Datasheet

1
2
3
4
5 7
8
9
10
11
12
13
14
600
500
400
300
200
100
0
-100
-200
-300
-400
-500
-600
pH
mV
10°C (74.04 mV/pH)
25°C (59.15 mV/pH)
0°C (54.20 mV/pH)
ACID BASE
0
2
4
7
10
12
14
+414 mV
-414 mV
-177 mV
0 mV
+177 mV
pH
LMP7701, LMP7702, LMP7704
SNOSAI9H –SEPTEMBER 2005–REVISED MARCH 2013 www.ti.com
Figure 54. Output Voltage of a pH Electrode
The temperature dependence of a typical pH electrode is shown in Figure 55. As is evident, the output voltage
changes with changes in temperature.
Figure 55. Temperature Dependence of a pH Electrode
The schematic shown in Figure 56 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 the need to convert 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, R
T
in Figure 56, to –V
+
in order to be able to read temperatures
below 0°C. R
T
is not needed if temperatures are not expected to go below zero.
The output of pH electrodes is usually large enough that it does not require much amplification; however, due to
the very high impedance, the output of a pH electrode needs to be buffered before it can go to an ADC. Since
most ADCs are operated on single supply, the output of the pH electrode also needs to be level shifted. Amplifier
A1 buffers the output of the pH electrode with a moderate gain of +2, while A2 provides the level shifting. V
OUT
at
the output of A2 is given by: V
OUT
= −2V
pH
+ 1.024V.
The LM4140A is a precision, low noise, voltage reference used to provide the level shift needed. The ADC used
in this application is the ADC12032 which is a 12-bit, 2 channel converter with multiplexers on the inputs and a
serial output. The 12-bit ADC enables users to measure pH with an accuracy of 0.003 of a pH unit. Adequate
power supply bypassing and grounding is extremely important for ADCs. Recommended bypass capacitors are
shown in Figure 56. It is common to share power supplies between different components in a circuit. To minimize
the effects of power supply ripples caused by other components, the op amps need to have bypass capacitors
on the supply pins. Using the same value capacitors as those used with the ADC are ideal. The combination of
these three values of capacitors ensures that AC noise present on the power supply line is grounded and does
not interfere with the amplifiers' signal.
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