ADS1274
ADS1278
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SBAS367–JUNE 2007
FUNCTIONAL DESCRIPTION
controlled. Furthermore, the digital filters are
synchronized to start the convolution phase at the
same modulator clock cycle. This design results in
excellent phase match among the ADS1274/78
channels.
The ADS1274/78 is a delta-sigma ADC consisting of
four/eight independent converters that digitize
four/eight input signals in parallel.
The converter is composed of two main functional
blocks to perform the ADC conversions: the
modulator and the digital filter. The modulator
samples the input signal together with sampling the
reference voltage to produce a 1's density output
stream. The density of the output stream is
proportional to the analog input level relative to the
reference voltage. The pulse stream is filtered by the
internal digital filter where the output conversion
result is produced.
Figure 35 shows the inter-device channel sample
matching for the ADS1274 and ADS1278.
The phase match of one 4-channel ADS1274 to that
of another ADS1274 (eight or more channels total)
may not have the same degree of sampling match.
As a result of manufacturing variations, differences in
internal propagation delay of the internal CLK signal
coupled with differences of the arrival of the external
CLK signal to each device may cause larger sampling
match errors. Equal length CLK traces or external
clock distribution devices can be used to reduce the
sampling match error between devices.
In operation, the input signal is sampled by the
modulator at a high rate (typically 64x higher than the
final output data rate). The quantization noise of the
modulator is moved to a higher frequency range
where the internal digital filter removes it.
Oversampling results in very low levels of noise
within the signal passband.
FREQUENCY RESPONSE
The digital filter sets the overall frequency response.
The filter uses a multi-stage FIR topology to provide
linear phase with minimal passband ripple and high
stop band attenuation. The filter coefficients are
identical to the coefficients used in the ADS1271. The
oversampling ratio of the digital filter (that is, the ratio
of the modulator sampling to the output data rate, or
fMOD/fDATA) is a function of the selected mode, as
shown in Table 2.
Since the input signal is sampled at a very high rate,
input signal aliasing does not occur until the input
signal frequency is at the modulator sampling rate.
This architecture greatly relaxes the requirement of
external antialiasing filters because of the high
modulator sampling rate.
SAMPLING APERTURE MATCHING
Table 2. Oversampling Ratio versus Mode
The ADS1274/78 converters operate from the same
CLK input. The CLK input controls the timing of the
modulator sampling instant. The converter is
designed such that the sampling skew, or modulator
sampling aperture match between channels, is
MODE SELECTION
High-Speed
OVERSAMPLING RATIO (fMOD/fDATA)
64
128
64
High-Resolution
Low-Power
Low-Speed
64
21
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