+V

ADC

-

+

GAIN = 50x

ADC122S021

LMP7701

SENSOR

+V

-

+

0V

0V

INPUT VOLTAGE (V)

V+

0

0

V+

OUTPUT SATURATION

LM7705

SNVS420B – NOVEMBER 2008 – REVISED MARCH 2013

www.ti.com

Figure 27. Limitation of the Output of an Amplifier

The introduction of operational amplifiers with output Rail-to-rail drive capabilities is a strong improvement and

the (output) performance of op amps is for many applications no longer a limiting factor. For example, Texas

Instruments' LMP7701 (a typical rail-to-rail op amp), has an output drive capability of only 50 mV over all

temperatures for a 10 k

Ω load resistance. This is close to the lower supply voltage rail.

However, for true zero output applications with a single supply, the saturation voltage of the output stage is still a

limiting factor. This limitation has a negative impact on the functionality of true zero output applications. This is

illustrated in Figure 28.

Figure 28. Output Limitation for Single Supply True Zero Output Aapplication

In the following section, two applications will be discussed, showing the limitations of the output stage of an op

amp in a single supply configuration.

•

A single stage true zero amplifier, with a 12 bit ADC back end.

•

A dual stage true zero amplifier, with a 12 bit ADC back end.

One-stage, Single Supply True Zero Amplifier

This application shows a sensor with a DC output signal, amplified by a single supply op amp. The output voltage

of the op amp is converted to the digital domain using an Analog to Digital Converter (ADC). Figure 29 shows the

basic setup of this application.

Figure 29. Sensor with DC Output and a Single Supply Op Amp

12

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