Note that no pull-up resistor is shown on the SCL line. In this
simple case, the resistor is not needed; the microcontroller
can simply leave the line on output, and set it to one or zero
as appropriate. It can do this because the ADS1100 never
drives its clock line low. This technique can also be used with
multiple devices, and has the advantage of lower current
consumption due to the absence of a resistive pull-up.
I2C Pull-Up Resistors
1kΩ to 10kΩ (typ.)
VDD
ADS1100A0
Microcontroller or
Microprocessor
with I2C Port
VIN+
VIN–
VDD
1
2
6
5
GND
SCL
SDA
SCL
SDA
3
4
If there are any devices on the bus that may drive their clock
lines low, the above method should not be used; the SCL line
should be high-Z or zero and a pull-up resistor provided as
usual. Note also that this cannot be done on the SDA line in
any case, because the ADS1100 does drive the SDA line low
from time to time, as all I2C devices do.
ADS1100A1
VIN+
VIN–
VDD
1
2
6
5
GND
Some microcontrollers have selectable strong pull-up circuits
built in to their GPIO ports. In some cases, these can be
switched on and used in place of an external pull-up resistor.
Weak pull-ups are also provided on some microcontrollers,
but usually these are too weak for I2C communication. If
there is any doubt about the matter, test the circuit before
committing it to production.
SCL
SDA
3
4
ADS1100A2
NOTE: ADS1100 power
and input connections
omitted for clarity.
VIN+
VIN–
VDD
1
2
6
5
GND
SCL
SDA
3
4
SINGLE-ENDED INPUTS
Although the ADS1100 has a fully differential input, it can
easily measure single-ended signals. A simple single-ended
connection scheme is shown in Figure 7. The ADS1100 is
configured for single-ended measurement by grounding ei-
ther of its input pins, usually VIN–, and applying the input
signal to VIN+. The single-ended signal can range from –0.2V
to VDD + 0.3V. The ADS1100 loses no linearity anywhere in
its input range. Negative voltages cannot be applied to this
circuit because the ADS1100 inputs can only accept positive
voltages.
FIGURE 5. Connecting Multiple ADS1100s.
USING GPIO PORTS FOR I2C
Most microcontrollers have programmable input/output pins
that can be set in software to act as inputs or outputs. If an
I2C controller is not available, the ADS1100 can be con-
nected to GPIO pins, and the I2C bus protocol simulated, or
bit-banged, in software. An example of this for a single
ADS1100 is shown in Figure 6.
VDD
ADS1100
VDD
Microcontroller or
Microprocessor
with I2C Port
VIN+
VIN–
1
2
6
5
GND
VDD
0V - VDD
ADS1100
Single-Ended
SCL
SDA
SCL
SDA
3
4
VIN+
VIN–
1
2
6
5
GND
VDD
Filter Capacitor
33pF to 100pF
(typ.)
SCL
SDA
3
4
Output
Codes
0-32767
NOTE: ADS1100 power
and input connections
omitted for clarity.
FIGURE 6. Using GPIO with a Single ADS1100.
FIGURE 7. Measuring Single-Ended Inputs.
Bit-banging I2C with GPIO pins can be done by setting the
GPIO line to zero and toggling it between input and output
modes to apply the proper bus states. To drive the line low,
the pin is set to output a zero; to let the line go high, the pin
is set to input. When the pin is set to input, the state of the
pin can be read; if another device is pulling the line low, this
will read as a zero in the port’s input register.
The ADS1100 input range is bipolar differential with respect
to the reference, i.e. ±VDD. The single-ended circuit shown in
Figure 7 covers only half the ADS1100 input scale because
it does not produce differentially negative inputs; therefore,
one bit of resolution is lost. The Burr-Brown DRV134 bal-
anced line driver from Texas Instruments can be employed
to regain this bit for single-ended signals.
ADS1100
SBAS239B
13
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BB [ BURR-BROWN CORPORATION ]
