LTC1061

12

1061fe

+

Σ

∫

∫

AGND

R1

N

BP

LP

1061 F20

+

–

S

R2

R3

R4

; Q =

= –

100(50)

R2

R1

R3

R2

–R2/R1

1 + (R2/R4)

R3

R1

2

f

→

()

R4

100(50)

R4

–R2/R1

1 + (R2/R4)

–

1

0

–90

–50

–30

–10

10

100

1061 F19

–80

–70

–60

–40

–20

4

STANDARD 1%

RESISTOR VALUES

R11 = 30.9k

R31 = 16.2k

R22 = 10.5k

R42 = 10k

R33 = 28.7k

R21 = 10k

R41 = 26.7k

R32 = 100k

R23 = 10k

R43 = 12.7k

LOWPASS SUCH AS (1/2

200kHz

500kHz

1MHz

0

–10

1.6

1061 F18

–30

–60

–70

0.2

0.8

1.2

2.0

–50

–40

–20

0

0.4 0.6

1.0

1.4

1.8

STANDARD 1%

RESISTOR VALUES

R11 = 84.5k

R31 = 31.6k

R22 = 10k

R42 = 97.6k

R33 = 300k

R21 = 10.2k

R41 = 63.4k

R32 = 232k

R23 = 20k

R43 = 80.6k

NOTE: CONNECT 39pF AND

100pF ACROSS R21 AND R22

RESPECTIVELY.

ODES OF OPERATIO

Mode 2 – This is a combination of Mode 1 and Mode 3,

Figure 20. With Mode 2, the clock-to-center frequency

compared to Mode 3 and for applications requiring 2nd

Mode 2 provides less sensitivity to resistor tolerances. As

in Mode 1, Mode 2 has a notch output which directly

depends on the clock frequency and therefore the notch

the 2nd order section.

Figure 18 shows the plotted amplitude responses of a 6th

order notch filter operating again with a clock-to-center

notch frequency ratio of 250:1. The theoretical notch

depth is 70dB and when the notch is centered at 1kHz its

width is 50Hz. Two small, noncritical capacitors were used

across the R21 and R22 resistors of Figure 16, to band-

limit the first two highpass outputs such that the practical

notch depth will approach the theoretical value. With these

two fixed capacitors, the notch frequency can be swept

within a 3:1 range.

When the circuit of Figure 16 is used to realize lowpass

filter and at the same time eliminates any small clock

feedthrough. This is shown in Figure 19 where the ampli-

tude response of the filter is plotted for 3 different cutoff

frequencies. When the clock frequency equals or exceeds

1MHz, the stopband notches lose their depth due to the

finite bandwidth of the internal op amps and to the small

crosstalk between the different sides of the LTC1061. The

lowpass filter, however, does not lose its passband accu-

racy and it maintains nearly all of its attenuation slope. The

theoretical performance of the 7th order lowpass filter of

Figure 19 is 0.2dB passband ripple, 1.5:1 stopband-to-

cutoff frequency ratio, and 73dB stopband attenuation.

Without any tuning, the obtained results closely approxi-

mate the textbook response.

Figure 19. Frequency Responses of a 7th Order Lowpass Elliptic

Filter Realized with Figure 16 Topology

Figure 18. 6th Order Band Reject Filter Operating with a Clock-

to-Center Notch Frequency Ratio of 250:1. The Ratio of 0dB to

the – 65dB Notch Width is 8:1.

Figure 20. Mode 2: 2nd Order Filter Providing

Notch, Bandpass, Lowpass