LTC6241CS8 datasheet(14/24 Pages) LINER

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LTC6241/LTC6242

62412f

APPLICATIO S I FOR ATIO

Ampliﬁer Characteristics

Figure 1 is a simpliﬁed schematic of the LTC6241, 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 LTC6241 has 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 LTC6241 exhibits 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 LTC6241 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

6241 F01

VIN

ITAIL

VIN

–

V+

V–

DESD5

DIFFERENTIAL

DRIVE

GENERATOR

BIAS

DESD6

V+

DESD2

V+

DESD4

V–

DESD1

V–

DESD3

+2.5V

–2.5V

6241 F02

–

1/2

LTC6241

VIN

VOUT

VOUT AND VIN OF FOLLOWER WITH LARGE INPUT OVERDRIVE

VDD =

+2.5V

VSS =

–2.5V

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