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LTC1069-7

Linear Phase 8th Order Lowpass Filter

Linear Technology 

LTC1069-7

PIN FUNCTIONS

ANALOG GROUND

PLANE

1 AGND

8

VOUT

VOUT

V+

2 V+

V– 7

V–

0.1μF

3

LTC1069-7

NC

NC

6

0.1μF

VIN 4 VIN

CLK 5

STAR

SYSTEM

GROUND

DIGITAL

GROUND

PLANE

1k

CLOCK

SOURCE

1069 F02

Figure 2. Connections for Dual Supply Operation

CLK (Pin 5): Clock Input. Any TTL or CMOS clock source

with a square wave output and 50% duty cycle (±10%) is

an adequate clock source for the device. The power supply

for the clock source should not necessarily be the filter’s

power supply. The analog ground of the filter should only

be connected to the clock’s ground at a single point. Table

1 shows the clock’s low and high level threshold value for

a dual or single supply operation. A pulse generator can

be used as a clock source provided the high level on-time

is greater than 0.42μs (VS = ± 5V). Sine waves less than

100kHz are not recommended for clock sources because

excessive slow clock rise or fall times generate internal

clock jitter. The maximum clock rise or fall time is 1μs. The

clock signal should be routed from the right side of the IC

package to avoid coupling into any input or output analog

signal path. A 1k resistor between the clock source and the

clock input (Pin 5) will slow down the rise and fall times

of the clock to further reduce charge coupling, Figure 1.

Table 1. Clock Source High and Low Thresholds

POWER SUPPLY

HIGH LEVEL

Dual Supply = ± 5V

1.5V

Single Supply = 10V

6.5V

Single Supply = 5V

1.5V

LOW LEVEL

0.5V

5.5V

0.5V

VOUT (Pin 8): Filter Output. Pin 8 is the output of the filter,

and it can source 23mA or sink 16mA. The total harmonic

distortion of the filter will degrade when driving coaxial

cables or loads less than 20k without an output buffer.

APPLICATIONS INFORMATION

Temperature Behavior

The power supply current of the LTC1069-7 has a positive

temperature coefficient. The GBW product of its internal

op amps is nearly constant and the speed of the device

does not degrade at high temperatures.

Clock Feedthrough

The clock feedthrough is defined as the RMS value of the

clock frequency and its harmonics that are present at the

filter’s output (Pin 8). The clock feedthrough is tested with

the input (Pin 4) shorted to the AGND pin and depends on

PC board layout and on the value of the power supplies.

With proper layout techniques the values of the clock

feedthrough are shown on Table 2.

Table 2. Clock Feedthrough

VS

CLOCK FEEDTHROUGH

5V

400μVRMS

±5V

850μVRMS

Any parasitic switching transients during the rising and

falling edges of the incoming clock are not part of the

clock feedthrough specifications. Switching transients

have frequency contents much higher than the applied

clock; their amplitude strongly depends on scope probing

techniques as well as grounding and power supply

bypassing. The clock feedthrough can be reduced by

adding a single RC lowpass filter at the output (Pin 8) of

the LTC1069-7.

10697fa

7

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