Negative input voltages must be level-shifted. A good candi-
date for this function is the Texas Instruments THS4130
differential amplifier, which can output fully differential sig-
nals. This device can also help recover the lost bit noted
previously for single-ended positive signals. Level shifting
can also be performed using the DRV134.
WHEATSTONE BRIDGE SENSOR
The ADS1100 has a fully differential high-impedance input
stage and internal gain circuitry, which makes it a good
candidate for bridge-sensor measurement. An example is
shown in Figure 9.
LOW-SIDE CURRENT MONITOR
VDD
Figure 8 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 ADS1100.
Bridge
Sensor
E+
V–
V+
E–
11.5kΩ
ADS1100
5V
5V
V
VDD
VIN+
VIN–
1
2
6
5
FS = 0.63V
Load
GND
SCL
VDD
OPA335
(1)
R3
I2C
4.7µF
ADS1100
SDA
3
4
49.9kΩ
(2)
RS
1kΩ
–5V
G = 12.5
(PGA Gain = 8)
5V FS
NOTE: (1) Pull-down resistor to allow accurate swing to 0V.
(2) RS is sized for a 50mV drop at full-scale current.
I2C I/O
FIGURE 9. Measuring a Wheatstone Bridge Sensor.
FIGURE 8. Low-Side Current Measurement.
The Wheatstone bridge sensor is connected directly to the
ADS1100 without intervening instrumentation amplifiers; a
single, small input capacitor provides rejection of high-fre-
quency interference. The excitation voltage of the bridge is
the power supply, which is also the ADS1100 reference
voltage. The measurement is, therefore, ratiometric. In this
circuit, the ADS1100 would typically be operated at a gain of
8. The input range in this case is ±0.75 volts.
It is suggested that the ADS1100 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 0.75V. If the shunt resistor is sized to provide
a maximum voltage drop of 50mV at full-scale current, the
full-scale input to the ADS1100 is 0.63V.
ADS1100
14
SBAS239B
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