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LTC2480
2480fd
applicaTions inForMaTion
SDO remains high impedance and any externally gener-
ated SCK clock pulses are ignored by the internal data
out shift register.
In order to shift the conversion result out of the device,
CS must first be driven LOW. EOC is seen at the SDO pin
of the device once CS is pulled LOW. EOC changes in real
time from HIGH to LOW at the completion of a conversion.
This signal may be used as an interrupt for an external
microcontroller. Bit 23 (EOC) can be captured on the first
rising edge of SCK. Bit 22 is shifted out of the device on
the first falling edge of SCK. The final data bit (bit 0) is
shifted out on the falling edge of the 23rd SCK and may
be latched on the rising edge of the 24th SCK pulse. On
the falling edge of the 24th SCK pulse, SDO goes HIGH
indicating the initiation of a new conversion cycle. This
bit serves as EOC (bit 23) for the next conversion cycle.
Table 4 summarizes the output data format.
As long as the voltage on the IN+ and IN– pins is main-
tained within the –0.3V to (VCC + 0.3V) absolute maximum
operating range, a conversion result is generated for any
differential input voltage VIN from –FS = –0.5 VREF/GAIN
to +FS = 0.5 VREF/GAIN. For differential input voltages
greater than +FS, the conversion result is clamped to the
value corresponding to the +FS + 1LSB. For differential
inputvoltagesbelow–FS,theconversionresultisclamped
to the value corresponding to –FS – 1LSB.
Conversion Clock
A major advantage the delta-sigma converter offers over
conventional type converters is an on-chip digital filter
(commonly implemented as a SINC or Comb filter). For
high resolution, low frequency applications, this filter is
typicallydesignedtorejectlinefrequenciesof50Hzor60Hz
plus their harmonics. The filter rejection performance is
directly related to the accuracy of the converter system
clock.TheLTC2480incorporatesahighlyaccurateon-chip
oscillator. This eliminates the need for external frequency
setting components such as crystals or oscillators.
Frequency Rejection Selection (fO)
TheLTC2480internaloscillatorprovidesbetterthan110dB
normal mode rejection at the line frequency and all its
harmonics (up to the 255th) for 50Hz ±2% or 60Hz ±2%,
or better than 87dB normal mode rejection from 48Hz to
62.4Hz. The rejection mode is selected by writing to the
on-chip configuration register and the default mode at
POR is simultaneous 50Hz/60Hz rejection.
When a fundamental rejection frequency different from
50Hz or 60Hz is required or when the converter must be
synchronized with an outside source, the LTC2480 can
operate with an external conversion clock. The converter
automatically detects the presence of an external clock
signal at the fO pin and turns off the internal oscillator.
The frequency fEOSC of the external signal must be at least
10kHz to be detected. The external clock signal duty cycle
is not significant as long as the minimum and maximum
specifications for the high and low periods tHEO and tLEO
are observed.
While operating with an external conversion clock of a
frequency fEOSC, the LTC2480 provides better than 110dB
normal mode rejection in a frequency range of fEOSC/5120
±4% and its harmonics. The normal mode rejection as a
function of the input frequency deviation from fEOSC/5120
is shown in Figure 3.
DIFFERENTIAL INPUT SIGNAL FREQUENCY
DEVIATION FROM NOTCH FREQUENCY fEOSC/5120(%)
–12
–8
–4
0
4
8
12
NORMAL
MODE
REJECTION
(dB)
2480 F03
–80
–85
–90
–95
–100
–105
–110
–115
–120
–125
–130
–135
–140
Figure 3. LTC2480 Normal Mode Rejection When Using
an External Oscillator
Whenever an external clock is not present at the fO pin,
the converter automatically activates its internal oscilla-
tor and enters the internal conversion clock mode. The
LTC2480 operation will not be disturbed if the change of
conversion clock source occurs during the sleep state
or during the data output state while the converter uses