10

LTC1264-7

Filter Input Pin (2)

The input pin is connected internally through a 50k resistor

tied to the inverting input of an op amp.

Filter Output Pins (9, 6)

Pin 9 is the specified output of the filter; it can typically

source 3mA and 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

PI FU CTIO S

distortion an output buffer is required. A noninverting

buffer, Figure 4, can be used provided that its input

common-mode range is well within the filter’s output

swing. Pin 6 is an intermediate filter output providing an

unspecified 6th order lowpass filter. Pin 6 should not be

loaded.

External Connection Pins (7, 14)

Pins 7 and 14 should be connected together. In a printed

circuit board the connection should be done under the IC

package through a short trace surrounded by the analog

ground plane.

NC Pin (1, 5, 8, 13)

Pins 1, 5, 8 and 13 are not connected to any internal circuit

point on the device and should be preferably tied to analog

ground.

Figure 4. Buffer for Filter Output

1k

1264-7 F04

LT1220

S

APPLICATI

I FOR ATIO

Clock Feedthrough

Clock feedthrough is defined as the RMS value of the clock

frequency and its harmonics that are present at the filter’s

output pin (9). The clock feedthrough is tested with the

input pin (2) grounded and it 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

in Table 8.

clock feedthrough, if bothersome, can be greatly reduced

by adding a simple R/C lowpass network at the output of

the filter pin (9). This R/C will completely eliminate any

switching transients.

Wideband Noise

The wideband noise of the filter is the total RMS value of

the device’s noise spectral density and it is used to

determine the operating signal-to-noise ratio. Most of its

frequency contents lie within the filter passband and it

cannot be reduced with post filtering. For instance, the

LTC1264-7 wideband noise at

145

to the filter’s cutoff frequency. The total wideband noise

(

The clock feedthrough specifications are not part of the

wideband noise.

Speed Limitations

To avoid op amp slew rate limiting at maximum clock

frequencies, the signal amplitude should be kept below a

specified level as shown in Table 9.

Any parasitic switching transients during the rise and fall

edges of the incoming clock are not part of the clock

feedthrough specifications. Switching transients have fre-

quency contents much higher than the applied clock; their

amplitude strongly depends on scope probing techniques

as well as grounding and power supply bypassing. The

Note: The clock feedthrough at 25:1 is imbedded in the wideband

noise of the filter. Clock waveform is a square wave.

Table 8. Clock Feedthrough

25:1

50:1

Single 5V

100

100

±5V

100

400

±7.5V

120

1000