12

TP2111/TP2111N/TP2112/TP2114

Nanopower 300nA, 1.8V, Rail-to-Rail Input/Output Op-amps

REV1.0

www.3peakic.com

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.

R1

10M

Ω

R2

10M

Ω

COAX

BATTERY

3V

(DURACELL

DL1620)

ALL COMPONENTS CONTAJNED WITHIN THE pH PROBE

GENERAL PURPOSE

COMBINATION

pH PROBE

(CORNING 476540)

pH

PROBE

To

ADC/AFE/MCU

TP211x

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

input bias current, high CMRR, and high PSRR are other factors which make these amplifiers excellent choices for this

application.

100k

Ω

1%

100

Ω

1%

100k

Ω

1%

10M

Ω

1%

2

O

I

2

1 in Air ( 21% O )

0.7

OUT

DD

VV

I

uA





Oxygen Sensor

City Technology

4OX2

TP211x

Figure 6