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LTC1069-1IN8

Low Power, 8th Order Progressive Elliptic, Lowpass Filter

Linear Technology 

LTC1069-1

PIN FUNCTIONS

AGND (Pin 1): Analog Ground. The quality of the analog

signal ground can affect the filter performance. For either

single or dual supply operation, an analog ground plane

surrounding the package is recommended. The analog

ground plane should be connected to any digital ground

at a single point. For dual supply operation Pin 1 should

be connected to the analog ground plane.

For single supply operation Pin 1 should be bypassed to

the analog ground plane with a 0.47μF or larger capaci-

tor. An internal resistive divider biases Pin 1 to 1/2 the

total power supply. Pin 1 should be buffered if used to

bias other ICs. Figure 1 shows the connections for single

supply operation.

V+, V– (Pins 2, 7): Power Supply Pins. The V+ (Pin 2) and

the V– (Pin 7) should be bypassed with a 0.1μF capacitor

to an adequate analog ground. The filter’s power supplies

should be isolated from other digital or high voltage analog

supplies. A low noise linear supply is recommended. Using

switching power supplies will lower the signal-to-noise

ratio of the filter. Unlike previous monolithic filters, the

power supplies can be applied at any order, that is, the

positive supply can be applied before the negative supply

and vice versa. Figure 2 shows the connection for dual

supply operation.

NC (Pins 3, 6): No Connection. Pins 3 and 6 are not con-

nected to any internal circuity; they should be preferably

tied to ground.

VIN (Pin 4): Filter Input Pin. The filter input pin is internally

connected to the inverting input of an op amp through a

43k resistor.

CLK (Pin 5): Clock Input Pin. 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 be

connected to clock’s ground at a single point only. Table 1

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

dual or a 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 signal because excessive

slow clock rise or fall times generate internal clock jitter.

The maximum clock rise or fall 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

Single Supply = 3.3V

1.2V

LOW LEVEL

0.5V

5.5V

0.5V

0.5V

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

filter and it can source or sink 1mA. Driving coaxial cables

or resistive loads less than 20k will degrade the total har-

monic distortion of the filter. When evaluating the device’s

dynamic range, a buffer is required to isolate the filter’s

output from coax cables and instruments.

0.47μF

V+

0.1μF

VIN

1 AGND VOUT 8

2 V+

V– 7

LTC1069-1

3 NC

NC 6

4 VIN

CLK 5

VOUT

ANALOG GROUND PLANE

STAR

SYSTEM

GROUND

DIGITAL

GROUND

PLANE

1k

CLOCK

SOURCE

10691 F01

Figure 1. Connections for Single Supply Operation

6

V+

0.1μF

1 AGND VOUT 8

2 V+

V– 7

LTC1069-1

3 NC

NC 6

VOUT

V–

0.1μF

VIN 4 VIN

CLK 5

ANALOG GROUND PLANE

STAR

SYSTEM

GROUND

DIGITAL

GROUND

PLANE

1k

CLOCK

SOURCE

10691 F02

Figure 2. Connections for Dual Supply Operation

10691fa

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