LMP7708MA/NOPB National Semiconductor, LMP7708MA/NOPB Datasheet - Page 16

IC AMP RRIO PREC DUAL 8SOIC

LMP7708MA/NOPB

Manufacturer Part Number
LMP7708MA/NOPB
Description
IC AMP RRIO PREC DUAL 8SOIC
Manufacturer
National Semiconductor
Series
LMP®, PowerWise®r
Datasheet

Specifications of LMP7708MA/NOPB

Amplifier Type
General Purpose
Number Of Circuits
2
Output Type
Rail-to-Rail
Slew Rate
5.9 V/µs
Gain Bandwidth Product
15MHz
Current - Input Bias
0.2pA
Voltage - Input Offset
37µV
Current - Supply
1.7mA
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
8-SOIC (3.9mm Width)
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
-3db Bandwidth
-
Other names
*LMP7708MA/NOPB
LMP7708MA

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Quantity:
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TOTAL NOISE CONTRIBUTION
The LMP7707/LMP7708/LMP7709 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 4 shows the typical input noise of the LMP7707/
LMP7708/LMP7709 as a function of source resistance. The
total noise at the input can be calculated using Equation 1.
Where:
e
e
e
referred current noise or e
e
The input current noise of the LMP7707/LMP7708/LMP7709
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 4, at lower R
is dominated by the amplifier’s input voltage noise. Once R
is 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.
ni
n
i
t
is the voltage drop across source resistance due to input
is the thermal noise of the source resistance
denotes the input referred voltage noise
is the total noise on the input.
FIGURE 3. Input of the LMP7707
FIGURE 4. Total Input Noise
i
= R
S
S
* i
. As mentioned before, the
n
S
values, the total noise
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(1)
S
16
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 5, 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. The
LMP7707/LMP7708/LMP7709 have very low input bias cur-
rent, typically 200 fA.
USAGE OF DECOMPENSATED AMPLIFIERS
This section discusses the differences between compensated
and decompensated op amps and presents the advantages
of decompensated amplifiers. In high gain applications de-
compensated amplifiers can be used without any changes
compared to standard amplifiers. However, for low gain ap-
plications special frequency compensation measures have to
be taken to ensure stability.
Feedback circuit theory is discussed in detail, in particular as
it applies to decompensated amplifiers. Bode plots are pre-
sented for a graphical explanation of stability analysis. Two
solutions are given for creating a feedback network for de-
compensated amplifiers when relatively low gains are re-
quired: A simple resistive feedback network and a more
advanced frequency dependent feedback network with im-
proved noise performance. Finally, a design example is pre-
sented resulting in a practical application. The results are
compared to fully compensated amplifiers (National Semi-
conductors LMP7701/LMP7702/LMP7704).
COMPENSATED AMPLIFIERS
A (fully) compensated op amp is designed to operate with
good stability down to gains of ±1. For this reason, the com-
pensated op amp is also called a unity gain stable op amp.
Figure 6 shows the Open Loop Response of a compensated
amplifier.
BIAS
*R
S
less than the input voltage noise of the amplifier,
BIAS
FIGURE 5. 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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