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SBAS282D − JUNE 2003 − REVISED MARCH 2004
SINGLE-ENDED INPUTS
Although the ADS1112 has two differential inputs, it can
easily measure three single-ended signals. A single-
ended connection scheme is shown in Figure 8. The
ADS1112 is configured for single-ended measurement by
grounding the AIN3 pin and applying the input signals to
any of AIN0, AIN1, or AIN2. Then the data is read out of
one of the inputs based on the selection on the
configuration register. The single-ended signal can range
from 0V to 2.048V. The ADS1112 loses no linearity
anywhere in its input range. Negative voltages cannot be
applied to this circuit because the ADS1112 can only
accept positive voltages.
Figure 9. Low-Side Current Measurement
It is suggested that the ADS1112 be operated at a gain of
8. The gain of the OPA335 can then be set lower. For a gain
of 8, the op amp should be set up to give a maximum output
voltage of no greater than 0.256V. If the shunt resistor is
sized to provide a maximum voltage drop of 50mV at
full-scale current, the full-scale input to the ADS1112 is
0.2V.
The ADS1112 is fabricated in
a small-geometry,
low-voltage process. The analog inputs feature protection
diodes to the supply rails. However, the current-handling
ability of these diodes is limited, and the ADS1112 can be
permanently damaged by analog input voltages that
remain more than approximately 300mV beyond the rails
for extended periods. One way to protect against
overvoltage is to place current-limiting resistors on the
input lines. The ADS1112 analog inputs can withstand
momentary currents of as large as 10mA.
Figure 8. Measuring Single-Ended Inputs
The ADS1112 input range is bipolar differential with
respect to the reference, that is, 2.048V. The single-ended
circuit shown in Figure 8 covers only half the ADS1112
input scale because it does not produce differentially
negative inputs; therefore, one bit of resolution is lost. If
AIN3 is set to a higher voltage, negative single-ended
voltage can be measured.
The previous paragraph does not apply to the I2C ports,
which can both be driven to 6V regardless of the supply.
If the ADS1112 is driven by an op amp with high-voltage
supplies, such as 12V, protection should be provided,
even if the op amp is configured so that it does not output
out-of-range voltages. Many op amps seek to one of the
supply rails immediately when power is applied, usually
before the input has stabilized; this momentary spike can
damage the ADS1112. This incremental damage results in
slow, long-term failure—which can be disastrous for
permanently installed, low-maintenance systems.
LOW-SIDE CURRENT MONITOR
Figure 9 shows a circuit for a low-side shunt-type current
monitor. The circuit reads the voltage across a shunt
resistor, which is sized as small as possible while still
giving a readable output voltage. This voltage is amplified
by an OPA335 low-drift op amp, and the result is read by
the ADS1112.
If an op amp or other front-end circuitry is used with the
ADS1112, its performance characteristics must be taken
into account.
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