LTC1563-3 Linear Technology, LTC1563-3 Datasheet - Page 7

• Manufacturer Part Number: LTC1563-3
• Description: Active RC/ 4th Order Lowpass Filter Family
• Manufacturer: Linear Technology

APPLICATIONS
Functional Description
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:
where f
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.
By simply utilizing different valued resistors, gain and
other transfer functions are achieved. For these applica-
tions, the resistor value calculation gets more difficult. The
tables of formulas provided later in this section make this
task much easier. For best results, design these filters
using FilterCAD
Linear Technology Filter Applications group for assis-
tance.
Cutoff Frequency (f
The LTC563-X has both a maximum f
mum f
mode and 25.6kHz in the Low Power mode) is set by the
speed of the LTC1563-X’s op amps. At the maximum f
the gain is also limited to unity.
A minimum f
reliably obtaining large valued, precision resistors. As the
desired f
When f
reliable, precise 1M resistance between two points on a
printed circuit board is somewhat difficult. For example, a
1M resistor with 20M of stray, layout related resistance
R = 10k(256kHz/f
C
C
C
limit. The maximum f
C
is the desired cutoff frequency. For many appli-
is 2.56kHz, the resistors are 1M. Obtaining a
decreases, the resistor value required increases.
C
TM
is dictated by the practical limitation of
Version 3.0 (or newer) or contact the
U
C
C
) and Gain limitations
)
INFORMATION
U
C
limit (256kHz in High Speed
W
C
limit and a mini-
U
C
,
FilterCAD is trademark of Linear Technology Corporation.
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 f
At intermediate f
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
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.
For gains greater than unity, the gain should be distributed
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.
Output Loading: Resistive and Capacitive
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.
Resistive loading affects the maximum output signal swing
and signal distortion. If the output load is excessive, the
output swing is reduced and distortion is increased. All of
the output voltage swing testing on the LTC1563-X is done
with R22 = 100k and a 10k load resistor. For best undistorted
output swing, the output load resistance should be greater
than 10k.
C
.
C
LTC1563-2/LTC1563-3
, the gain is limited by one of the two
7