10

FN6293.5

July 27, 2009

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 5.5V supply. The

EL8171 and EL8172 also deliver excellent DC and AC

specifications while consuming only 65µA 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.

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. The inverting inputs and

FB- inputs have ESD diodes to the V-rail, and the non-inverting

inputs and FB+ terminals have ESD diodes to the V+ rail. The

EL8172 has additional back-to-back diodes across the input

terminals and also across the feedback terminals. If overdriving

the inputs is necessary, the external input current must never

exceed 5mA. On the other hand, the EL8171 has no clamps to

limit the differential voltage on the input terminals allowing

higher differential input voltages at lower gain applications. It is

recommended however, that the input terminals of the EL8171

are not overdriven beyond 1V to avoid offset drift. An external

series resistor may be used as an external 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 (IREC) 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 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 drive the output

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 less than 35mA.

Pin Descriptions

EL8171/EL8172

PIN NAME

EQUIVALENT CIRCUIT

PIN FUNCTION

1

DNC

Do Not Connect; Internal connection - Must be left floating.

2

IN-

Circuit 1A, Circuit 1B

High impedance input terminals. EL8172 input circuit is shown in Circuit 1A, and

the EL8171 input circuit is shown in Circuit 1B. EL8171: to avoid offset drift, it is

recommended that the terminals are not overdriven beyond 1V and the input

current must never exceed 5mA.

3

IN+

Circuit 1A, Circuit 1B

4

V-

Circuit 3

Negative supply terminal.

5

FB-

Circuit 1A, Circuit 1B

High impedance feedback terminals. EL8172 input circuit is shown in Circuit 1A,

and the EL8171 input circuit is shown in Circuit 1B. EL8171: to avoid offset drift, it

is recommended that the terminals are not overdriven beyond 1V and the input

current must never exceed 5mA.

8

FB+

Circuit 1A, Circuit 1B

7

V+

Circuit 3

Positive supply terminal.

6

VOUT

Circuit 2

Output Voltage.

IN-

V+

V-

V+

V-

OUT

CIRCUIT 2

CIRCUIT 1A

V+

V-

CAPACITIVELY

COUPLED

ESD CLAMP

CIRCUIT 3

V+

V-

CIRCUIT 1B

FB-

IN-

FB-

IN+

FB+

IN+

FB+

EL8171, EL8172