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LMP7702 数据手册 - National
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LMP7702数据手册
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Application Information (Continued)
The schematic shown in Figure 12 is a typical circuit which
can be used for pH measurement. The LM35 is a precision
integrated circuit temperature sensor. This sensor is differ-
entiated from similar products because it has an output
voltage linearly proportional to Celcius measurement, with-
out 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 12,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 are usually large enough that
they don’t 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.
LM4140A is a precision, low noise, voltage reference used to
provide the level shift needed. The ADC used in this appli-
cation is the ADC12032 which is a 12-bit, 2 channel con-
verter 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 12. 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 com-
bination 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.
20127361
FIGURE 11. Temperature Dependence of a pH
Electrode
20127362
FIGURE 12. pH Measurement Circuit
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