LT6003CDC datasheet(15/20 Pages) LINER

15 / 20 page

LT6003/LT6004/LT6005

600345f

APPLICATIONS INFORMATION

Gain

The open-loop gain is almost independent of load when

the output is sourcing current. This optimizes performance

in single supply applications where the load is returned

to ground. The typical performance curve of Open-Loop

Gain for various loads shows the details.

Start-Up and Output Saturation Characteristics

Micropower op amps are often not micropower during

start-up or during output saturation. This can wreak havoc

on limited current supplies, in the worst case there may

not be enough supply current available to take the system

up to nominal voltages. Unlike the LT6003/LT6004/LT6005,

when the output saturates, some op amps may draw

excessive current and pull down the supplies, compromis-

ing rail-to-rail performance. Figure 2 shows the start-up

characteristics of the LT6003/LT6004/LT6005 for three

limiting cases. The circuits are shown in Figure 3. One

circuit creates a positive offset forcing the output to come

up saturated high. Another circuit creates a negative offset

forcing the output to come up saturated low, while the last

circuit brings the output up at 1/2 supply. In all cases, the

supply current is well controlled and is not excessive when

the output is on either rail.

Adaptive Filter

The circuit of Figure 4 shows the LT6005 applied as a

micropower adaptive ﬁlter, which automatically adjusts

the time constant depending on the signal level. Op amp

A1 buffers the input onto the RC which has either a 1ms

or 20ms time constant depending on the state of switch

S1. The signal is then buffered to the output by op amp

A2. Op amps A3 and A4 are conﬁgured as gain-of-40

difference ampliﬁers, gaining up the difference between

the buffered input voltage and the output. When there is

no difference, the outputs of A3 and A4 will be near zero.

When a positive signal step is applied to the input, the

output of A3 rises. When a negative signal step is applied

to the input, the output of A4 rises. These voltages are fed

to the LT6700-2 comparator which has a built in 400mV

reference. If the input step exceeds 10mV, the output

of the difference ampliﬁers will exceed 400mV and the

comparator output (wired in OR gate fashion) falls low.

This turns on S1, reducing the time constant and speed-

ing up the settling. The overall effect is that the circuit

provides “slow ﬁltering” with “fast settling”. Waveforms

are shown in the accompanying photo, for a 100mV input

step. The fast 1ms time constant is obvious in the output

waveform, while the slow time constant is discernible as

the slow ramping sections. That the slow time constant

is discernible at all is due to delay time in the difference

ampliﬁer and comparator functions.

Figure 2. Start-Up Characteristics

SUPPLY VOLTAGE (V)

1.2

600345 F02

1.0

0.8

0.6

0.4

0.2

0.5

1.5

3.5

4.5

2.5

OUTPUT LOW

OUTPUT HIGH

OUTPUT AT VS/2

TA = 25°C

Figure 3. Circuits for Start-Up Characteristics

–

30mV

Output High

–

VS/2

600345 F03

Output at VS/2

–

30mV

Output Low

LT6003CDC Datasheet(PDF) 15 Page - Linear Technology

LT6003CDC Datasheet(HTML) 15 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Descarga

Enlace URL