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LM7705

SNVS420B – NOVEMBER 2008 – REVISED MARCH 2013

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Dual Supply, True Zero Amplifiers

The limitations of the output stage of the op amp, as indicated in both examples, can be omitted by using a dual

supply op amp. The output stage of the used op amp can then still swing from 50 mV of the supply rails.

However, the functional output range of the op amp is now from ground level to a value near the positive supply

rail. Figure 31 shows the output drive of an amplifier in a true zero output voltage application.

Figure 31. Amplifier output drive with a dual supply

Disadvantages of this solution are:

•

The usage of a dual supply instead of a simple single supply is more expensive.

•

A dual supply voltage for the op amps requires parts that can handle a larger operating range for the supply

voltage. If the op amps used in the current solution can’t handle this, a redesign can be required.

A better solution is to use the LM7705. This low noise negative bias generator has some major advantages with

respect to a dual supply solution:

•

Operates with only a single positive supply, and is therefore a much cheaper solution.

•

The LM7705 generates a negative supply voltage of only

−0.23V. This is more than enough to create a True-

zero output for most op amps.

•

In many applications, this “small” extension of the supply voltage range can be within the abs max rating for

many op amps, so an expensive redesign is not necessary.

In the next section a typical amplifier application will be evaluated. The performance of an amplifier will be

measured in a single supply configuration. The results will be compared with an amplifier using a LM7705

supplying a negative voltage to the bias pin.

TYPICAL AMPLIFIER APPLICATION

This section shows the measurement results of a true zero output amplifier application with an analog to digital

converter (ADC) used as back-end. The biasing of the op amp can be done in two ways:

•

A single supply configuration

•

A single supply in combination with the LM7705, extending the negative supply from ground level to a fixed -

0.23 Voltage.

Basic Setup

The basic setup of this true zero output amplifier is given in Figure 32. The LMP7701 op amp is configured as a

voltage follower to demonstrate the output limitation, due to the saturation of the output stage. The negative

power supply pin of the op amp can be connected to ground level or to the output of the negative bias generator,

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