LTC1264CS datasheet(10/16 Pages) LINER

10 / 16 page

LTC1264

APPLICATI

I FOR ATIO

Operating Limits

The Typical Maximum Q vs Clock Frequency and Band-

pass Gain Error graphs, under Typical Performance Char-

acteristics, define an upper limit of operating Q for each

LTC1264 2nd order section. These graphs indicate the

power supply, fCLK and Q value conditions under which a

filter implemented with an LTC1264 will remain stable

when operated at temperatures of 85

°C or less. For a 2nd

order section, a bandpass gain error of 3dB or less is

arbitrarily defined as a condition for stability.

When the passband gain error begins to exceed 1dB, the

use of capacitor CC will reduce the gain error (capacitor CC

is connected from the lowpass node to the inverting node

of a 2nd order section). Please refer to Figures 4 through

9. The value of CC can be best determined experimentally,

and as a guide it should be about 5pF for each 1dB of gain

error and not to exceed 15pF. When operating LTC1264

very near the limits defined by the Typical Performance

Characteristics graphs, passband gain variations of 2dB

or more should be expected.

Speed Limitations

To avoid op amp slew rate limiting, the signal amplitude

should be kept below a specified level as shown in Table 2.

Table 2. Maximum VIN vs VS and Clock

MAXIMUM CLOCK

MAXIMUM VIN

±7.5V

4MHz to 5MHz

0.5VRMS fIN ≥ 400kHz

±5V

3MHz to 4MHz

0.5VRMS fIN ≥ 250kHz

Single 5V

1MHz to 2MHz

0.35VRMS fIN ≥ 160kHz

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 pins. The clock feedthrough is tested with the

filter’s input grounded and it depends on PC board layout

and on the value of the power supplies. With proper layout

techniques, the typical values of clock feedthrough are

listed under Electrical Characteristics.

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

clock feedthrough, if bothersome, can be greatly reduced

by adding a simple RC lowpass network at the final filter

output. This RC 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.

The total wideband noise (

µVRMS)isnearlyindependentof

the value of the clock. The clock feedthrough specifica-

tions are not part of the wideband noise.

For a specific filter design, the total noise depends on the

Q of each section and the cascade sequence. Table 3

shows typical 2nd order section noise (gain = 1) for Q

values and supplies operating at 25

°C. Noise increases by

20% at the highest operating temperatures.

Table 3. 2nd Order Section Noise (

µVRMS) for Modes 1, 1b,

2 or 3 (R2 = R4)

QVS = ±2.5V

VS = ±5V

VS = ±7.5V

140

µVRMS

250

µVRMS

360

µVRMS

475

µVRMS

115

590

µVRMS

110

135

Aliasing

Aliasing is an inherent phenomenon of switched-capacitor

filters and it occurs when the frequency of input signals

approaches the sampling frequency. The input signals

that produce the strongest aliased components have a

frequency, fIN, such as (fSAMPLING – fIN) falls into the

filter’s passband. For the LTC1264 the sampling fre-

quency is twice fCLK. If the input signal spectrum is not

band-limited, aliasing may occur.

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