LTC1164

10

1164fa

Mode 3

Mode 3 is the second of the primary modes. In Mode 3, the

ratio of the external clock frequency to the center

frequency of each 2nd order section can be adjusted above

or below 50:1 or 100:1. Side D of the LTC1164 can only be

connected in Mode 3. Figure 7 illustrates Mode 3, the

classical state variable configuration, providing highpass,

bandpass, and lowpass 2nd order filter functions. Mode 3

is slower than Mode 1. Mode 3 can be used to make high

order all-pole bandpass, lowpass, highpass and notch

filters.

When the internal clock-to-center frequency ratio is set at

50:1, the design equations for Q and bandpass gain are

different from the 100:1 case. This was done to provide

speed without penalizing the noise performance.

SECONDARY MODES

Mode 1b

Mode 1b is derived from Mode 1. In Mode 1b, Figure 8, two

additional resistors R5 and R6, are added to alternate the

amount of voltage feedback from the lowpass output into

the input of the SA (or SB or SC) switched capacitor

summer. This allows the filter clock-to-center frequency

ratio to be adjusted beyond 50:1 or 100:1. Mode 1b

maintains the speed advantages of Mode 1.

Mode 2

Mode 2 is a combination of Mode 1 and Mode 3, as shown

in Figure 9. With Mode 2, the clock-to-center frequency

ratio, fCLK/fO, is always less than 50:1 or 100:1. The

advantage of Mode 2 is that it provides less sensitivity to

resistor tolerances than does Mode 3. As in Mode 1,

Mode 2 has a notch output which depends on the clock

frequency, and the notch frequency is therefore less than

the center frequency, fO.

When the internal clock-to-center frequency ratio is set at

50:1, the design equations for Q and bandpass gain are

different from the 100:1 case.

Figure 8. Mode 1b: 2nd Order Filter Providing Notch,

Bandpass, Lowpass

Figure 7. Mode 3: 2nd Order Filter Providing Highpass,

Bandpass, Lowpass

LTC1164 • MOO02

R3

R4

R2

HP

S

BP

LP

R1

AGND

R2/R1;

1/4 LTC1164

fo =

; Q =

; HOHP = –

R3/R1;

HOBP = –

HOBP = –

MODE 3 (100:1):

R4/R1

HOLP = –

R4/R1

; HOLP = –

R2/R1;

HOLP = –

fCLK

100

–

+

+

–

Σ

∫

∫

VIN

CC

R3

R2

R2

R4

fo =

; Q =

MODE 3 (50:1):

; THEN CALCULATE R1 TO SET

THE DESIRED GAIN

R3 =

R2

NOTE: THE 50:1 EQUATIONS FOR MODE 3 ARE DIFFERENT FROM THE EQUATIONS

FOR MODE 3 OPERATION OF THE LTC1059, LTC1060 AND LTC1061. START WITH

fo, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:

fCLK

50

R2

R4

R2

R4

R3/R1

1 – (R3/16R4)

1.005 (√R2/R4)

(R2/R3) – (R2/16R4);

+

1.005

Q

R2

R4

R2

16R4

LTC1164 • MOO03

R3

R6

R5

R2

NS

BP

LP

R1

AGND

fo =

; Q =

= –

;

; (R5//R6) < 5k

Ω

HON1 (f → 0) = HON2 f →

HOBP = –

; HOLP =;

fCLK

100(50)

–

+

+

–

Σ

∫

∫

VIN

fCLK

2

R3

R2

R3

R1

R2

R1

; fn = fo

R6

R5 + R6

– R2/R1

R6/(R5 + R6)

R6

R5 + R6

()

ODES OF OPERATIO