LMP7701_06 NSC [National Semiconductor], LMP7701_06 Datasheet - Page 18

• Manufacturer Part Number: LMP7701_06
• Description: Precision, CMOS Input, RRIO, Wide Supply Range Amplifiers
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
TOTAL NOISE CONTRIBUTION
The LMP7701/LMP7702/LMP7704 have 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 LMP7701/
LMP7702/LMP7704 as a function of source resistance
where:
e
e
referred current noise or e
e
e
Where:
The input current noise of the LMP7701/LMP7702/LMP7704
is so low that it will not become the dominant factor in the
total noise unless source resistance exceeds 300 MΩ, which
is an unrealistically high value.
As is evident in Figure 8, at lower R
dominated by the amplifier’s input voltage noise. Once R
larger than a few kilo-Ohms, then the dominant noise factor
becomes the thermal noise of R
current noise will not be the dominant noise factor for any
practical application.
n
i
t
ni
is the voltage drop across source resistance due to input
shows the thermal noise of the source resistance
denotes the input referred voltage noise
shows the total noise on the input.
FIGURE 8. Total Input Noise
i
= R
S
S
* i
. As mentioned before, the
n
S
values, total noise is
(Continued)
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S
is
18
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 resis-
tance as shown in Figure 9, where V
The last term, I
To prevent errors introduced to the system due to this volt-
age, 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
the amplifier, so that it will not become the dominant noise
factor.
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 volt-
age 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 differ-
ent pH sensors. Temperature is also an important factor in a
pH electrode 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.
The temperature dependence of a typical pH electrode is
shown in Figure 11. As is evident, the output voltage
changes with changes in temperature.
FIGURE 10. Output Voltage of a pH Electrode
FIGURE 9. Noise Due to I
BIAS
BIAS
*R
*R
S
S
, shows the voltage drop across R
less than the input voltage noise of
IN
+
= V
BIAS
S
– I
BIAS
20127359
*R
20127360
S
S
.