LT6003/LT6004/LT6005
APPLICATIONS INFORMATION
Gain
V
V
V
S
S
S
30mV
V /2
S
+
–
+
–
+
–
The open-loop gain is almost independent of load when
theoutputissourcingcurrent.Thisoptimizesperformance
in single supply applications where the load is returned to
ground. The Typical Performance Characteristics curve of
Open-Loop Gain for various loads shows the details.
30mV
600345 F03
Output High
Output Low
Output at V /2
S
Figure 3. Circuits for Start-Up Characteristics
Start-Up and Output Saturation Characteristics
Adaptive Filter
Micropower op amps are often not micropower during
start-uporduringoutputsaturation. Thiscanwreakhavoc
on limited current supplies. In the worst case there may
not be enough supply current available to take the system
uptonominalvoltages.UnliketheLT6003/LT6004/LT6005,
when the output saturates, some op amps may draw
excessivecurrentandpulldownthesupplies,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
upsaturatedhigh. Anothercircuitcreatesanegativeoffset
forcing the output to come up saturated low, while the last
circuit brings the output up at 1/2 supply. In all cases, the
supplycurrentiswellcontrolledandisnotexcessivewhen
the output is on either rail.
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
for a 100mV input step are shown in the accompanying
photo. 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.
1.2
T
= 25°C
A
OUTPUT AT V /2
S
OUTPUT HIGH
OUTPUT LOW
1.0
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
SUPPLY VOLTAGE (V)
600345 F02
Figure 2. Start-Up Characteristics
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14
LINEAR_DIMENSIONS [ Linear Dimensions ]
