ADS1259
SBAS424C –JUNE 2009–REVISED MARCH 2010
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ALIASING
CLOCK SOURCE
The low-pass characteristic of the digital filter repeats
at multiples of the modulator rate (fMOD = fCLK/8).
Figure 42 shows the responses plotted out to
7.3728MHz at the data rate of 14.4kSPS. Notice how
the responses near dc, 0.9216MHz, 1.8432MHz,
2.7698MHz, etc, are the same as given by f =
NfMOD ± fDATA where N = 0, 1, 2, etc. The digital filter
attenuates high-frequency noise on the ADS1259
inputs up to the frequency where the response
repeats. However, noise or frequency components
existing in the signal where the response repeats
alias into the passband. Often, a simple RC antialias
filter is sufficient to reject these input frequencies.
There are three ways to provide the ADS1259 clock:
the internal oscillator, an external clock, or an
external crystal/ceramic resonator. The ADS1259
selects the clock source automatically. Figure 43
shows the clock select block. If either external clock
sources are present, the internal oscillator is disabled
and the external clock source is selected. If no
external clock is present, the internal oscillator is
selected. The ADS1259 continuously monitors the
clock source. The clock source can be polled by the
EXTCLK bit (bit 6 of register CONFIG2), 0 = internal
oscillator, 1 = external clock.
The data rate and corresponding filter notches scale
by the accuracy of clock frequency. Consideration
should be given to the clock accuracy and the
corresponding effect to the notch frequency locations.
0
-20
-40
-60
Clock
Detect
XTAL1/CLKIN
-80
ENB
Internal
Oscillator
-100
-120
-140
XTAL2
S0
S1
SEL
MUX
System Clock
0
0.9
1.8
2.8
3.7
4.6
5.5
6.5
7.4
Frequency (MHz)
Figure 43. Equivalent Circuitry of the Clock
Source
Figure 42. Frequency Response to 7.3728MHz
(Data Rate = 14400SPS)
Internal Oscillator
Figure 44 shows the internal oscillator connection.
XTAL1/CLKIN is grounded and XTAL2 is not
connected (floating). The internal oscillator draws
approximately 40mA from the DVDD supply. Note that
the internal oscillator has ±2% accuracy over
temperature. The oscillator accuracy has
a
corresponding effect on line-cycle notch frequency
locations.
XTAL1/CLKIN
XTAL2
Figure 44. Internal Oscillator Connection
20
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