EL8178

FN7504 Rev 7.00

Page 9 of 13

May 14, 2008

Application Information

Introduction

The EL8178 is a rail-to-rail input and output (RRIO),

micropower, precision, single supply op amp with an enable

feature. This amplifier is designed to operate from single

supply (2.4V to 5.5V) or dual supply (±1.2V to ±2.75V) while

drawing only 55µA of supply current.The device achieves rail-

to-rail input and output operation while eliminating the

drawbacks of many conventional RRIO op amps.

Rail-to-Rail Input

The PFET input stage of the EL8178 has an input

common-mode voltage range that includes the negative and

positive supplies without introducing offset errors or degrading

performance like some existing rail-to-rail input op amps. Many

rail-to-rail input stages use two differential input pairs: a long-

tail PNP (or PFET) and an NPN (or NFET). Severe penalties

result from using this topology. As the input signal moves from

one supply rail to the other, the op amp switches from one

input pair to the other causing changes in input offset voltage

and an undesired change in the input offset current’s

magnitude and polarity.

The EL8178 achieves rail-to-rail input performance without

sacrificing important precision specifications and without

degrading distortion performance. The EL8178's input offset

voltage exhibits a smooth behavior throughout the entire

common-mode input range.

Rail-to-Rail Output

A pair of complementary MOSFET devices achieve rail-to-rail

output swing. The NMOS sinks current to swing the output in the

negative direction, while the PMOS sources current to swing the

output in the positive direction. The EL8178 with a 100k

 load

swings to within 3mV of the supply rails.

Results of Overdriving the Output

Caution should be used when overdriving the output for long

periods of time. Overdriving the output can occur in three ways:

1. The input voltage times the gain of the amplifier exceeds the

supply voltage by a large value.

2. The output current required is higher than the output stage can

deliver.

3. Operating the device in slew rate limit. These conditions can

1µV/hr of exposure under these conditions.

Enable/Disable Feature

The EL8178 features an active low EN pin that when pulled up

The EN pin has an internal pull-down, so an undriven pin pulls

to the negative rail, thereby enabling the op amp by default.

For applications where the EN pin is not being used, it is

recommended that the EN pin be permanently tyed to ground.

The high impedance output during disable allows for

connecting multiple EL8178s together to implement a Mux

Amp. The outputs are connected together and activating the

appropriate EN pin selects the desired channel. If utilizing non-

unity gain op amp configurations, then the loading effects of

the disabled amplifiers’ feedback networks must be considered

Pin Descriptions

SO PIN

NUMBER

SOT-23 PIN

NUMBER

PIN NAME

EQUIVALENT

CIRCUIT

DESCRIPTION

1

NC

No internal connection

2

4

IN-

Circuit 1

Amplifier’s inverting input

3

3

IN+

Circuit 1

Amplifier’s non-inverting input

4

2

V-

Circuit 4

Negative power supply

5

NC

No internal connection

6

1

OUT

Circuit 3

Amplifier’s output

7

6

V+

Circuit 4

Positive power supply

85

EN

Circuit 2

Amplifier’s enable pin with internal pull-down; Logic “1” selects the disabled

state; Logic “0” selects the enabled state.

EN

OUT

CIRCUIT 3

CIRCUIT 2

CAPACITIVELY

COUPLED

ESD CLAMP

CIRCUIT 4

V+

V+

V+

V-

V-

V-

IN-

V+

V-

CIRCUIT 1

IN+