LTC6362H データシートの表示（PDF） - Linear Technology

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LTC6362H

Precision, Low Power Rail-to-Rail Input/Output Differential Op Amp/SAR ADC Driver

Linear Technology 

LTC6362

APPLICATIONS INFORMATION

Input Common Mode Voltage Range

The LTC6362’s input common mode voltage (VICM) is

defined as the average of the two input pins, V+IN and

V–IN. The inputs of the LTC6362 are capable of swinging

rail-to-rail and as such the valid range that can be used for

VICM is V– to V+. However, due to external resistive divider

action of the gain and feedback resistors, the effective

range of signals that can be processed is even wider. The

input common mode range at the op amp inputs depends

on the circuit configuration (gain), VOCM and VCM (refer to

Figure 1). For fully differential input applications, where

VINP = –VINM, the common mode input is approximately:

VICM

=

V+IN

+

2

V–IN

≈

VOCM

• RI

RI +RF

+

VCM

• RF

RI +RF

With single-ended inputs, there is an input signal compo-

nent to the input common mode voltage. Applying only

VINP (setting VINM to zero), the input common voltage is

approximately:

VICM

=

V+IN

+

2

V–IN

≈ VOCM

• RI

RI +RF

+

VCM

•

RF

RI +RF

+

VINP

2

• RF

RI +RF

This means that if, for example, the input signal (VINP)

is a sine, an attenuated version of that sine signal also

appears at the op amp inputs.

VINP +–

VCM +– VINM +–

RI

V+IN

RF

VOCM

+

VOCM

–

RI V–IN

RF

V–OUT

6362 F01 V+OUT

Figure 1. Definitions and Terminology

Input Bias Current

Input bias current varies according to VICM. For common

mode voltages ranging from 0.2V above the negative

supply to 1.1V below the positive supply, input bias

current follows ∆IB/∆VICM = 75nA/V, with IB at VICM = 2.5V

typically below 75nA on a 5V supply. For common mode

voltages ranging from 1.1V below the positive supply to

0.2V below the positive supply, input bias current follows

∆IB/∆VICM = 25nA/V, with IB at VICM = 4.5V typically below

75nA on a 5V supply. Operating within these ranges allows

the amplifier to be used in applications with high source

resistances where errors due to voltage drops must be

minimized. For applications where VICM is within 0.2V of

either rail, input bias current may reach values over 1µA.

Input Impedance and Loading Effects

The low frequency input impedance looking into the VINP

or VINM input of Figure 1 depends on how the inputs are

driven. For fully differential input sources (VINP = –VINM),

the input impedance seen at either input is simply:

RINP = RINM = RI

For single-ended inputs, because of the signal imbalance

at the input, the input impedance actually increases over

the balanced differential case. The input impedance looking

into either input is:

RINP

=

RINM

=

1–





1

2 

RI

•





RF

RI + RF





Input signal sources with non-zero output impedances can

also cause feedback imbalance between the pair of feedback

networks. For the best performance, it is recommended

that the input source output impedance be compensated.

If input impedance matching is required by the source, a

termination resistor R1 should be chosen (see Figure 2)

such that:

R1=

RINM

RINM

•RS

– RS

According to Figure 2, the input impedance looking into

the differential amp (RINM) reflects the single-ended source

case, given above. Also, R2 is chosen as:

R2

=

R1||RS

=

R1• RS

R1+ RS

6362fa

11

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