CS5501 Datasheet, PDF (24/54 Pages) Cirrus Logic – Non-aliasing, 16- & 20-bit A/D Converters

CS5501 CS5503
CS5501/CS5503
Anti-Alias Considerations
The digital filter in the CS5501/CS5503 does not
provide rejection around integer multiples of the
oversampling rate [(N*CLKIN)/256, where
N = 1,2,3,...]. That is, with a 4.096 MHz master
clock the noise on the analog input signal within
the narrow ±10 Hz bands around the 16 kHz,
32 kHz, 48 kHz, etc., passes unfiltered to the digi-
tal output. Most broadband noise will be very
well filtered because the CS5501/CS5503 use a
very high oversampling ratio of 800 (16 kHz:
2x10 Hz). Broadband noise is reduced by:
e_out = e_in √2f_−₃_d_B⁄f_s
e_out = 0.035 e_in
where e_in and e_out are rms noise terms referred to
the input. Since f_-3dB equals CLKIN/409,600 and
f_s equals CLKIN/256, the digital filter reduces
white, broadband noise by 96.5% independent of
the CLKIN frequency. For example, a typical op-
erational amplifier’s 50µV rms noise would be
reduced to 1.75µV rms (0.035 LSB’s rms at the
16-bit level in the CS5501 and 0.4 LSB’s rms at
the 20-bit level in the CS5503).
Simple high frequency analog filtering in the sig-
nal conditioning circuitry can aid in removing
energy at multiples of the sampling rate.
Bits of
Output
Accuracy
9
10
11
12
13
14
15
16
17
18
19
20
Filter
Cycles
340
356
389
435
459
475
486
495
500
504
506
507
CLKIN
Cycles
348,160
364,544
398,336
445,440
470,016
486,400
497,664
506,880
512,000
516,096
518,144
519,168
Table 5. Settling Time of the 6 Pole Low Pass Filter in
the CS5501 to 1/2 LSB Accuracy with a Full Scale
Step Input
2244
Post Filtering
Post filtering is useful to enhance the noise per-
formance of the CS5503. With a constant input
voltage the output codes from the CS5503 will
exhibit some variation due to noise. The CS5503
has typically 1.6 LSB₂₀ rms noise in its output
codes. Additional variation in the output codes
can arise due to noise from the input signal source
and from the voltage reference. Post filtering
(digital averaging) will be necessary to achieve
less than 1 LSB p-p noise at the 20-bit level. The
CS5503 has peak noise less than the 18-bit level
without additional filtering if care is exercised in
the design of the voltage reference and the input
signal condition circuitry. Noise in the bandwidth
from dc to 10 Hz on both the AIN and VREF
inputs should be minimized to ensure maximum
performance. As the amount of noise will be
highly system dependent, a specific recommenda-
tion for post filtering for all applications cannot be
stated. The following guidelines are helpful. Real-
ize that the digital filter in the CS5503, like any
other low pass filter, acts as an information stor-
age unit. The filter retains past information for a
period of time even after the input signal has
changed. The implication of this is that immedi-
ately sequential 20-bit updates to the serial port
contain highly correlated information. To most ef-
ficiently post filter the CS5503 output data,
uncorrelated samples should be used. Samples
which have sufficiently reduced correlation can be
obtained if the CS5503 is allowed to execute 200
filter cycles between each subsequent data word
collected for post filtering.
The character of the noise in the data will influ-
ence the post filtering requirements. As a general
rule, averaging N uncorrelated data samples will
reduce noise by 1/√N. While this rule assumes
that the noise is white (which is true for the
CS5503 but not true for all real system signals
between dc and 10Hz), it does offer a starting
point for developing a post filtering algorithm for
removing the noise from the data. The algorithm
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