Cirrus Logic CS5508 User Manual
Page 20
If the CS5505/6/7/8 is operated at a clock rate
other than 32.768 kHz, the filter characteristics,
including the comb filter zeros, will scale with
the operating clock frequency. Therefore, opti-
mum rejection of line frequency interference will
o c c ur with th e CS5 50 5 /6 /7 /8 ru nn ing a t
32.768 kHz. The CS5505/6/7/8 can be used with
external clock rates from 30 kHz to 163 kHz.
Anti-Alias Considerations for Spectral
Measurement Applications
Input frequencies greater than one half the out-
put word rate (CONV = 1) may be aliased by
the converter. To prevent this, input signals
should be limited in frequency to no greater than
one half the output word rate of the converter
(when CONV =1). Frequencies close to the
modulator sample rate (XIN/2) and multiples
thereof may also be aliased. If the signal source
includes spectral components above one half the
output word rate (when CONV = 1) these com-
ponents should be removed by means of low-
pass filtering prior to the A/D input to prevent
aliasing. Spectral components greater than one
half the output word rate on the VREF inputs
(VREF+ and VREF-) may also be aliased. Fil-
tering of the reference voltage to remove these
spectral components from the reference voltage
is desirable.
Crystal Oscillator
The CS5505/6/7/8 is designed to be operated us-
ing a 32.768 kHz "tuning fork" type crystal. One
end of the crystal should be connected to the
XIN input. The other end should be attached to
XOUT. Short lead lengths should be used to
minimize stray capacitance. Figure 12 illustrates
the gate oscillator, and a simplified version of
the control logic used on the chip.
Over the industrial temperature range (-40 to
+85
°C) the on-chip gate oscillator will oscillate
Start
Conversion
D Q
CLK
D Q
CLK
Input
Mux
Decoder
4
3
2
1
T
Q
Modulator
Sample
Clock
XTL=32.768 kHz
A0
A1
CONV
CS5505/6
CAL
D Q
CLK
D Q
CLK
Start
Calibration
S Q
S Q
R
R
R
Channel A0 A1
1
1
0
0
1
0
1
0
10 M
Ω
22.5 pF
15 pF
gm ~~19 umho
XOUT
XIN
Figure 12. Gate Oscillator and Control Logic
CS5505/6/7/8
20
DS59F4
CS5505/6/7/8
20
DS59F7