LTC1563-2/LTC1563-3
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W U U
APPLICATIONS INFORMATION
Output Loading: Resistive and Capacitive
near the cutoff frequency. Poor layout can give 0.5dB to
1dB of excess peaking.
The op amps of the LTC1563-X have a rail-to-rail output
stage. To obtain maximum performance, the output load-
ing effects must be considered. Output loading issues can
be divided into resistive effects and capacitive effects.
To minimize the effects of parasitic layout capacitance, all
of the resistors for section A should be placed as close as
possible to the SA pin. Place the R31 resistor first so that
it is as close as possible to the SA pin on one end and as
close as possible to the INVA pin on the other end. Use the
same strategy for the layout of section B, keeping all of the
resistors as close as possible to the SB node and first
placing R32 between the SB and INVB pins. It is also best
if the signal routing and resistors are on the same layer as
the part without any vias in the signal path.
Resistiveloadingaffectsthemaximumoutputsignalswing
and signal distortion. If the output load is excessive, the
output swing is reduced and distortion is increased. All of
theoutputvoltageswingtestingontheLTC1563-Xisdone
withR22=100kanda10kloadresistor.Forbestundistorted
outputswing, theoutputloadresistanceshouldbegreater
than 10k.
Figure 1 illustrates a good layout using the LTC1563-X
with surface mount 0805 size resistors. An even tighter
layout is possible with smaller resistors.
Capacitive loading on the output reduces the stability of
the op amp. If the capacitive loading is sufficiently high,
the stability margin is decreased to the point of oscillation
at the output. Capacitive loading should be kept below
30pF. Good, tight layout techniques should be maintained
at all times. These parts should not drive long traces and
must never drive a long coaxial cable. When probing the
LTC1563-X,alwaysusea10xprobe.Neverusea1xprobe.
A standard 10x probe has a capacitance of 10pF to 15pF
while a 1x probe’s capacitance can be as high as 150pF.
The use of a 1x probe will probably cause oscillation.
R11
V
IN
LTC1563-X
V
OUT
R12
For larger capacitive loads, a series isolation resistor can
be used between the part and the capacitive load. If the
load is too great, a buffer must be used.
1653 F01
Figure 1. PC Board Layout
Layout Precautions
Single Pole Sections and Odd Order Filters
The LTC1563-X is an active RC filter. The response of the
filter is determined by the on-chip capacitors and the
external resistors. Any external, stray capacitance in par-
allel with an on-chip capacitor, or to an AC ground, can
alter the transfer function.
The LTC1563 is configured to naturally form even ordered
filters (2nd, 4th, 6th and 8th). With a little bit of work,
singlepolesectionsandoddorderfiltersareeasilyachieved.
To form a single pole section you simply use the op amp,
the on-chip C1 capacitor and two external resistors as
shown in Figure 2. This gives an inverting section with the
gain set by the R2-R1 ratio and the pole set by the R2-C1
time constant. You can use this pole with a 2nd order
section to form a noninverting gain 3rd order filter or as a
stand alone inverting gain single pole filter.
Capacitance to an AC ground is the most likely problem.
Capacitance on the LPA or LPB pins does not affect the
transfer function but does affect the stability of the op
amps. Capacitance on the INVA and INVB pins will affect
the transfer function somewhat and will also affect the
stability of the op amps. Capacitance on the SA and SB
pins alters the transfer function of the filter. These pins are
the most sensitive to stray capacitance. Stray capacitance
on these pins results in peaking of the frequency response
Figure 3 illustrates another way of making odd order
filters. The R1 input resistor is split into two parts with an
additional capacitor connected to ground in between the
resistors.This“TEE”networkformsasinglerealpole.RB1
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Linear [ Linear ]
