13

LTC1061

ODES OF OPERATIO

higher frequency notch provided by the side A of the

LTC1061. As shown in Figure 22, the highpass corner

frequency is 3.93kHz and the higher notch frequency is

3kHz while the filter operates with a 300kHz clock. The

center frequencies, Qs, and notches of Figure 22, when

topology of Figure 16, this approach uses lower and more

restricted clock frequencies. The obtained notch in Mode

2 is shallower although the topology is more efficient.

Output Noise

The wideband RMS noise of the LTC1061 outputs is nearly

independent from the clock frequency. The LTC1061

noise when operating with

±2.5V supply is lower, as Table

3 indicates. The noise at the bandpass and lowpass

outputs increases rough as the

√Q. Also the noise in-

creases when the clock-to-center frequency ratio is al-

tered with external resistors to exceed the internally set

100:1 or 50:1 ratios. Under this condition, the noise

increases square root-wise.

Output Offsets

The equivalent input offsets of the LTC1061 are shown in

Figure 23. The DC offset at the filter bandpass output is

outputs (Notch and LP) depend on the mode of operation

and external resistor ratios. Table 4 illustrates this.

It is important to know the value of the DC output offsets,

especially when the filter handles input signals with large

dynamic range. As a rule of thumb, the output DC offsets

increase when:

1. The Qs decrease

done by decreasing either the (R2/R4) or the R6/(R5

+ R6) resistor ratios.

Figure 21 shows the side A of the LTC1061 connected in

Mode 2 while sides B and C are in Mode 3a. This topology

can be used to synthesize elliptic bandpass, highpass and

notch filters. The elliptic highpass of Figure 17 is synthe-

sized again, Figure 22, but the clock is now locked onto the

Figure 22. 6th Order Elliptic Highpass Filter Operating with a

Clock-to-Cutoff Frequency Ratio of 75:1, and Using the Topology

of Figure 21.

1

–10

9

1061 F22

–30

–60

–80

3

5

7

0

–50

–40

–20

0

–70

–90

2

4

68

10

STANDARD 1%

RESISTOR VALUES

R11 = 54.9k

R31 = 34.8k

R22 = 68.1k

R42 = 10k

R33 = 75k

R21 = 24.3k

R41 = 10k

R32 = 18.2k

R23 = 10k

R43 = 14k

NOTE: FOR CLOCK FREQUEN-

CIES ABOVE 300kHz, ADD

A CAPACITOR C ACROSS

R21 AND R22 SUCH AS

(1/2

20

19

18

17

16

15

14

13

12

11

1

2

3

4

5

6

7

8

9

10

LTC1061 F21

LTC1061

R43

R33

CLOCK INPUT

R32

R22

R42

R31

R41

R21

R11

R23

Figure 21. LTC1061 with Side A is Connected in Mode 2 While

Side B, C are in Mode 3a. Topology is Useful for Elliptic

Highpass, Notch and Bandpass Filters.