SM72501 Datasheet, PDF (19/28 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

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SM72501
SNIS157C – JANUARY 2011 – REVISED APRIL 2013
1000
100
e_ni
e_n
10
e_t
e_i
1
0.1
10 100 1k 10k 100k 1M 10M
R_S (:)
Figure 48. Total Input Noise
HIGH IMPEDANCE SENSOR INTERFACE
Many sensors have high source impedances that may range up to 10 MΩ. 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 49,
where V_IN+ = V_S – I_BIAS*R_S
The last term, I_BIAS*R_S, shows the voltage drop across R_S. 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 I_BIAS*R_S less than the input voltage noise of the amplifier, so that it will not become the
dominant noise factor.
SENSOR
R_S
V_S
+
-
V⁺
I_B
V_IN+
+
-
V^-
Figure 49. Noise Due to I_BIAS
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 has to 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.
Figure 50 shows a typical output voltage spectrum of a pH electrode. Note that 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.
ACID
0
2
4
BASE
7
10 12
14 pH
+414 mV
+177 mV 0 mV
-177 mV
-414 mV
Figure 50. Output Voltage of a pH Electrode
The temperature dependence of a typical pH electrode is shown in Figure 51. As is evident, the output voltage
changes with changes in temperature.
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