LTC6244 datasheet(14/24 Pages) LINER

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LTC6244

6244f

APPLICATIO S I FOR ATIO

Ampliﬁer Characteristics

Figure 1 is a simpliﬁed schematic of the LTC6244, 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.

The input common mode voltage range extends from

V– to V+ – 1.5V. In unity gain voltage follower applications,

exceeding this range by applying a signal that reaches 1V

from the positive supply rail can create a low level instability

at the output. Loading the ampliﬁer with several hundred

micro-amps will reduce or eliminate the instability.

ESD

The LTC6244 has reverse-biased ESD protection diodes on

all input and outputs as shown in Figure 1. These diodes

protect the ampliﬁer for ESD strikes to 4kV. If these pins

are forced beyond either supply, unlimited current will

ﬂow through these diodes. If the current transient is less

than 1 second 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 Chacteristics.

Noise

The LTC6244 exhibits low 1/f noise in the 0.1Hz to 10Hz

region. This 1.5µVP-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 inap-

propriate due to their input sampling characteristic.

In the frequency region above 1kHz the LTC6244 also

shows good noise voltage performance. In this frequency

region, noise can easily be dominated by the total source

Figure 1. Simpliﬁed Schematic

Figure 2. Unity Gain Follower Test Circuit

6244 F01

VIN

ITAIL

VIN

–

V+

V–

DESD5

DIFFERENTIAL

DRIVE

GENERATOR

BIAS

DESD6

V+

DESD2

V+

DESD4

V–

DESD1

V–

DESD3

+2.5V

Ω

–2.5V

6244 F02

–

1/2

LTC6244

VIN

VOUT

VOUT AND VIN OF FOLLOWER WITH LARGE INPUT OVERDRIVE

V+

2.5V

V–

–2.5V

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