12

FN7490.1

March 9, 2006

higher differential input voltages at lower gain applications. It

is recommended however, that the input terminals of the

EL8173 is 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 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.

Input Bias Cancellation/Compensation

Inside the EL8170 and EL8173 is 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/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 performance curves section.

Output Stage and Output Voltage Range

A pair of complementary MOSFET devices drives the output

VOUT to within a few millivolts 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 EL8170

and EL8173 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 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

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

the gain resistor RG.

In Figure 41, 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-op amp instrumentation amplifier, a finite

series resistance seen at the REF terminal does not degrade

the EL8170 and EL8173's high CMRR performance

eliminating the need for an additional external buffer

amplifier. Figure 42 uses the FB+ pin to provide a high

impedance REF terminal.

.

1

3

2

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

VS+

VS-

EN

2.9V to 5V

RF

RG

VOUT

EN_BAR

EL8170/3

VCM

VIN/2

VIN/2

FIGURE 41. GAIN IS SET BY TWO EXTERNAL RESISTORS,

VOUT

1

--------

+







VIN

=

FIGURE 42. GAIN SETTING AND REFERENCE CONNECTION

1

3

2

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

VS+

VS-

EN

2.9V to 5V

RF

RG

VOUT

EN_BAR

EL8170/3

VCM

VIN/2

VIN/2

REF

2.9V to 5V

R1

R2

VOUT

1

--------

+







VIN

()

1

--------

+







VREF

()

+

=

EL8170, EL8173