LMP7701MFX/NOPB National Semiconductor, LMP7701MFX/NOPB Datasheet - Page 18

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LMP7701MFX/NOPB

Manufacturer Part Number
LMP7701MFX/NOPB
Description
IC OP AMP PREC RRIO SGL SOT23-5
Manufacturer
National Semiconductor
Series
LMP®r
Datasheet

Specifications of LMP7701MFX/NOPB

Amplifier Type
General Purpose
Number Of Circuits
1
Output Type
Rail-to-Rail
Slew Rate
1.1 V/µs
Gain Bandwidth Product
2.5MHz
Current - Input Bias
0.2pA
Voltage - Input Offset
37µV
Current - Supply
790µA
Current - Output / Channel
86mA
Voltage - Supply, Single/dual (±)
2.7 V ~ 12 V, ±1.35 V ~ 6 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
SOT-23-5, SC-74A, SOT-25
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
-3db Bandwidth
-
Other names
LMP7701MFX
www.national.com
TOTAL NOISE CONTRIBUTION
The LMP7701/LMP7702/LMP7704 have very low input bias
current, very low input current noise, and very low input volt-
age 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
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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 resistance
as shown in Figure 9, where V
The last term, I
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
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 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.
The temperature dependence of a typical pH electrode is
shown in Figure 11. As is evident, the output voltage changes
with changes in temperature.
BIAS
FIGURE 10. Output Voltage of a pH Electrode
*R
S
less than the input voltage noise of the amplifier,
BIAS
FIGURE 9. Noise Due to I
*R
S
, shows the voltage drop across R
IN
+
= V
S
– I
BIAS
BIAS
*R
S
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. To

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