LTC6240HVHS8 datasheet(17/28 Pages) LINER

部件名	LTC6240HVHS8

功能描述	Single/Dual/Quad 18MHz, Low Noise, Rail-to-Rail Output, CMOS Op Amps

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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17 / 28 page

LTC6240/LTC6241/LTC6242

17

624012fc

APPLICATIO S I FOR ATIO

Ampliﬁer Characteristics

Figure 1 is a simpliﬁed schematic of the ampliﬁer, which

has a pair of low noise input transistors M1 and M2. A

simple folded cascode Q1, Q2 and R1, R2 allow the input

stage to swing to the negative rail, while performing level

shift to the Differential Drive Generator. Low offset voltage

is accomplished by laser trimming the input stage.

Capacitor C1 reduces the unity cross frequency and im-

proves the frequency stability without degrading the gain

bandwidth of the ampliﬁer. Capacitor Cm sets the overall

ampliﬁer gain bandwidth. The differential drive generator

supplies signals to transistors M3 and M4 that swing the

output from rail-to-rail.

The photo of Figure 2 shows the output response to an

input overdrive with the ampliﬁer connected as a voltage

follower. If the negative going input signal is less than

a diode drop below V–, no phase inversion occurs. For

input signals greater than a diode drop below V–, limit the

current to 3mA with a series resistor RS to avoid phase

inversion.

ESD

The LTC6240/LTC6241/LTC6242 have reverse-biased ESD

protection diodes on all input and outputs as shown in

Figure 1. If these pins are forced beyond either supply,

unlimited current will ﬂow through these diodes. If the

current is transient and limited to one hundred milliamps

or less, no damage to the device will occur.

The ampliﬁer input bias current is the leakage current of

these ESD diodes. This leakage is a function of the tem-

perature and common mode voltage of the ampliﬁer, as

shown in the Typical Performance Curves.

Noise

The LTC6240/LTC6241/LTC6242 exhibit exceptionally

low 1/f noise in the 0.1Hz to 10Hz region. This 550nVP-P

noise allows these op amps to be used in a wide variety

of high impedance low frequency applications, where

Zero-Drift ampliﬁers might be inappropriate due to their

charge injection.

In the frequency region above 1kHz the LTC6240/LTC6241/

LTC6242 also show good noise voltage performance. In

this frequency region, noise can easily be dominated by

the total source resistance of the particular application.

Speciﬁcally, these ampliﬁers exhibit the noise of a 3.1kΩ

resistor, meaning it is desirable to keep the source and

feedback resistance at or below this value, i.e. RS + RG||RFB

≤ 3.1kΩ. Above this total source impedance, the noise

voltage is not dominated by the ampliﬁer.

Noise current can be estimated from the expression in =

√2qIB, where q = 1.6 • 10

–19 coulombs. Equating √4kTRΔf

and R√2qIBΔf shows that for source resistors below 50GΩ

the ampliﬁer noise is dominated by the source resistance.

See the Typical Characteristics curve Noise Current vs

Frequency.

Figure 1. Simpliﬁed Schematic

Figure 2. Unity Gain Follower Test Circuit

R2

Q2

6241 F01

VIN

+

ITAIL

VIN

–

VO

V+

V–

CM

DESD5

DIFFERENTIAL

DRIVE

GENERATOR

BIAS

DESD6

V+

DESD2

V+

DESD4

V–

DESD1

V–

DESD3

R1

Q1

M2

M1

M3

M4

C1

+2.5V

–2.5V

6241 F02

–

+

LTC6240

RS

VIN

VOUT

VOUT AND VIN OF FOLLOWER WITH LARGE INPUT OVERDRIVE

VDD =

+2.5V

VSS =

–2.5V

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