11

FN6153.3

February 11, 2008

Applications Information

Introduction

The ISL28136 and ISL28236 are single and dual channel

Bi-CMOS rail-to-rail input, output (RRIO) micropower

precision operational amplifiers. The parts are designed to

operate from single supply (2.4V to 5.5V) or dual supply

(±1.2V to ±2.75V). The parts have an input common mode

range that extends 0.25V above the positive rail and down to

the negative supply rail. The output operation can swing

within about 3mV of the supply rails with a 100k

Ω load.

Rail-to-Rail Input

Many rail-to-rail input stages use two differential input pairs,

a long-tail PNP (or PFET) and an NPN (or NFET). Severe

penalties have to be paid for this circuit topology. As the

input signal moves from one supply rail to another, the

operational amplifier switches from one input pair to the

other causing drastic changes in input offset voltage and an

undesired change in magnitude and polarity of input offset

current.

The ISL28136 and ISL28236 achieve input rail-to-rail

operation without sacrificing important precision

specifications and degrading distortion performance. The

devices’ input offset voltage exhibits a smooth behavior

throughout the entire common-mode input range. The input

bias current versus the common-mode voltage range gives

an undistorted behavior from typically down to the negative

rail to 0.25V higher than the positive rail.

Rail-to-Rail Output

A pair of complementary bi-polar devices are used to

achieve the rail-to-rail output swing. The PNP sinks current

to swing the output in the negative direction. The NPN

sources current to swing the output in the positive direction.

The ISL28136 and ISL28236 with a 100k

Ω load will swing to

within 3mV of the positive supply rail and within 3mV of the

negative supply rail.

Results of Over-Driving the Output

Caution should be used when over-driving the output for long

periods of time. Over-driving the output can occur in two ways.

1) The input voltage times the gain of the amplifier exceeds the

supply voltage by a large value or, 2) the output current

required is higher than the output stage can deliver. These

as much as 1µV/hr. of exposure under these conditions.

IN+ and IN- Input Protection

All input 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. They also contain

back-to-back diodes across the input terminals (see “Pin

the input differential voltage is expected to exceed 0.5V, an

external series resistor must be used to ensure the input

currents never exceed 5mA (Figure 38).

Enable/Disable Feature

The ISL28136 offers an EN pin that disables the device

when pulled up to at least 2.0V. In the disabled state (output

in a high impedance state), the part consumes typically 10µA

at room temperature. The EN pin has an internal pull-down.

If left open, the EN pin will pull to the negative rail and the

device will be enabled by default. The EN pin should never

be left floating. When not used, the EN pin should either be

left floating or connected to the V- pin.

By disabling the part, multiple ISL28136 parts can be

connected together as a MUX. In this configuration, the

outputs are tied together in parallel and a channel can be

selected by the EN pin. The loading effects of the feedback

resistors of the disabled amplifier must be considered when

multiple amplifier outputs are connected together. Note that

feed through from the IN+ to IN- pins occurs on any Mux

Amp disabled channel where the input differential voltage

best suited for small signal applications. If large signals are

keep the feed through current low enough to minimize the

impact on the active channel. See“Limitations of the

Differential Input Protection” on page 11 for more details.

Limitations of the Differential Input Protection

If the input differential voltage is expected to exceed 0.5V, an

external current limiting resistor must be used to ensure the

input current never exceeds 5mA. For non-inverting unity gain

applications, the current limiting can be via a series IN+ resistor,

or via a feedback resistor of appropriate value. For other gain

configurations, the series IN+ resistor is the best choice, unless

sufficiently large to limit the input current to 5mA.

Large differential input voltages can arise from several

sources:

1) During open loop (comparator) operation. Used this way,

the IN+ and IN- voltages don’t track, so differentials arise.

2) When the amplifier is disabled but an input signal is still

the varying IN+ signal creates a differential voltage. Mux

Amp applications are similar, except that the active channel

FIGURE 38. INPUT CURRENT LIMITING

-

+

ISL28136, ISL28236