LTC1563-2/ LTC1563-3
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APPLICATIONS INFORMATION
Functional Description
in parallel, yields a net effective resistance of 952k and an
error of –5%. Note that the gain is also limited to unity at
the minimum fC.
The LTC1563-2/LTC1563-3 are a family of easy-to-use,
4th order lowpass filters with rail-to-rail operation. The
LTC1563-2, with a single resistor value, gives a unity-gain
filter approximating a Butterworth response. The
LTC1563-3, with a single resistor value, gives a unity-gain
filter approximating a Bessel (linear phase) response. The
proprietary architecture of these parts allows for a simple
unity-gain resistor calculation:
At intermediate fC, the gain is limited by one of the two
reasons discussed above. For best results, design filters
with gain using FilterCAD Version 3 (or newer) or contact
the Linear Technology Filter Applications Group for assis-
tance.
DC Offset, Noise and Gain Considerations
R = 10k(256kHz/fC)
The LTC1563-X is DC offset trimmed in a 2-step manner.
First, section A is trimmed for minimum DC offset. Next,
section B is trimmed to minimize the total DC offset
(section A plus section B). This method is used to give the
minimum DC offset in unity gain applications and most
higher gain applications.
where fC is the desired cutoff frequency. For many appli-
cations, this formula is all that is needed to design a filter.
For example, a 50kHz filter requires a 51.2k resistor. In
practice, a 51.1k resistor would be used as this is the
closest E96, 1% value available.
The LTC1563-X is constructed with two 2nd order sec-
tions. The output of the first section (section A) is simply
fed into the second section (section B). Note that section
A and section B are similar, but not identical. The parts are
designed to be simple and easy to use.
Forgains greaterthanunity, thegainshouldbedistributed
such that most of the gain is taken in section A, with
section B at a lower gain (preferably unity). This type of
gain distribution results in the lowest noise and lowest DC
offset. For high gain, low frequency applications, all of the
gain is taken in section A, with section B set for unity-gain.
In this configuration, the noise and DC offset is dominated
by those of section A. At higher frequencies, the op amps’
finite bandwidth limits the amount of gain that section A
can reliably achieve. The gain is more evenly distributed in
this case. The noise and DC offset of section A is now
multiplied by the gain of section B. The result is slightly
higher noise and offset.
By simply utilizing different valued resistors, gain and
other transfer functions are achieved. For these applica-
tions,theresistorvaluecalculationgets moredifficult.The
tables of formulas provided later in this section make this
task much easier. For best results, design these filters
TM
using FilterCAD Version 3.0 (or newer) or contact the
Linear Technology Filter Applications group for assis-
tance.
Cutoff Frequency (fC) and Gain limitations
Output Loading: Resistive and Capacitive
The LTC563-X has both a maximum fC limit and a mini-
mumfC limit.ThemaximumfC limit(256kHzinHighSpeed
mode and 25.6kHz in the Low Power mode) is set by the
speed of the LTC1563-X’s op amps. At the maximum fC,
the gain is also limited to unity.
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.
Resistiveloadingaffects themaximumoutputsignalswing
and signal distortion. If the output load is excessive, the
output swing is reduced and distortion is increased. All of
theoutputvoltageswingtestingontheLTC1563-Xis done
withR22=100kanda10kloadresistor.Forbestundistorted
outputswing, theoutputloadresistanceshouldbegreater
than 10k.
A minimum fC is dictated by the practical limitation of
reliably obtaining large valued, precision resistors. As the
desiredfC decreases,theresistorvaluerequiredincreases.
When fC is 2.56kHz, the resistors are 1M. Obtaining a
reliable, precise 1M resistance between two points on a
printed circuit board is somewhat difficult. For example, a
1M resistor with 20MΩ of stray, layout related resistance
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