TS1002/TS1004
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 is one example.
The circuit in Figure
1
illustrates
a
typical
implementation used to amplify the output of an
oxygen detector. Either amplifier makes an
Figure 2: A Simple, Single-pole Active Low-Pass Filter.
If additional attenuation is needed, a two-pole
Sallen-Key filter can be used to provide the
additional attenuation as shown in Figure 3.
For best results, the filter’s cutoff frequency should
be 8 to 10 times lower than the amplfier’s crossover
Figure 1: A Nanopower, Precision Oxygen Gas Sensor
Amplifier.
excellent choice for this application as it only draws
0.6µA of supply current per amplifier and operates
on supply voltages down to 0.65V. With the
components shown in the figure, the circuit
consumes less than 0.7 μA 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 TCVOS, low input bias
current, high CMRR, and high PSRR are other
factors which make these amplifiers excellent
choices for this application. Since oxygen sensors
typically exhibit an operating life of one to two years,
an oxygen sensor amplifier built around one of these
Figure 3: A Nanopower 2-Pole Sallen-Key Low-Pass Filter.
frequency. Additional operational amplifier phase
margin shift can be avoided if the amplifier
bandwidth-to-signal bandwidth ratio is greater than
8.
The design equations for the 2-pole Sallen-Key low-
pass filter are given below with component values
selected to set a 400Hz low-pass filter cutoff
frequency:
R1 = R2 = R = 1MΩ
C1 = C2 = C = 400pF
Q = Filter Peaking Factor = 1
f–3dB = 1/(2 x π x RC) = 400 Hz
R3 = R4/(2-1/Q); with Q = 1, R3 = R4.
amplifiers can operate from
a conventionally-
available single 1.5-V alkaline AA battery for over
290 years! At such low power consumption from a
single cell, the oxygen sensor could be replaced
over 150 times before the battery requires replacing!
A Single +1.5 V Supply, Two Op Amp
Instrumentation Amplifier
NanoWatt, Buffered Single-pole Low-Pass Filters
When receiving low-level signals, limiting the
bandwidth of the incoming signals into the system is
often required. As shown in Figure 2, the simplest
way to achieve this objective is to use an RC filter at
the noninverting terminal of the amplifier.
The amplifiers’ ultra-low supply current and ultra-low
voltage operation make them ideal for battery-
powered applications such as the instrumentation
amplifier shown in Figure 4 using a TS1002.
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TS1002_4DS r1p0
RTFDS
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