SM72501/NOPB National Semiconductor, SM72501/NOPB Datasheet - Page 17

• Manufacturer Part Number: SM72501/NOPB
• Description: IC AMP CMOS INPUT SOT-23-5
• Manufacturer: National Semiconductor
• Series: SolarMagic™r

Specifications of SM72501/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: 200nA
• Voltage - Input Offset: 37µV
• Current - Supply: 790µA
• Current - Output / Channel: 86mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 12 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -

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
function of source resistance where:
e
e
referred current noise or e
e
e
Where:
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
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.
shows the typical input noise of the SM72501 as a
FIGURE 8. Total Input Noise
Figure
8, at lower R
i
= R
S
* i
S
. As mentioned before, the
n
S
values, total noise is
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is
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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
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
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
with changes in temperature.
BIAS
FIGURE 10. Output Voltage of a pH Electrode
*R
Figure
S
shows a typical output voltage spectrum of a pH
less than the input voltage noise of the amplifier,
Figure
BIAS
FIGURE 9. Noise Due to I
11. As is evident, the output voltage changes
*R
9, where V
S
, shows the voltage drop across R
IN
+
= V
S
– I
BIAS
BIAS
*R
S
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