Datasheet

 2012-2013 Microchip Technology Inc. DS20002286B-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, f
S
= 1 MHz, f
D
= 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 (V
IN
= 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 Section 2.0, Typical Performance
Curves for more information, see 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
percentage (%) compared to the ideal transfer function
defined by Equation 5-3. The specification
incorporates both PGA and ADC gain error
contributions but not the V
REF
contribution (it is
measured with an external V
REF
).
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 the 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 Oversampling Ratio
OSR
OSR<2:0>
000 32
001 64
010 128
011 256 (DEFAULT)
100 512
101 1024
110 2048
111 4096
SNR dB() 10
SignalPower
NoisePower
----------------------------------


log=
SINAD dB() 10
SignalPower
Noise HarmonicsPower+
---------------------------------------------------------------------


log=
SINAD dB() 10 10
SNR
10
-----------


10
THD–
10
----------------


+log=