6

FN7504.2

October 5, 2005

Applications Information

Introduction

The EL8178 is a rail-to-rail input and output micro-power

precision single supply operational amplifier with an enable

feature. The device achieves rail-to-rail input and output

operation and eliminates the concerns introduced by a

conventional rail-to-rail I/O operational Amplifier.

Rail-to-Rail Input

The PFET input stage of the EL8178 has an input common-

mode voltage range that goes from negative supply to

positive supply without introducing offset errors or degrading

performance associated with a conventional rail-to-rail input

operational amplifier. Many rail-to-rail input stages use two

differential input pairs, a long-tail PNP (or PFET) and an

NPN (or NFET). Severe penalties have to be paid for this

topology. As the input signal moves from one supply rail to

another, the operational amplifier switches from one input

pair to the other causing drastic changes in input offset

voltage and an undesired change in magnitude and polarity

of input offset current.

The EL8178 achieves input rail-to-rail 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 achieves rail-to-

rail output swing. The NMOS sinks current to swing the

output in the negative direction. The PMOS sources current

to swing the output in the positive direction. The EL8178 with

a 100k

Ω load will swing to within 3mV of the supply rails.

Enable/Disable Feature

The EL8178 offers an EN pin. The active low enable pin

disables the device when pulled up to at least 2.2V. Upon

disable the part consumes typically 3µA, while the output is

in a high impedance state. The EN also has an internal pull

down. If left open, the EN pin will pull to negative rail and the

device will be enabled by default. The high impedance at

output during disable allows multiple EL8178s to be

connected together as a MUX. The outputs are tied together

in parallel and a channel can be selected by the EN pin.

Typical Applications

FIGURE 17. PH PROBE AMPLIFIER

A general-purpose combination pH probe has extremely

high output impedance typically in the range of 10G

Ω to

12G

Ω. Low loss and expensive Teflon cables are often used

to connect the PH probe to the meter electronics. The above

circuit provides a low-cost alternative solution using the

EL8178 and a low-cost coax cable. The EL8178 PMOS high

impedance input senses the pH probe output signal and

buffers it to drive the coax cable. Its rail-to-rail input nature

also eliminates the need for a bias resistor network required

by other amplifiers in the same application.

FIGURE 18. THERMOCOUPLE AMPLIFIER

Thermocouples are the most popular temperature-sensing

device because of their low cost, interchangeability, and

ability to measure a wide range of temperatures. The

EL8178 is used to convert the differential thermocouple

voltage into single-ended signal with 10X gain. The

EL8178's rail-to-rail input characteristic allows the

thermocouple to be biased at ground and the converter to

run from a single 5V supply.

-

+

3V

+

V+

V-

EL8178

COAX

GENERAL

PURPOSE

COMBINATION

pH PROBE

-

+

5V

+

V+

V-

EL8178

K TYPE

THERMOCOUPLE

10k

Ω

10k

Ω

100k

Ω

100k

Ω

410µV/°C

EL8178