12-03-2014, 03:34 PM
ANALOG edge SM Designing with pH Electrodes
[attachment=60070]
Application Note AN-1852
A pH electrode measures hydrogen ion (H+) activity and
produces an electrical potential or voltage. The operation of
the pH electrode is based on the principle that an electric
potential develops when two liquids of different pH come
into contact at opposite sides of a thin glass membrane.
This was originally discovered in 1906 by Max Cremer1.
His discovery laid the foundation for Fritz Haber and
Zygmunt Klemensiewicz, who published their findings in
1909, to create the first glass electrode which measured
hydrogen activity2. Today, modern pH electrodes use the
same principles to measure pH in a variety of applications
including water treatment, chemical processing, medical
instrumentation, and environmental test systems.
[b]pH Electrode Characteristics[/b]
When designing with a pH electrode, as with any sensor,
it is important to understand the sensor characteristics
and how they affect a specific application. These charac-
teristics include whether the sensor is active or passive,
unipolar or bipolar, and whether it has a voltage or
current output. Sensor sensitivity, linearity, full scale
range, and source impedance should also be considered.
The pH electrode is a passive sensor which means no
excitation source (voltage or current) is required. Because
the electrode’s output can swing above and below the
reference point, it is classified as a bipolar sensor. It pro-
duces a voltage output which is linearly dependent upon
the pH of the solution being measured.
An Optimum pH-Electrode Circuit
The important sensor characteristics described need to be
accounted for in order to design a circuit which will condition the
sensor signal so that it can be faithfully utilized by other compo-
nents (such as an ADC, microcontroller, etc.) along the signal
path. First, because the pH electrode produces a bipolar signal
and most applications operate on a single supply, the signal will
have to be level shifted. Second, due to the high impedance of
the electrode, a high-input impedance buffer will be required.
Finally, the temperature of the measured solution must be known
in order to compensate for the electrode’s sensitivity variation over
temperature.
Amplifier Selection
The specific design challenges of the pH electrode
impose the need to select an amplifier which does
not degrade the overall system performance. It is best
to start with an understanding of what amplifier
parameters contribute most to the voltage error in
a pH-electrode application. The most significant
parameter to consider is the amplifier’s input-bias
current. This is because even a small input-bias cur-
rent can produce a large voltage error when injected
into the very high impedance of a pH electrode.
[attachment=60070]
Application Note AN-1852
A pH electrode measures hydrogen ion (H+) activity and
produces an electrical potential or voltage. The operation of
the pH electrode is based on the principle that an electric
potential develops when two liquids of different pH come
into contact at opposite sides of a thin glass membrane.
This was originally discovered in 1906 by Max Cremer1.
His discovery laid the foundation for Fritz Haber and
Zygmunt Klemensiewicz, who published their findings in
1909, to create the first glass electrode which measured
hydrogen activity2. Today, modern pH electrodes use the
same principles to measure pH in a variety of applications
including water treatment, chemical processing, medical
instrumentation, and environmental test systems.
[b]pH Electrode Characteristics[/b]
When designing with a pH electrode, as with any sensor,
it is important to understand the sensor characteristics
and how they affect a specific application. These charac-
teristics include whether the sensor is active or passive,
unipolar or bipolar, and whether it has a voltage or
current output. Sensor sensitivity, linearity, full scale
range, and source impedance should also be considered.
The pH electrode is a passive sensor which means no
excitation source (voltage or current) is required. Because
the electrode’s output can swing above and below the
reference point, it is classified as a bipolar sensor. It pro-
duces a voltage output which is linearly dependent upon
the pH of the solution being measured.
An Optimum pH-Electrode Circuit
The important sensor characteristics described need to be
accounted for in order to design a circuit which will condition the
sensor signal so that it can be faithfully utilized by other compo-
nents (such as an ADC, microcontroller, etc.) along the signal
path. First, because the pH electrode produces a bipolar signal
and most applications operate on a single supply, the signal will
have to be level shifted. Second, due to the high impedance of
the electrode, a high-input impedance buffer will be required.
Finally, the temperature of the measured solution must be known
in order to compensate for the electrode’s sensitivity variation over
temperature.
Amplifier Selection
The specific design challenges of the pH electrode
impose the need to select an amplifier which does
not degrade the overall system performance. It is best
to start with an understanding of what amplifier
parameters contribute most to the voltage error in
a pH-electrode application. The most significant
parameter to consider is the amplifier’s input-bias
current. This is because even a small input-bias cur-
rent can produce a large voltage error when injected
into the very high impedance of a pH electrode.