Datasheet

© 2011 Microchip Technology Inc. DS25048B-page 25
MCP3903
5.4 SINC
3
Filter
All ADCs present in the MCP3903 include a decimation
filter that is a third-order sinc (or notch) filter. This filter
processes the multi-bit bitstream into 16 or 24 bits
words (depending on the WIDTH configuration bit). The
settling time of the filter is 3 DMCLK periods. It is
recommended to discard unsettled data to avoid data
corruption which can be done easily by setting the
DR_LTY bit high in the STATUS/COM register.
The resolution achievable at the output of the sinc filter
(the output of the ADC) is dependant on the OSR and
is summarized in the following table:
For 24 -bit output mode (WIDTH = 1), the output of the
sinc filter is padded with least significant zeros for any
resolution less than 24 bits.
For 16-bit output modes, the output of the sinc filter is
rounded to the closest 16-bit number in order to
conserve only 16-bit words and to minimize truncation
error.
The gain of the transfer function of this filter is 1 at each
multiple of DMCLK (typically 1 MHz) so a proper
anti-aliasing filter must be placed at the inputs to
attenuate the frequency content around DMCLK, and
keep the desired accuracy over the baseband of the
converter. This anti-aliasing filter can be a simple
first-order RC network, with a sufficiently low time
constant to generate high rejection at DMCLK
frequency.
EQUATION 5-1: SINC FILTER TRANSFER
FUNCTION H(Z)
Where:
The Normal-Mode Rejection Ratio (NMRR), or gain of
the transfer function, is shown in the following equation:
EQUATION 5-2: MAGNITUDE OF
FREQUENCY RESPONSE
H(f)
or:
where:
Figure 5-2 shows the sinc filter frequency response:
FIGURE 5-2: SINC Filter Response with
MCLK = 4 MHz, OSR = 64, PRESCALE = 1.
TABLE 5-2: ADC RESOLUTION VS. OSR
OSR<1:0> OSR
ADC
Resolution
(bits)
No Missing
Codes
00 32 17
01 64 20
10 128 23
11 256 24
Hz()
1z
OSR–
–
OSR 1 z
1–
–()
---------------------------------
⎝⎠
⎜⎟
⎛⎞
3
=
z
2πfj
DMCLK
----------------------
⎝⎠
⎛⎞
exp=
NMRR f()
c π
f
DRCLK
--------------------⋅
⎝⎠
⎛⎞
sin
c
π
f
DMCLK
----------------------⋅
⎝⎠
⎛⎞
sin
----------------------------------------------
3
=
NMRR f()
c π
f
f
D
-----⋅
⎝⎠
⎛⎞
sin
c π
f
f
S
----⋅
⎝⎠
⎛⎞
sin
-----------------------------
3
=
cx()sin
x()sin
x
---------------=
-120
-100
-80
-60
-40
-20
0
20
1 10 100 1000 10000 100000 1000000
Input Frequency (Hz)
Magnitude (dB)