7

LTC1069-6

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Figure 4b

FREQUENCY (kHz)

1

1

2

17

1069-6 F04a

0

–1

–2

5

9

13

319

7

11

15

21

85

°C

–40

°C

Figure 4c

FREQUENCY (kHz)

1

1

2

25

1069-6 F04c

0

–1

–2

7

13

19

428

10

16

22

31

85

°C

–40

°C

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 AGND (Pin 1) 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 in Table 2.

Table 2. Clock Feedthrough

CLOCK FEEDTHROUGH

3.3V

100

5V

170

10V

350

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 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

clock feedthrough can be reduced by adding a single RC

lowpass filter at the Output (Pin 8).

Wideband Noise

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

the device’s noise spectral density and determines the

operating signal-to-noise ratio. The frequency contents of

the wideband noise lie within the filter’s passband. The

wideband noise cannot be reduced by adding post filter-

ing. The total wideband noise is nearly independent of the

clock frequency and depends slightly on the power supply

voltage (see Table 3). The clock feedthrough specifica-

tions are not part of the wideband noise.

Table 3. Wideband Noise

WIDEBAND NOISE

3.3V

118

5V

123

±5V

127

Aliasing

Aliasing is an inherent phenomenon of sampled data

systems and occurs for input frequencies approaching the

sampling frequency. The internal sampling frequency of

the LTC1069-6 is 100 times its cutoff frequency. For

input of an LTC1069-6 operating with a 50kHz clock, a

1.5kHz, 484

output. Table 4 shows details.

INPUT FREQUENCY

OUTPUT LEVEL

OUTPUT FREQUENCY

(Relative to Input)

(Aliased Frequency)

(kHz)

(dB)

(kHz)

96 (or 104)

–78.3

4.0

97 (or 103)

–70.4

3.0

98 (or 102)

– 80.6

2.0

98.5 (or 101.5)

– 46.3

1.5

99 (or 101)

– 2.8

1.0

99.5 (or 100.5)

– 1.38

0.5