SM72501 Datasheet, PDF (17/20 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

TOTAL NOISE CONTRIBUTION
The SM72501 has very low input bias current, very low input
current noise, and very low input voltage noise. As a result,
these amplifiers are ideal choices for circuits with high
impedance sensor applications.
Figure 8 shows the typical input noise of the SM72501 as a
function of source resistance where:
e_n denotes the input referred voltage noise
e_i is the voltage drop across source resistance due to input
referred current noise or e_i = R_S * i_n
e_t shows the thermal noise of the source resistance
e_ni shows the total noise on the input.
Where:
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 9, 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.
The input current noise of the SM72501 is so low that it will
not become the dominant factor in the total noise unless
source resistance exceeds 300 MΩ, which is an unrealisti-
cally high value.
As is evident in Figure 8, at lower R_S values, total noise is
dominated by the amplifier's input voltage noise. Once R_S is
larger than a few kilo-Ohms, then the dominant noise factor
becomes the thermal noise of R_S. As mentioned before, the
current noise will not be the dominant noise factor for any
practical application.
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FIGURE 9. 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 solu-
tion. 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 so-
lution, pH = 7, and positive and negative voltages for acidic
or alkaline solutions. This means that the output of a pH elec-
trode 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 sen-
sors. Temperature is also an important factor in a pH elec-
trode reading. The output voltage of the senor will change with
temperature.
Figure 10 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 elec-
trode has an output voltage of 59.15 mV/pH at 25°C.
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FIGURE 8. Total Input Noise
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FIGURE 10. Output Voltage of a pH Electrode
The temperature dependence of a typical pH electrode is
shown in Figure 11. As is evident, the output voltage changes
with changes in temperature.
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