Datasheet
© 2011 Microchip Technology Inc. DS22192D-page 27
MCP3901
5.5 SINC
3
Filter
Both ADCs present in the MCP3901 include a
decimation filter that is a third-order sinc (or notch)
filter. This filter processes the multi-bit bitstream into
16 or 24-bit words (depending on the WIDTH
Configuration bit). The settling time of the filter is
3 DMCLK periods. It is recommended that unsettled
data be discarded 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 with 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. This will
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 given by the following equation:
EQUATION 5-2: MAGNITUDE OF
FREQUENCY RESPONSE
H(f)
or:
where:
Figure 5-3 shows the sinc filter frequency response:
FIGURE 5-3: SINC Filter Response with
MCLK = 4 MHz, OSR = 64, PRESCALE = 1.
TABLE 5-3: 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
DMCLK
----------------------
⋅
⎝⎠
⎛⎞
sin
c
π
f
DRCLK
--------------------
⋅
⎝⎠
⎛⎞
sin
----------------------------------------------
3
=
NMRR f()
c
π
f
f
S
----
⋅
⎝⎠
⎛⎞
sin
c
π
f
f
D
-----
⋅
⎝⎠
⎛⎞
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)