LTC1069-7IS8(1996) Просмотр технического описания (PDF) - Linear Technology

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LTC1069-7IS8
(Rev.:1996)

Linear Phase 8th Order Lowpass Filter

Linear Technology 

LTC1069-7

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 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) of the LTC1069-7.

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. Most of the wideband

noise frequency contents lie within the filter passband.

The wideband noise cannot be reduced by adding post

filtering. 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 speci-

fications are not part of the wideband noise.

Table 3. Wideband Noise

VS

4.75V

± 5V

WIDEBAND NOISE

125µVRMS

140µVRMS

Aliasing

Aliasing is an inherent phenomenon of sampled data

systems and it occurs for input frequencies approaching

the sampling frequency. The internal sampling frequency

of the LTC1069-7 is 50 times its fCUTOFF frequency. For

instance if a 48kHz, 100mVRMS signal is applied at the

input of an LTC1069-7 operating with a 50% duty cycle

25kHz clock, a 2kHz, 741µVRMS alias signal will appear at

the filter output. Table 4 shows details.

Table 4. Aliasing

INPUT FREQUENCY

VIN = 1VRMS

OUTPUT LEVEL

Relative to Input

fCLK/fC = 25:1, fCUTOFF = 1kHz

40kHz (or 60kHz)

–59.9dB

47kHz (or 53kHz)

–54.2dB

48kHz (or 52kHz)

–42.6dB

48.5kHz (or 51.5kHz)

–18.3dB

49kHz (or 52kHz)

–2.9dB

49.5kHz (or 50.5kHz)

–0.65dB

OUTPUT FREQUENCY

Aliased Frequency

10kHz

3kHz

2kHz

1.5kHz

1.0kHz

0.5kHz

Speed Limitations

To avoid op amp slew rate limiting, the signal amplitude

should be kept below a specified level as shown in Table 5.

Table 5. Maximum VIN vs VS and Clock

VS

MAXIMUM CLOCK

MAXIMUM VIN

5V

≥ 2.5MHz

340mVRMS (fIN ≥ 200kHz)

±5V

≥ 4.5MHz

1.2VRMS (fIN ≥ 400kHz)

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.

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