LTC1069-7

8

10697fa

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

specifications are not part of the wideband noise.

Table 3. Wideband Noise

VS

CLOCK FEEDTHROUGH

4.75V

125μVRMS

±5V

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

APPLICATIONS INFORMATION

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

OUTPUT FREQUENCY

Aliased Frequency

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

40kHz (or 60kHz)

–59.9dB

10kHz

47kHz (or 53kHz)

–54.2dB

3kHz

48kHz (or 52kHz)

–42.6dB

2kHz

48.5kHz (or 51.5kHz)

–18.3dB

1.5kHz

49kHz (or 52kHz)

–2.9dB

1.0kHz

49.5kHz (or 50.5kHz)

–0.65dB

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)