LMP7702MASLASHNOPB datasheet(28/44 Pages) TI1

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ACID

BASE

+414 mV

-414 mV

-177 mV

0 mV

+177 mV

SENSOR

VIN+

LMP7701, LMP7702, LMP7704

SNOSAI9I – SEPTEMBER 2005 – REVISED NOVEMBER 2015

www.ti.com

Typical Application (continued)

9.2.1 Design Requirements

pH electrodes are very high impedance sensors. As their name indicates, they are used to measure the pH of a

solution. They usually do this by generating an output voltage which is proportional to the pH of the solution. pH

electrodes are calibrated so that they have zero output for a neutral solution, pH = 7, and positive and negative

voltages for acidic or alkaline solutions. This means that the output of a pH electrode is bipolar and must be level

shifted to be used in a single supply system. The rate of change of this voltage is usually shown in mV/pH and is

different for different pH sensors. Temperature is also an important factor in a pH electrode reading. The output

voltage of the senor will change with temperature.

9.2.2 Detailed Design Procedure

Many sensors have high source impedances that may range up to 10 M

Ω. The output signal of sensors often

needs to be amplified or otherwise conditioned by means of an amplifier. The input bias current of this amplifier

can load the sensor's output and cause a voltage drop across the source resistance as shown in Figure 50,

where VIN

+ = V

S – IBIAS*RS

The last term, IBIAS*RS, shows the voltage drop across RS. To prevent errors introduced to the system due to this

voltage, an op amp with very low input bias current must be used with high impedance sensors. This is to keep

the error contribution by IBIAS*RS less than the input voltage noise of the amplifier, so that it will not become the

dominant noise factor.

Figure 50. Noise Due to IBIAS

Figure 51 shows a typical output voltage spectrum of a pH electrode. The exact values of output voltage will be

different for different sensors. In this example, the pH electrode has an output voltage of 59.15 mV/pH at 25°C.

Figure 51. Output Voltage of a pH Electrode

The temperature dependence of a typical pH electrode is shown in Figure 52. As is evident, the output voltage

changes with changes in temperature.

The schematic shown in Figure 49 is a typical circuit which can be used for pH measurement. The LM35 is a

precision integrated circuit temperature sensor. This sensor is differentiated from similar products because it has

an output voltage linearly proportional to Celcius measurement, without converting the temperature to Kelvin. The

LM35 is used to measure the temperature of the solution and feeds this reading to the Analog to Digital

Converter, ADC. This information is used by the ADC to calculate the temperature effects on the pH readings.

The LM35 needs to have a resistor, RT in Figure 49, to –V

+ to be able to read temperatures less than 0°C. R

T is

not needed if temperatures are not expected to be less than zero.

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LMP7702MA/NOPB Datasheet(PDF) 28 Page - Texas Instruments

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