CN0326 Datasheet, PDF (2/7 Pages) Analog Devices – Devices Connected

CN-0326
Circuit Note
CIRCUIT DESCRIPTION
Fundamentals of pH Measurements
The pH value is a measure of the relative amount of hydrogen
and hydroxide ions in an aqueous solution. In terms of molar
concentrations, water at 25°C contains 1 × 10^−7 moles/liter of
hydrogen ions and the same concentration of hydroxide ions. A
neutral solution is one in which the hydrogen ion concentration
exactly equals the hydroxide ion concentration. pH is another
way of expressing the hydrogen ion concentration and is
defined as follows:
pH  – log( H ^ )
Therefore, if the hydrogen ion concentration is 1.0 × 10^−2
moles/liter, the pH is 2.00.
The pH electrodes are electrochemical sensors used by many
industries but are of particular importance to the water and waste-
water industry. The pH probe consists of a glass measuring
electrode and a reference electrode, which is analogous to a battery.
When the probe is place in a solution, the measuring electrode
generates a voltage depending on the hydrogen activity of the
solution, which is compared to the potential of the reference
electrode. As the solution becomes more acidic (lower pH) the
potential of the glass electrode becomes more positive (+mV) in
comparison to the reference electrode; and as the solution becomes
more alkaline (higher pH) the potential of the glass electrode
becomes more negative (−mV) in comparison to the reference
electrode. The difference between these two electrodes is the meas-
ured potential. A typical pH probe ideally produces 59.154 mV/pH
units at 25^oC. This is expressed in the Nernst equation as follows
E  a – ^{2.303 R T  273.1}   pH – pH _ISO 
nF
where:
E = voltage of the hydrogen electrode with unknown activity
ܽ = ±30 mV, zero point tolerance
T = ambient temperature in ^oC
n = 1 at 25 ^oC, valence (number of charges on ion)
F = 96485 coulombs/mol, Faraday constant
R = 8.314 volt-coulombs /°K mol, Avogadro's number
pH = hydrogen ion concentration of an unknown solution
pHISO = 7, reference hydrogen ion concentration
The equation shows that the voltage generated is dependent on
the acidity or alkalinity of the solution and varies with the hydrogen
ion activity in a known manner. The change in temperature of
the solution changes the activity of its hydrogen ions. When the
solution is heated, the hydrogen ions move faster which result
in an increase in potential difference across the two electrodes.
In addition, when the solution is cooled, the hydrogen activity
decreases causing a decrease in the potential difference. Electrodes
are designed ideally to produce a zero volt potential when
placed in a buffer solution with a pH of 7.
A good reference on the theory of pH is pH Theory and Practice,
Radiometer Analytical SAS, Villeurbanne Cedex, France.
Circuit Details
The design provides a complete solution for pH sensor with
temperature compensation. The circuit has three critical stages:
the pH probe buffer, the ADC, and the digital and power
isolator as shown in Figure 1.
The AD8603, a precision micro power (50 μA maximum) and
low noise (22 nV/√Hz) CMOS operational amplifier configured
as a buffer to the input of one of the channels of the AD7793.
The AD8603 has a typical input bias current of 200 fA that
provides an effective solution to the pH probe that has high
internal resistance.
The pH sensing and temperature compensation system is based on
the AD7793, 24-bit sigma-delta (Σ-Δ) with. It has three differential
analog inputs and has an on-chip, low noise, programmable gain
amplifier (PGA) that ranges from unity gain to 128. The AD7793
consumes only a maximum of 500 μA making it suitable for any
low power applications. It has a low noise, low drift internal band
gap reference and can accept external differential reference. The
output data rate from the part is software programmable and
can be varied from 4.17 Hz to 470 Hz.
The ADuM5401, quad-channel digital isolator with an
integrated dc-to-dc converter provides the digital signal and
power isolation between the microcontroller and the AD7793
digital lines. The iCoupler chip-scale transformer technology is
used to isolate the logic signals and the power feedback path in
the dc-to-dc converter.
Buffer for pH Sensor Interface
The electrode of a typical pH probe is made up of glass that
creates an extremely high resistance that can range from 1 MΩ
to 1 GΩ and acts as a resistance in series with the pH voltage
source as shown in Figure 2.
pH SENSOR
pH
V_OUT
1GΩ
210µA
J1
I_BIAS
3.3V_ISO
1MΩ _AD8603
10kΩ
1µF
+1.05V
5kΩ
10kΩ
1µF
IOUT2
AD7793
AIN1(+)
AIN1(–)
AIN2(–)
RFIN(+)/AIN3(+)
RFIN(–)/AIN3(–)
GND
Figure 2. pH Sensor and Buffer Interface to ADC (Simplified Schematic: All
Connections, RTD, and Decoupling Not Shown.)
The buffer amplifier bias current flowing through this series
resistance introduces an offset error in the system. To isolate the
circuit from this high source resistance, a buffer amplifier with
high input impedance and very low input bias current is needed
for this application. The AD8603 is used as a buffer amplifier for
this application as shown in Figure 2. The low input current of
the AD8603 minimizes the voltage error produced by the bias
current flowing through the electrode resistance.
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