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LTC1563-2/LTC1563-3

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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:

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, other

transfer functions and higher cutoff frequencies are

achieved. For these applications, the resistor value calcu-

lation 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 Version 3.0 (or

newer) or contact the Linear Technology Filter Applica-

tions group for assistance.

mode and 25.6kHz in the Low Power mode) is set by the

the gain is also limited to unity.

reliably obtaining large valued, precision resistors. As the

reliable, precise 10M resistance between two points on a

printed circuit board is somewhat difficult. For example, a

10M resistor with only 200M

Ω of stray, layout related

resistance in parallel, yields a net effective resistance of

9.52M and an error of – 5%. Note that the gain is also

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

assistance.

While the simple formula and the tables in the applications

section deliver good approximations of the transfer func-

tions, a more accurate response is achieved using FilterCAD.

FilterCAD calculates the resistor values using an accurate

and complex algorithm to account for parasitics and op

amp limitations. A design using FilterCAD will always yield

the best possible design. By using the FilterCAD design

tool you can also achieve filters with cutoff frequencies

beyond 256kHz. Cutoff frequencies up to 360kHz are

attainable.

Contact the Linear Technology Filter Applications Group

for a copy the FilterCAD software. FilterCAD can also be

downloaded from our website at www.linear.com.

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.