LT1352 datasheet(12/20 Pages) LINER

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LT6020/LT6020-1

60201fa

For more information www.linear.com/LT6020

applicaTions inForMaTion

Figure 1. Settling Time Is Essentially Flat

smaller inputs the LT6020 slew rate approaches the slew

rate more common in traditional micropower amplifiers.

Input Bias Current

The design of the input stage of the LT6020 is more so-

phisticated than that shown in the Simplified Schematic.

It uses both NPN and PNP input differential amplifiers to

sense the input differential voltage. As a result the speci-

fied input bias current can flow in or out of the input pins.

Multiplexer Applications/High Dynamic Input

Impedance

The LT6020 has features which make it desirable for

multiplexer applications, such as the application featured

on the back page of this data sheet. When the channels of

the multiplexer are cycled, the output of the multiplexer

can produce large voltage transitions. Normally, bipolar

amplifiers have back-to-back diodes between the inputs,

whichwillturnonwhentheinputtransientvoltageexceeds

0.7V, causing a large transient current to be conducted

from the amplifier output stage back into the input driving

circuitry. The driving circuitry then needs to absorb this

current and settle before the amplifier can settle. The

LT6020 uses 5.5V Zener diodes to protect its inputs which

dramaticallyincreasesitsinputimpedancewithinputsteps

as large as 5V.

Achieving Rail-to-Rail Operation without

Rail-to-Rail Inputs

The LT6020 output is able to swing close to each power

supply rail, but the input stage is limited to operating

between V– + 1.2V and V+ – 1.4V. For many inverting

applications and noninverting gain applications, this is

largely inconsequential. Figure 2 shows the basic op amp

configurations, what happens to the op amp inputs and

whether or not the op amp must have rail-to-rail inputs.

The circuit of Figure 3 shows an extreme example of the

inverting case. The input voltage at the 100k resistor can

swing ±13.5V and the LT6020 will output an inverted,

OUTPUT STEP (VP-P)

60201 F01

0.0015%

AV = 1

0.01%

The supply current of the LT6020 increases with large

differential input voltages. Normally, this does not impact

the low power nature of the LT6020 because the ampli-

fier is forcing the two inputs to be at the same potential.

Conditionswhichcausedifferentialinputvoltagetoappear

should be avoided in order to preserve the low power dis-

sipation of the LT6020. This includes but is not limited

to: operation as a comparator, excessive loading on the

output and overdriving the input.

Enhanced Slew Rate

The LT6020 uses a proprietary input stage which provides

an enhanced slew rate without sacrificing input precision

specssuchasinputoffsetvoltage,commonmoderejection

and noise. The unique input stage of the LT6020 allows the

output to quickly slew to its final value when large signal

input steps are applied. This enhanced slew characteristic

allows the LT6020 to quickly settle the output to 0.0015%

independent of input step size as shown in Figure 1. Typi-

calmicropoweramplifierscannotprocesslargeamplitude

signals with this speed. As shown in the Typical Perfor-

mance curves, when the LT6020 is configured in unity

gain and a 10V step is applied to the input the output will

slew at 5V/µs. In this same configuration, a 5V input step

will slew the output at 2.4V/µs. Furthermore, a 0.7V input

step will lower the slew rate to 0.2V/µs. Note that for these

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