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LTC1064-4C

Low Noise, 8th Order, Clock Sweepable Cauer Lowpass Filter

Linear Technology 

LTC1064-4

PI FU CTIO S (Pin Numbers Refer to the 14-Pin Package)

INV C, COMP1, INV A, COMP2 (Pins 1, 6, 7 and 13): To

obtain a Cauer response with minimum passband ripple

and cutoff frequencies above 20kHz, compensating com-

ponents are required. Figure 6 uses ±7.5V power supplies

and compensation components to achieve up to 40kHz

sweepable cutoff frequencies and ±0.1dB passband ripple.

Table 7 lists the typical amplitude response of Figure 6.

Figure 7 illustrates the compensation scheme required to

obtain a 100kHz cutoff frequency; Graph 4 and Tables 8

and 9 list the typical response of Figure 7 for 25°C and

125°C ambient temperature. As shown the ripple in-

creases at high temperatures but still a ±0.25dB figure

can be obtained for ambient temperatures below 70°C.

VIN, VOUT (Pins 2, 9): The input Pin 2 is connected to a 12k

resistor tied to the inverting input of an op amp. Pin 2 is

protected against static discharge. The device’s output,

Pin 9, is the output of an op amp which can typically

source/sink 3mA/1mA. Although the internal op amps are

unity gain stable, driving long coax cables is not recom-

mended.

When testing the device for noise and distortion, the

output, Pin 9, should be buffered (Figure 4). The op amp

power supply wire (or trace) should be connected

directly to the power source. To eliminate any output

clock feedthrough, Pin 9 should be buffered with a simple

R, C lowpass filter (Figure 5). The cutoff frequency of the

output filter should be fCLK/3.

AGND (Pins 3, 5): For dual supply operation these pins

should be connected to a ground plane. For single supply

operation both pins should be tied to one half supply

(Figure 2).

V+, V– (Pins 4, 12): Should be bypassed with a 0.1µF

capacitor to an adequate analog ground. Low noise,

nonswitching power supplies are recommended. To avoid

latchup when the power supplies exhibit high turn-on

transients, a 1N5817 Schottky diode should be added

from the V+ and V– pins to ground (Figures 1 and 2).

INV A, R(h, I) (Pins 7, 14): A very short connection

between Pin 7 and Pin 14 is recommended. This connec-

tion should be preferably done under the IC package. In a

breadboard, use a one inch, or less, shielded coaxial cable;

the shield should be grounded. In a PC board, use a one

inch trace or less; surround the trace by a ground plane.

NC (Pin 8 ): Pin 8 is not internally connected, it should be

preferably grounded.

50/100 Ratio (Pin 10): For an fCLK/fC ratio of 50:1,

Pin 10 should be tied to V+. For an fCLK/f –3dB ratio of

100:1, Pin 10 should be tied to V–. When Pin 10 is at

midsupplies (i.e. ground), the filter response is neither

Cauer nor transitional. Table 6 illustrates this response.

Bypassing Pin 10 with a 0.1µF capacitor reduces the

already small clock feedthrough.

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