⎛⎞

log

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