MCP3918 datasheet(25/88 Pages) MICROCHIP

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DS20005287A-page 25

MCP3918

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,

with

MCLK = 4 MHz,

PRESCALE = 1,

AMCLK = 4 MHz, fS = 1 MHz, and fD = 3.90625 ksps.

The bits in Table 4-3, available in the CONFIG0

(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_CH0 register with a

24-bit calibration word.

The offset on the MCP3918 has a low temperature

coefficient.

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 in

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_CH0 register with

a 24-bit calibration word.

The gain error on the MCP3918 has a low temperature

coefficient.

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 MCP3918 ADC, the signal-to-noise ratio is a

ratio of the output fundamental signal power to noise

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

the input is a sine wave at a predetermined frequency

(see Equation 4-4). 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 4-4:

SIGNAL-TO-NOISE RATIO

4.10

Signal-to-Noise and Distortion

Ratio (SINAD)

The most important figure of merit for the analog

performance of the ADC present on the MCP3918 is

the

Signal-to-Noise

and

Distortion

(SINAD)

specification.

The 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

(see

Equation 4-5).

The

SINAD

specification depends mainly on the OSR and DITHER

settings.

EQUATION 4-5:

SINAD EQUATION

The calculated combination of SNR and THD per the

following formula also yields SINAD (see Equation 4-

6).

EQUATION 4-6:

SINAD, THD AND SNR

RELATIONSHIP

TABLE 4-3:

MCP3918 OVERSAMPLING

RATIO SETTINGS

CONFIG0

Oversampling Ratio

(OSR)

OSR<2:0>

000

001

010

128

011

256 (Default)

100

512

101

1024

110

2048

111

4096

SNR dB



SignalPower

NoisePower

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





log

SINAD dB



SignalPower

Noise

HarmonicsPower

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





log

SINAD dB



SNR

-----------





THD

–

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





log

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