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11
LT1638/LT1639
16389fc
APPLICATIONS INFORMATION
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TYPICAL APPLICATIONS
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With 1.2MHz bandwidth, Over-The-Top capability, re-
verse-battery protection and rail-to-rail input and output
features, the LT1638/LT1639 are ideal candidates for
general purpose applications.
The lowpass slope limiting filter in Figure 1 limits the
maximum dV/dT (not frequency) that it passes. When the
input signal differs from the output by one forward diode
drop, D1 or D2 will turn on. With a diode on, the voltage
across R2 will be constant and a fixed current, V
DIODE
/R2,
will flow through capacitor C1, charging it linearly instead
of exponentially. The maximum slope that the circuit will
pass is equal to V
DIODE
divided by (R2)(C1). No matter
how fast the input changes the output will never change
any faster than the dV/dT set by the diodes and (R2)(C).
nonlinear common mode rejection. If the op amp is oper-
ating inverting there is no common mode induced distor-
tion. If the op amp is operating in the PNP input stage
(input is not within 0.8V of V
+
), the CMRR is very good,
typically 98dB. When the LT1638 switches between input
stages there is significant nonlinearity in the CMRR. Lower
load resistance increases the output crossover distortion,
but has no effect on the input stage transition distortion.
For lowest distortion the LT1638/LT1639 should be oper-
ated single supply, with the output always sourcing
current and with the input voltage swing between ground
and (V
+
– 0.8V). See the Typical Performance Character-
istics curves.
Gain
The open-loop gain is almost independent of load when
the output is sourcing current. This optimizes perfor-
mance 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.
V
OUT
V
IN
Response of Slope Limiting Filter
Figure 2. Lowpass Slope Limiting Filter with 0 TC
Figure 1. Lowpass Slope Limiting Filter
1638/39 TA02
A modification of this application is shown in Figure 2
using references instead of diodes to set the maximum
slope. By using references, the slope is independent of
temperature. A scope photo shows a 1V
P-P
, 2kHz input
signal with a 2V pulse added to the sine wave; the circuit
passes the 2kHz signal but limits the slope of the pulse.
–
+
1/2 LT1638
C1
V
OUT
1638/39 F01
R2
D2
D1
R1
V
IN
FOR R1 = 10k, R2 = 100k, C1 = 1000pF
V
OUT(MAX)
=
d
dt
V
D
(R2)(C1)
V
OUT(MAX)
= 0.006V/µs
d
dt