SBAS367F
–
JUNE 2007
–
REVISED FEBRUARY 2011
FUNCTIONAL DESCRIPTION
controlled.
Furthermore,
the
digital
filters
are
synchronized to start the convolution phase at the
The ADS1274/78 is a delta-sigma ADC consisting of
same modulator clock cycle. This design results in
four/eight
independent
converters
that
digitize
excellent phase match among the ADS1274/78
four/eight input signals in parallel.
channels.
The converter is composed of two main functional
shows the inter-device channel sample
blocks
to
perform
the
ADC
conversions:
the
matching for the ADS1274 and ADS1278.
modulator and the digital filter. The modulator
samples the input signal together with sampling the
The phase match of one 4-channel ADS1274 to that
reference voltage to produce a 1s density output
of another ADS1274 (eight or more channels total)
stream.
The
density
of
the
output
stream
is
may not have the same degree of sampling match.
proportional to the analog input level relative to the
As a result of manufacturing variations, differences in
reference voltage. The pulse stream is filtered by the
internal propagation delay of the internal CLK signal
internal digital filter where the output conversion
coupled with differences of the arrival of the external
result is produced.
CLK signal to each device may cause larger sampling
match errors. Equal length CLK traces or external
In operation, the input signal is sampled by the
clock distribution devices can be used to reduce the
modulator at a high rate (typically 64x higher than the
sampling match error between devices.
final output data rate). The quantization noise of the
modulator is moved to a higher frequency range
FREQUENCY RESPONSE
where
the
internal
digital
filter
removes
it.
Oversampling results in very low levels of noise
The digital filter sets the overall frequency response.
within the signal passband.
The filter uses a multi-stage FIR topology to provide
linear phase with minimal passband ripple and high
Since the input signal is sampled at a very high rate,
stop band attenuation. The filter coefficients are
input signal aliasing does not occur until the input
identical to the coefficients used in the
. The
signal frequency is at the modulator sampling rate.
oversampling ratio of the digital filter (that is, the ratio
This architecture greatly relaxes the requirement of
of the modulator sampling to the output data rate, or
external antialiasing filters because of the high
f
MOD
/f
DATA
) is a function of the selected mode, as
modulator sampling rate.
shown in
.
SAMPLING APERTURE MATCHING
Table 3. Oversampling Ratio versus Mode
The ADS1274/78 converters operate from the same
MODE SELECTION
OVERSAMPLING RATIO (f
MOD
/f
DATA
)
CLK input. The CLK input controls the timing of the
High-Speed
64
modulator
sampling
instant.
The
converter
is
High-Resolution
128
designed such that the sampling skew, or modulator
Low-Power
64
sampling
aperture
match
between
channels,
is
Low-Speed
64
©
2007
–
2011, Texas Instruments Incorporated
21
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