CN-0326

Circuit Note

Rev. 0 | Page 2 of 7

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

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:

)

log(

–





H

pH

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

 

ISO

pH

pH

nF

T

R

a

E

–

1

.

273

303

.

2

–







where:

E = voltage of the hydrogen electrode with unknown activity

= ±30 mV, zero point tolerance

F = 96485 coulombs/mol, Faraday constant

R = 8.314 volt-coulombs /°K mol, Avogadro's number

pH = hydrogen ion concentration of an unknown solution

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.

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.

AD7793

AIN1(+)

AIN1(–)

AIN2(–)

RFIN(+)/AIN3(+)

RFIN(–)/AIN3(–)

GND

IOUT2

10kΩ

10kΩ

1µF

1µF

5kΩ

1MΩ

1GΩ

AD8603

210µA

+1.05V

pH SENSOR

pH

J1