TP2111/TP2111N/TP2112/TP2114
Nanopower 300nA, 1.8V, Rail-to-Rail Input/Output Op-amps
Buffered Chemical Sensor (pH) Probe
The TP211x OPA has input bias current in the fA range. This is ideal in buffering high impedance chemical sensors
such as pH probe. As an example, the circuit in Figure 5 eliminates expansive low-leakage cables that that is
required to connect pH probe to metering ICs such as ADC, AFE and/or MCU. A TP211x OPA and a lithium battery
are housed in the probe assembly. A conventional low-cost coaxial cable can be used to carry OPA’s output signal to
subsequent ICs for pH reading.
BATTERY
3V
(DURACELL
DL1620)
GENERAL PURPOSE
COMBINATION
pH PROBE
COAX
(CORNING 476540)
TP211x
R1
10MΩ
To
pH
PROBE
ADC/AFE/MCU
R2
10MΩ
ALL COMPONENTS CONTAJNED WITHIN THE pH PROBE
Figure 5: Buffer pH Probe
Portable Gas Sensor Amplifier
Gas sensors are used in many different industrial and medical applications. Gas sensors generate a current that is
proportional to the percentage of a particular gas concentration sensed in an air sample. This output current flows
through a load resistor and the resultant voltage drop is amplified. Depending on the sensed gas and sensitivity of the
sensor, the output current can be in the range of tens of microamperes to a few milli-amperes. Gas sensor datasheets
often specify a recommended load resistor value or a range of load resistors from which to choose.
There are two main applications for oxygen sensors – applications which sense oxygen when it is abundantly present
(that is, in air or near an oxygen tank) and those which detect traces of oxygen in parts-per-million concentration. In
medical applications, oxygen sensors are used when air quality or oxygen delivered to a patient needs to be monitored.
In fresh air, the concentration of oxygen is 20.9% and air samples containing less than 18% oxygen are considered
dangerous. In industrial applications, oxygen sensors are used to detect the absence of oxygen; for example,
vacuum-packaging of food products.
The circuit in Figure 6 illustrates a typical implementation used to amplify the output of an oxygen detector. With the
components shown in the figure, the circuit consumes less than 300nA of supply current ensuring that small
form-factor single- or button-cell batteries (exhibiting low mAh charge ratings) could last beyond the operating life of
the oxygen sensor. The precision specifications of these amplifiers, such as their low offset voltage, low VOS TC, low
input bias current, high CMRR, and high PSRR are other factors which make these amplifiers excellent choices for this
application.
10MΩ
1%
100kΩ
1%
VOUT
TP211x
Oxygen Sensor
City Technology
4OX2
100kΩ
1%
I
100Ω
1%
O2
VOUT 1Vin Air ( 21% O2 )
IDD 0.7uA
Figure 6
REV1.0
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