Datasheet

OUT MAX
1
ADC ADC
2
=
TEST
OUT MAX
REF
3.5 V
1
ADC ADC
2 V 2
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ADS1293
SNAS602B –FEBRUARY 2013–REVISED MARCH 2013 www.ti.com
Table 3. Channel 1 Switch Configuration
VBAT_MONI_CHx CALx POSx NEGx MODE
1 X X X CHx is in battery voltage monitoring mode
0 11 X X CHx input shorted
0 01 X X CHx input connected to positive reference
0 10 X X CHx input connected to negative reference
CHx positive input connected to pin INx and negative input connected to pin
0 00 INx INy
INy
Battery Monitoring
The battery voltage monitoring mode is enabled by setting bit VBAT_MONI_CHx = 1 in the FLEX_VBAT_CN
register. Also, the instrumentation amplifier of the selected channel must be shut down by setting
SHDN_INA_CHx = 1 in the AFE_SHDN_CN register. In this mode, the positive input, POSx, of the sigma-delta
modulator will sample the voltage supplied on the VDD pin. At the same time, the negative input, NEGx, of the
sigma-delta modulator will sample the reference voltage, V
REF
, generated on or provided to the CVREF pin. As a
result, the output signal of the sigma-delta modulator is a measure for (V
BAT
-V
REF
). In this operation, the sigma-
delta modulator works with a modified gain factor, and the battery voltage,V
BAT
, can be calculated as follows:
(1)
In this equation, V
REF
equals 2.4V if the internal reference voltage generator is used, and ADC
MAX
represents the
maximum output code of the ADC, which would correspond to a theoretical 2.4V signal at the input of the sigma-
delta modulator. The value of ADC
MAX
is dependent on the configuration of the digital filters, and the
corresponding ADC
MAX
values are listed in Table 5 through Table 8.
The battery monitoring mode is targeted for battery operated systems within a voltage range of 2.4V to 4.8V. The
battery monitoring mode cannot be used when the ADS1293 is powered from a regulated 5V supply because it
risks saturating the sigma-delta modulator. There is a also a low battery alarm that is implemented independently
from the battery monitoring mode, which will trigger a battery alarm when the supply voltage is below 2.7V (see
the BATLOW description in Alarm Functions).
Test Mode
If the battery voltage monitoring function is not enabled, and if bit TSTx = 01 (see the Input Channel Selection
Registers section), then a positive DC test signal is provided to the input of the instrumentation amplifier. If TSTx
= 10, then that same test signal is provided but with negative polarity. The expected ADC output code can be
calculated as follows:
(2)
In Equation 2, the positive or negative DC test signal V
TEST
= V
REF
/12. Note that this test mode is not a gain
calibration since V
TEST
and V
REF
are generated by the same reference; however, it can be used as a self-test or
to measure gain mismatches between channels.
When TSTx = 11, the inputs of the instrumentation amplifier in the channel can be shorted to provide a zero test
signal. The expected ADC output code equation can be simplified to:
(3)
For both equations, the value of ADC
MAX
corresponding to a given decimation configuration can be obtained from
Table 5 through Table 8.
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