ADS1259
SBAS424C –JUNE 2009–REVISED MARCH 2010
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MODULATOR OVERLOAD BEHAVIOR
ADS1259 recovers as normal. Note that the linear
input range is ±100mV beyond the analog supply
voltages; with input levels greater than this range,
use care to limit the input current to 100mA peak
transient (10mA continuous).
The ADS1259 modulator is inherently stable and
therefore has predictable recovery behavior resulting
from an input overdrive condition. The modulator
does not exhibit the self-resetting behavior of other
modulator types, which often results in unstable
output conversion results when overdriven.
INPUT OUT-OF-RANGE DETECTION (FLAG)
The ADS1259 has a fast-responding out-of-range
circuit that triggers when the differential input exceeds
+105% or –105% of FSR (±1.05 VREF). The
out-of-range circuit latches the result of the
comparator output and appends the result as either
the LSB of conversion data or as bit 7 of the data
checksum byte. After the conversion data are read, or
after a new conversion is started, the comparator
latch is reset. Figure 33 and Figure 34 show the
detection block diagram and the detection operation,
respectively. See the Data Checksum Byte and FLAG
Bit section for more detail.
The ADS1259 modulator outputs a 1s density data
stream at 90% duty cycle with the positive full-scale
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
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 (one reading for the
sinc1 filter and two readings for completely settled
data).
AINP
å
IABSI
J
1.05 VREF
AINN
Q
fMOD/2
FLAG
K
Data Read
Reset
If the inputs are sufficiently overdriven to drive the
modulator to full duty cycle (that is, all 1s or all 0s),
the modulator enters a stable saturated state. The
digital output code may clip to +FS or –FS, again
depending on the duration. A small duration overdrive
may not always clip the output code. When the input
returns to the normal range, the modulator requires
up to 12 modulator clock cycles (fMOD) to exit
saturation and return to the linear region. The digital
filter requires two additional conversions (sinc1, more
for sinc2) for fully settled data.
Figure 33. Input Out-Of-Range Detect Block
Diagram
+105
(Conversions)
0
-105
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
1
FLAG
Bit
0
Figure 34. Input Out-Of-Range Detect Operation
14
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