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15632I Datasheet(PDF) 10 Page - Linear Technology |
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15632I Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1563-2/LTC1563-3 156323fa APPLICATIONS INFORMATION 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: R = 10k(256kHz/fC) 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. 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. Cutoff Frequency (fC) and Gain Limitations The LTC563-X has both a maximum fC limit and a mini- mum fC limit. The maximum fC limit (256kHz in High Speed 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. A minimum fC is dictated by the practical limitation of reliably obtaining large valued, precision resistors. As the desired fC decreases, the resistor value required increases. When fC is 256Hz, the resistors are 10M. Obtaining a 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 limited to unity at the minimum fC. 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 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. |
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