5.0 HALF-WAVE RECTIFIER WITH RAIL-TO-GROUND

OUTPUT SWING

Since the LMV981/LMV982 input common mode range in-

cludes both positive and negative supply rails and the output

can also swing to either supply, achieving half-wave rectifier

functions in either direction is an easy task. All that is needed

are two external resistors; there is no need for diodes or

matched resistors. The half wave rectifier can have either

positive or negative going outputs, depending on the way the

circuit is arranged.

In Figure 3 the circuit is referenced to ground, while in Figure

4 the circuit is biased to the positive supply. These configu-

rations implement the half wave rectifier since the LMV981/

LMV982 can not respond to one-half of the incoming wave-

form. It can not respond to one-half of the incoming because

the amplifier can not swing the output beyond either rail

therefore the output disengages during this half cycle. Dur-

ing the other half cycle, however, the amplifier achieves a

half wave that can have a peak equal to the total supply

voltage. R

I

should be large enough not to load the

LMV981/LMV982.

6.0 INSTRUMENTATION AMPLIFIER WITH

RAIL-TO-RAIL INPUT AND OUTPUT

Some manufactures make a non-“rail-to-rail”-op amp rail-to-

rail by using a resistive divider on the inputs. The resistors

divide the input voltage to get a rail-to-rail input range. The

problem with this method is that it also divides the signal, so

in order to get the obtained gain, the amplifier must have a

higher closed loop gain. This raises the noise and drift by the

internal gain factor and lowers the input impedance. Any

mismatch in these precision resistors reduces the CMRR as

well. The LMV981/LMV982 is rail-to-rail and therefore

doesn’t have these disadvantages.

Using three of the LMV981/LMV982 amplifiers, an instru-

mentation amplifier with rail-to-rail inputs and outputs can be

made as shown in Figure 5.

In this example, amplifiers on the left side act as buffers to

the differential stage. These buffers assure that the input

impedance is very high and require no precision matched

resistors in the input stage. They also assure that the differ-

ence amp is driven from a voltage source. This is necessary

to maintain the CMRR set by the matching R

The gain is set by the ratio of R

and R

ranges, the input and output are only limited by the supply

voltages. Remember that even with rail-to-rail outputs, the

output can not swing past the supplies so the combined

common mode voltages plus the signal should not be

greater that the supplies or limiting will occur. For additional

applications, see National Semiconductor application notes

AN–29, AN–31, AN–71, and AN–127.

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FIGURE 3. Half-Wave Rectifier with Rail-To-Ground Output Swing Referenced to Ground

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FIGURE 4. Half-Wave Rectifier with Negative-Going Output Referenced to V

CC

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FIGURE 5. Rail-to-rail instrumentation amplifier

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