CS5509 View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'CS5509' PDF : 23 Pages View PDF
CS5509
Figure 4. CS5509 Differential Nonlinearity Plot
AIN+
Vos
≤
100 mV
+
-
AIN-
Vos
≤
100 mV
+
-
15 pF
Internal
Bias
Voltage
15 pF
Figure 5. Analog Input Model
An equation for the maximum acceptable source
resistance is derived.
Rsmax = ----------------------------------------------------------–----1-----------------------------------------------------------
2XIN(15pF + CEXT) ln
-----------------------V----e------------------------
Ve
+
-1--5----p----F----(--1---0---0---m------V----)-
15pF + CEXT
This equation assumes that the offset voltage of the
buffer is 100 mV, which is the worst case. The val-
ue of Ve is the maximum error voltage which is ac-
ceptable. CEXT is the combination of any external
or stray capacitance.
For a maximum error voltage (Ve) of 10 µV in the
CS5509 (1/4LSB at 16-bits), the above equation in-
dicates that when operating from a 32.768 kHz
XIN, source resistances up to 110 kΩ are accept-
able in the absence of external capacitance
(CEXT=0).
The VREF+ and VREF- inputs have nearly the
same structure as the AIN+ and AIN- inputs.
Therefore, the discussion on analog input imped-
ance applies to the voltage reference inputs as well.
Digital Filter Characteristics
The digital filter in the CS5509 is the combination
of a comb filter and a low pass filter. The comb fil-
ter has zeros in its transfer function which are opti-
mally placed to reject line interference frequencies
(50 and 60 Hz and their multiples) when the
CS5509 is clocked at 32.768 kHz. Figures 6, 7 and
8 illustrate the magnitude and phase characteristics
of the filter. Figure 6 illustrates the filter attenua-
tion from dc to 260 Hz. At exactly 50, 60, 100, and
120 Hz the filter provides over 120 dB of rejection.
Table 2 indicates the filter attenuation for each of
the potential line interference frequencies when the
converter is operating with a 32.768 kHz clock.
The converter yields excellent attenuation of these
interference frequencies even if the fundamental
line frequency should vary ± 1% from its specified
frequency. The -3 dB corner frequency of the filter
when operating from a 32.768 kHz clock is 17 Hz.
Figure 8 illustrates that the phase characteristics of
the filter are precisely linear phase.
If the CS5509 is operated at a clock rate other than
32.768kHz, the filter characteristics, including the
comb filter zeros, will scale with the operating
clock frequency. Therefore, optimum rejection of
DS125F2
13
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