9

FN6293.1

March 9, 2006

Description of Operation and Application

Information

Product Description

The EL8171 and EL8172 are micropower instrumentation

amplifiers (in-amps) which deliver rail-to-rail input

amplification and rail-to-rail output swing on a single 2.4V to

5V supply. The EL8171 and EL8172 also deliver excellent

DC and AC specifications while consuming only 60µA typical

supply current. Because EL8171 and EL8172 provide an

independent pair of feedback terminals to set the gain and to

adjust the output level, these in-amps achieve high common-

mode rejection ratio regardless of the tolerance of the gain

setting resistors. The EL8171 is internally compensated for a

minimum closed loop gain of 10 or greater, well suited for

moderate to high gains. For higher gains, the EL8172 is

internally compensated for a minimum gain of 100. An

ENABLE pin is used to reduce power consumption, typically

2.9µA, while the instrumentation amplifier is disabled.

Input Protection

All input and feedback terminals of the EL8171 and EL8172

have internal ESD protection diodes to both positive and

negative supply rails, limiting the input voltage to within one

diode drop beyond the supply rails. If overdriving the inputs

is necessary, the external input current must never exceed

5mA. External series resistor may be used as a protection to

limit excessive external voltage and current from damaging

the inputs.

Input Stage and Input Voltage Range

The input terminals (IN+ and IN-) of the EL8171 and EL8172

are single differential pair P-MOSFET devices aided by an

Input Range Enhancement Circuit to increase the headroom

of operation of the common-mode input voltage. The

feedback terminals (FB+ and FB-) also have a similar

topology. As a result, the input common-mode voltage range

of both the EL8171 and EL8172 is rail-to-rail. These in-amps

are able to handle input voltages that are at or slightly

beyond the supply and ground making these in-amps well

suited for single 5V or 3.3V low voltage supply systems.

There is no need then to move the common-mode input of

the in-amps to achieve symmetrical input voltage.

Output Stage and Output Voltage Range

A pair of complementary MOSFET devices drives the output

VOUT to within a few mV of the supply rails. At a 100k

Ω

load, the PMOS sources current and pulls the output up to

4mV below the positive supply, while the NMOS sinks

current and pulls the output down to 4mV above the negative

supply, or ground in the case of a single supply operation.

The current sinking and sourcing capability of the EL8171

and EL8172 are internally limited to 29mA.

Gain Setting

VIN, the potential difference across IN+ and IN-, is replicated

(less the input offset voltage) across FB+ and FB-. The

obsession of the EL8171 and EL8172 in-amp is to maintain

the differential voltage across FB+ and FB- equal to IN+ and

IN-; (FB+ - FB-) = (IN+ - IN-). Consequently, the transfer

function can be derived. The gain of the EL8171 and EL8172

is set by two external resistors, the feedback resistor RF, and

the gain resistor RG.

In Figure 28, the FB+ pin and one end of resistor RG are

connected to GND. With this configuration, the above gain

equation is only true for a positive swing in VIN; negative

input swings will be ignored and the output will be at ground.

Reference Connection

Unlike a three-opamp instrumentation amplifier, a finite

series resistance seen at the REF terminal does not degrade

the EL8171 and EL8172's high CMRR performance

eliminating the need for an additional external buffer

amplifier. Circuit 2 (Figure 29) uses the FB+ pin to provide a

high impedance REF terminal.

1

2

3

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

VS+

VS-

EN

2.9V to 5V

RF

RG

VOUT

EN_BAR

EL8171/2

VCM

VIN/2

VIN/2

FIGURE 28. CIRCUIT 1 - GAIN IS BY EXTERNAL RESISTORS

VOUT

1

--------

+







VIN

=

FIGURE 29. CIRCUIT 2 - GAIN SETTING AND REFERENCE

CONNECTION

1

2

3

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

VS+

VS-

EN

2.9V to 5V

RF

RG

VOUT

EN_BAR

EL8171/2

VCM

VIN/2

VIN/2

R1

R2

REF

2.9V to 5V

VOUT

1

--------

+







VIN

()

1

--------

+







VREF

()

+

=

EL8171, EL8172