11

FN7490.4

February 14, 2008

Description of Operation and Applications

Information

Product Description

The EL8170 and EL8173 are micropower instrumentation

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

The EL8170 and EL8173 also deliver excellent DC and AC

specifications while consuming only 65µA typical supply

current. Because the EL8170 and EL8173 provide an

independent pair of feedback terminals to set the gain and to

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

rejection ratio regardless of the tolerance of the gain setting

resistors. The EL8173 is internally compensated for a minimum

closed loop gain of 10 or greater, well suited for moderate to

high gains. For higher gains, the EL8170 is internally

compensated for a minimum gain of 100. An EN pin is used to

reduce power consumption, typically 2.6µA, while the

instrumentation amplifier is disabled.

Input Protection

All input and feedback terminals of the EL8170 and EL8173

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 EL8170 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 EL8173 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 EL8173 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 EL8170 and EL8173

are single differential pair bipolar PNP 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 EL8170 and EL8173 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

need to move the common-mode input of the in-amps to

achieve symmetrical input voltage.

Input Bias Cancellation, Input Bias Compensation

Both EL8170 and EL8173 feature an Input Bias

Cancellation/Compensation Circuit for both the input and

feedback terminals (IN+, IN-, FB+ and FB-), achieving a low

input bias current all throughout the input common-mode

range and the operating temperature range. While the PNP

bipolar input stages are biased with an adequate amount of

biasing current for speed and increased noise performance,

the Input Bias Cancellation/Compensation Circuit sinks most

of the base current of the input transistor leaving a small

portion as input bias current, typically 500pA. In addition, the

Input Bias Cancellation/Compensation Circuit maintains a

smooth and flat behavior of input bias current over the

common mode range and over the operating temperature

range. The Input Bias Cancellation, Input Bias Compensation

Circuit operates from input voltages of 10mV above the

negative supply to input voltages slightly above the positive

supply. See “Average Input Bias Current vs Common-Mode

Input Voltage” in the “Typical Performance Curves” beginning

on page 4.

Output Stage and Output Voltage Range

A pair of complementary MOSFET devices drives the output

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 EL8170

and EL8173 are internally limited to 26mA.

Gain Setting

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

objective of the EL8170 and EL8173 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 EL8170 and EL8173

EL8170, EL8173