lm6142qml National Semiconductor Corporation, lm6142qml Datasheet - Page 13

• Manufacturer Part Number: lm6142qml
• Description: 17 Mhz Rail-to-rail Input-output Operational Amplifiers
• Manufacturer: National Semiconductor Corporation

LM6142 Application Ideas
Typical Applications
ANALOG TO DIGITAL CONVERTER BUFFER
The high capacitive load driving ability, rail-to-rail input and
output range with the excellent CMR of 82 dB, make the
LM6142a good choice for buffering the inputs of A to D
converters.
3 OP AMP INSTRUMENTATION AMP WITH
RAIL-TO-RAIL INPUT AND OUTPUT
Using two LM6142 amplifiers, a 3 op amp instrumentation
amplifier with rail-to-rail inputs and rail to rail output can be
made. These features make these instrumentation amplifiers
ideal for single supply systems.
Some manufacturers use a precision voltage divider array of
5 resistors to divide the common-mode voltage to get an
input range of rail-to-rail or greater. The problem with this
method is that it also divides the signal, so to even get unity
gain, the amplifier must be run at high closed loop gains.
This raises the noise and drift by the internal gain factor and
lowers the input impedance. Any mismatch in these preci-
FIGURE 5.
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sion resistors reduces the CMR as well. Using two LM6142
amplifiers, all of these problems are eliminated.
In this example, amplifiers A and B act as buffers to the
differential stage (Figure 6). These buffers assure that the
input impedance is over 100MΩ and they eliminate the
requirement for precision matched resistors in the input
stage. They also assure that the difference amp is driven
from a voltage source. This is necessary to maintain the
CMR set by the matching of R1–R2 with R3–R4.
The gain is set by the ratio of R2/R1 and R3 should equal R1
and R4 equal R2. Making R4 slightly smaller than R2 and
adding a trim pot equal to twice the difference between R2
and R4 will allow the CMR to be adjusted for optimum.
With both rail to rail input and output ranges, the inputs and
outputs are only limited by the supply voltages. Remember
that even with rail-to-rail output, the output can not swing
past the supplies so the combined common mode voltage
plus the signal should not be greater than the supplies or
limiting will occur.
SPICE MACROMODEL
A SPICE macromodel of this and many other National Semi-
conductor op amps is available at no charge from the NSC
Customer Response Group at 800-272-9959.
FIGURE 6.
www.national.com
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