ISL28271, ISL28272

Rev X.00

Page 11 of 14

Application Information

Product Description

The ISL28271 and ISL28272 are dual channel micropower

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

input amplification and rail-to-rail output swing. The in-amps

also deliver excellent DC and AC specifications while

consuming only about 120µA for both channels. Because the

independent pair of feedback terminals set the gain and adjust

the output zero level, the ISL28271 and ISL28272 achieve high

CMRR regardless of the tolerance of the gain setting resistors.

The ISL28271 is internally compensated for a minimum gain of

10. The ISL28272 is internally compensated for a minimum

gain of 100.

EN pins are available to independently enable or disable a

channel. When all channels are off, current consumption is

down to typically 4µA.

Input Protection

All input terminals and feedback terminals have internal ESD

protection diodes to both positive and negative supply rails,

limiting the input voltage to within one diode beyond the supply

rails. Input signals originating from low impedance sources

should have current limiting resistors in series with the IN+ and

IN- pins to prevent damaging currents during power supply

sequencing and other transient conditions. The ISL28272 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.

External series resistors may be used as an external protection

to limit excessive external voltage and current from damaging

the inputs. On the other hand, the ISL28271 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 terminals of the ISL28271 are

not overdriven beyond 1V to avoid offset drift.

Input Stage and Input Voltage Range

The input terminals (IN+ and IN-) of the in-amps are a single

differential pair of CMOS 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 is rail-to-rail

regardless of the feedback terminal settings and regardless of

the gain settings. They are able to handle input voltages that

are at or slightly beyond the supply and ground sensing

making these in-amps well suited for single 5V down to 2.4V

supply systems.

The IREC enables rail-to-rail input amplification without the

problems usually associated with the dual differential stage

topology. The IREC ensures that there are no drastic changes

in offset voltage over the entire range of the input. See Input

Offset Voltage vs Common-Mode Input Voltage in performance

charts. IREC also cures the abrupt change and even reverse

polarity of the input bias current over the whole range of input.

Output Stage and Output Voltage Range

A Class AB common-source output stage drives the output.

The pair of complementary MOSFET devices drive the output

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



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

4mV below the positive supply. 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 are internally limited to 31mA.

When disabled, the outputs are in a high impedance state.

Gain Setting

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

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

function of the 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

in-amp gain is set by two external resistors, the feedback

In Figure 37, the FB+ pin and one end of resistor RG are

connected to GND. With this configuration, Equation 1 is only

true for a positive swing in VIN; negative input swings will be

ignored because the output will be at ground.

Reference Connection

Unlike a three op amp in-amp realization, a finite series

resistance seen at the REF terminal does not degrade the high

CMRR performance eliminating the need for an additional

external buffer amplifier. Figure 38 uses the FB+ pin to provide

a high impedance REF terminal.

+

-

+

-

IN+

IN-

FB+

FB-

V+

V-

EN

2.4V TO 5.5V

RF

RG

VOUT

EN

ISL28271

VCM

IN+

IN-

FIGURE 37. GAIN IS SET BY TWO EXTERNAL RESISTORS,

ISL28272

VOUT

1

--------

+







VIN

=

VIN

IN+ IN-

–

=

(EQ. 1)