LTC1061CS datasheet(7/16 Pages) LINER

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LTC1061

ODES OF OPERATIO

Description and Applications

1. Primary Modes: There are two basic modes of opera-

tion, Mode 1 and Mode 3. In Mode 1, the ratio of the

external clock frequency to the center frequency of each

2nd order section is internally fixed at 50:1 or 100:1. In

Mode 3, this ratio can be adjusted above or below 50:1 or

100:1. The side C of the LTC1061 can be connected only

in Mode 3. Figure 3 illustrates Mode 1 providing 2nd order

notch, lowpass, and bandpass outputs (for definition of

filter functions, refer to the LTC1060 data sheet). Mode 1

can be used to make high order Butterworth lowpass

filters; it can also be used to make low Q notches and for

cascading 2nd order bandpass functions tuned at the

same center frequency and with unity-gain. Mode 3,

Figure 4, is the classical state variable configuration pro-

viding highpass, bandpass and lowpass 2nd order filter

functions.

Since the input amplifier is within the resonant loop, its

phase shift affects the high frequency operation of the

filter and therefore, Mode 3 is slower than Mode 1. Mode

3 can be used to make high order all-pole bandpass,

lowpass, highpass and notch filters. Mode 3 as well as

Mode 1 is a straightforward mode to use and the filter’s

dynamics can easily be optimized. Figure 5 illustrates a 6th

order lowpass Butterworth filter operating with up to

40kHz cutoff frequency and with up to 200kHz input

frequency. Sides A, B are connected in Mode 1 while side

C is connected in Mode 3. The lower Q section was placed

in side C, Mode 3, to eliminate any early Q enhancement.

This could happen when the clock approaches 2MHz. The

measured frequency response is shown in Figure 6. The

attenuation floor is limited by the crosstalk between the

three different sections operating with a clock frequency

above 1MHz. The measured wideband noise was

150

µVRMS. For limited temperature range the filter of

Figure 5 works up to 2.5MHz clock frequency thus yielding

a 50kHz cutoff.

Figure 4. Mode 3: 2nd Order Filter Providing Highpass,

Bandpass, Lowpass

Figure 3. Mode 1: 2nd Order Filter Providing Notch,

Bandpass, Lowpass

R32

VOUT

R13

VIN

LTC1061 F05

LTC1061

R22

R12

R31

R21

R41

R33

R23

T2L CLOCK

< 2.5MHz

V –

R11

V +

HARMONIC DISTORTION WITH fCLK = 2MHz

fIN

2ND HARMONIC

10kHz, 1VRMS

20kHz, 1VRMS

30kHz, 1VRMS

40kHz, 1VRMS

–74dB

–62dB

STANDARD 1%

RESISTOR VALUES

R11 = 20k

R31 = 11k

R12 = 20k

R32 = 14k

R13 = 10k

R21 = 20k

R41 = 20k

R22 = 20k

R23 = 10k

R33 = 17.8k

Figure 5. 6th Order Butterworth Lowpass Filter with

Cutoff Frequency up to 45kHz

–

∫

AGND

VIN

1061 F03

–

1/3 LTC1061

fO =

; fn = fO

HOLP = –

; HOBP = –

; HON1 = –

Q =

fCLK

100(50)

–

∫

AGND

VIN

1061 F04

–

1/3 LTC1061

fO =

; Q =

HOHP = –

; HOBP = –

; HOLP = –

fCLK

100(50)

√R2

NOTE: ADD CC FOR Q > 5 AND fCLK > 1MHz, SUCH AS CC ≅

0.16

× 1.2MHz

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