LTC1563-3IGN#PBF Datasheet LTC1563-2IGN#TRPBF, LTC1563-2CGN#PBF, LTC1563-2IGN#PBF | P10-P12

LTC1563-2/LTC1563-3

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APPLICATIONS INFORMATION

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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:

R = 10k(256kHz/f

)

where f

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 (f

) and Gain Limitations

The LTC563-X has both a maximum f

limit and a mini-

mum f

limit. The maximum f

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 f

the gain is also limited to unity.

A minimum f

is dictated by the practical limitation of

reliably obtaining large valued, precision resistors. As the

desired f

decreases, the resistor value required increases.

When f

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 200MΩ 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 f

At intermediate f

, 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.

LTC1563-2/LTC1563-3

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APPLICATIONS INFORMATION

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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.

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, always use a 10x probe. Never use a 1x probe

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.

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.

Layout Precautions

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.

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

near the cutoff frequency. Poor layout can give 0.5dB to

1dB of excess peaking.

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.

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.

1653 F01

R11

LTC1563-X

R12

R32

R22

R21

R31

OUT

Figure 1. PC Board Layout

Single Pole Sections and Odd Order Filters

The LTC1563 is configured to naturally form even ordered

filters (2nd, 4th, 6th and 8th). With a little bit of work,

single pole sections and odd order filters are easily achieved.

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.

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” network forms a single real pole. RB1

LTC1563-2/LTC1563-3

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should be much larger than RA1 to minimize the interac-

tion of this pole with the 2nd order section. This circuit is

useful in forming dual 3rd order filters and 5th order filters

with a single LTC1563 part. By cascading two parts, 7th

order and 9th order filters are achieved.

DC GAIN = LTC1563-2: C1A = 53.9pF, C1B = 39.2pF

LTC1563-3: C1A = 35pF, C1B = 26.8pF

–R2

2π • R2 • C1

1563 F02

–

INV

1/2 LTC1563

AGND

(OPEN)

OUT

Figure 2

RA1 ≈

RB1

R2RB1RA1 R3

1563 F03

RA1 • RB1

RA1 + RB1

()

2π •C

–

INV

1/2 LTC1563

AGND

–

(OPEN)

INV

1/2 LTC1563

1563 F04

AGND

You can also use the TEE network in both sections of the

part to make a 6th order filter. This 6th order filter does not

conform exactly to the textbook responses. Textbook

responses (Butterworth, Bessel, Chebyshev etc.) all have

three complex pole pairs. This filter has two complex pole

pairs and two real poles. The textbook response always

has one section with a low Q value between 0.5 and 0.6. By

replacing this low Q section with two real poles (two real

poles are the same mathematically as a complex pole pair

with a Q of 0.5) and tweaking the Q of the other two

complex pole pair sections you end up with a filter that is

indistinguishable from the textbook filter. The Typical

Applications section illustrates a 100kHz, 6th order pseudo-

Butterworth filter. FilterCAD is a valuable tool for custom

filter design and tweaking textbook responses.

Figure 3

What To Do with an Unused Section

If the LTC1563 is used as a 2nd or 3rd order filter, one of

the sections is not used. Do not leave this section uncon-

nected. If the section is left unconnected, the output is left

to float and oscillation may occur. The unused section

should be connected as shown in Figure 4 with the INV pin

connected to the LP pin and the S pin left open.

Figure 4

APPLICATIONS INFORMATION

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P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

LTC1563-3IGN#PBF

Mfr. #:

Buy LTC1563-3IGN#PBF

Manufacturer:

Analog Devices Inc.

Description:

Active Filter Active RC, 4th Order Lpass Filt Fam

Lifecycle:

New from this manufacturer.

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LTC1563-3IGN#PBF

LTC1563-3IGN#PBF

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