Datasheet

ADS1158
www.ti.com SBAS429D –JUNE 2008– REVISED MARCH 2011
EXTERNAL MULTIPLEXER LOOP Use of the switch time delay register reduces the
effective channel data rate. Table 4 shows the actual
The external multiplexer loop consists of two
data rates derived from Equation 1, when using the
differential multiplexer output pins and two differential
switch time delay feature.
ADC input pins. The user may use external
components (buffering/filtering, single-ended to When pulse converting, where one channel is
differential conversion, etc.) to form a signal converted with each START pin pulse or each pulse
conditioning loop. For best performance, the ADC command, the application software may provide the
input should be buffered and driven differentially. required time delay between pulses. However, with
Chop = 1, the switch time delay feature may continue
To bypass the external multiplexer loop, connect the
to be necessary to allow for settling.
ADC input pins directly to the multiplexer output pins,
or select internal bypass connection (BYPASS = 0 of In estimating the time delay that may be required,
CONFIG0). Note that the multiplexer output pins are Table 5 lists the time delay-to-time constant ratio (t/τ)
active regardless of the bypass setting. and the corresponding final settled data in % and
number of bits.
SWITCH TIME DELAY
Table 5. Settling Time
When using the ADS1158 in the Auto-Scan mode,
FINAL SETTLING FINAL SETTLING
where the converter automatically switches from one
t/τ
(1)
(%) (Bits)
channel to the next, the settling time of the external
1 63 2
signal conditioning circuit becomes important. If the
channel does not fully settle after the multiplexer 3 95 5
channel is switched, the data may not be correct. The
5 99.3 7
ADS1158 provides a switch time delay feature which
7 99.9 10
automatically provides a delay after channel switching
10 99.995 14
to allow the channel to settle before taking a reading.
15 99.998 16
The amount of time delay required depends primarily
on the settling time of the external signal conditioning.
(1) Multiple time constants can be approximated by:
Additional consideration may be needed to account
(τ
1
2
+ τ
2
2
+…).
for the settling of the input source arising from the
transient generated from channel switching.
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