MCP3912 datasheet(24/82 Pages) MICROCHIP

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MCP3912

DS20005348A-page 24

 2014 Microchip Technology Inc.

4.5

OSR – Oversampling Ratio

This is the ratio of the sampling frequency to the output

data rate; OSR = DMCLK/DRCLK. The default

OSR<2:0>

256,

with

MCLK = 4 MHz,

PRESCALE = 1, AMCLK = 4 MHz, fS = 1 MHz and

fD = 3.90625 ksps. The OSR<2:0> bits in Table 4-3 in

the CONFIG0 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_CHn registers with a

24-bit calibration word.

The offset on the MCP3912 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 a per-

centage, 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_CHn registers

with a 24-bit calibration word.

The gain error on the MCP3912 has a low temperature

coefficient.

4.8

Integral Nonlinearity Error

Integral nonlinearity 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 MCP3912 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 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 Ratio And

Distortion (SINAD)

The most important Figure of Merit for analog

performance of the ADCs present on the MCP3912 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 harmonic’s 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:

MCP3912 OVERSAMPLING

RATIO SETTINGS

OSR<2:0>

Oversampling Ratio

OSR

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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