Datasheet

Amplitude(dB)
Frequency(Hz)
0
-20
-40
-60
-80
-100
-180
1 10
100 100k
1k 10k
-120
-140
-160
1HzResolution
V =20mV
IN DC
f
MOD
/2
MFLAG
100%FS
AINN
AINP
P
Q
IABSI
å
ThresholdTolerance: 2.5%Typical±
ADS1281
SBAS378D –AUGUST 2007–REVISED JUNE 2010 www.ti.com
The modulator is optimized for input signals within a If the inputs are sufficiently overdriven to drive the
4kHz passband. As Figure 21 shows, the noise modulator to full duty cycle, all 1s or all 0s, the
shaping of the modulator results in a sharp increase modulator enters a stable saturated state. The digital
in noise above 6kHz. The modulator has a chopped output code may clip to +FS or –FS, again depending
input structure that further reduces noise within the on the duration. A small duration overdrive may not
passband. The noise is moved out of the passband always clip the output code. When the input returns to
and appears at the chopping frequency (f
CLK
/512 = the normal range, the modulator requires up to 12
8kHz). The component at 6.5kHz is the tone modulator clock cycles (f
MOD
) to exit saturation and
frequency, shifted out of band by a 20mV offset. The return to the linear region. The digital filter requires an
frequency of the tone is proportional to the applied dc additional 62 conversion for fully settled data (linear
input and is given by V
IN
/0.003 (in kHz). phase FIR).
In the extreme case of over-range, either input is
overdriven exceeding that either analog supply
voltage plus an internal ESD diode drop. The internal
ESD diodes begin to conduct and the signal on the
input is clipped. If the differential input signal range is
not exceeded, the modulator remains in linear
operation. If the differential input signal range is
exceeded, the modulator is saturated but stable, and
outputs all 1s or 0s. When the input overdrive is
removed, the diodes recovery quickly and the
ADS1281 recovers as normal. Note that the linear
input range is ±100mV beyond the analog supply
voltages; with input levels above this, use care to limit
the input current to 100mA peak transient and 10mA
continuous.
Figure 21. Modulator Output Spectrum
MODULATOR OVER-RANGE DETECTION
(MFLAG)
The ADS1281 has a fast-responding over-range
MODULATOR OVER-RANGE
detection, indicating when the differential input
The ADS1281 modulator is inherently stable and,
exceeds approximately +100% or –100% full-scale.
therefore, has predictable recovery behavior that
The threshold tolerance is ±2.5%.The MFLAG output
results from an input overdrive condition. The
asserts high when in an over-range condition. As
modulator does not exhibit self-resetting behavior,
Figure 22 and Figure 23 illustrate, the absolute value
which often results in an unstable output data stream.
of the input is compared to 100% of range. The
output of the comparator is sampled at the rate of
The ADS1281 modulator outputs a 1s density data
f
MOD
/2, yielding the MFLAG output. The minimum
stream at 90% duty cycle with the positive full-scale
MFLAG pulse width is f
MOD
/2.
input signal applied (10% duty cycle with the negative
full-scale signal). If the input is overdriven past 90%
modulation, but below 100% modulation (10% and
0% for negative overdrive, respectively), the
modulator remains stable and continues to output the
1s density data stream. The digital filter may or may
not clip the output codes to +FS or –FS, depending
on the duration of the overdrive. When the input is
returned to the normal range from a long duration
overdrive (worst case), the modulator returns
Figure 22. Modulator Over-Range Block Diagram
immediately to the normal range, but the group delay
of the digital filter delays the return of the conversion
result to within the linear range (31 readings for linear
phase FIR). 31 additional readings (62 total) are
required for completely settled data.
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