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

LM6142/44 a good choice for buffering the inputs of A to D

converters.

3 OPAMP INSTRUMENTATION AMP WITH

RAIL-TO-RAIL INPUT AND OUTPUT

Using the LM6144, a 3 opamp 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 in-

put 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-

sion resistors reduces the CMR as well. Using the LM6144,

all of these problems are eliminated.

In this example, amplifiers A and B act as buffers to the dif-

ferential stage (

Figure 6). These buffers assure that the input

impedance is over 100 M

Ω and they eliminate the require-

ment 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 lim-

iting will occur.

SPICE MACROMODEL

A SPICE macromodel of this and many other National Semi-

conductor opamps is available at no charge from the NSC

Customer Response Group at 800-272-9959.

DS012057-13

FIGURE 6.

www.national.com

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