LTC1068 datasheet(13/28 Pages) LINER

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LTC1068 Series

APPLICATIONS INFORMATION

mine the operating signal-to-noise ratio. Most of its fre-

quency contents lie within the filter passband and cannot

be reduced with post filtering. For a notch filter the noise

of the filter is centered at the notch frequency.

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. Please refer

to the Noise vs Q graphs under the Typical Performance

Characteristics.

Aliasing

Aliasing is an inherent phenomenon of switched-capacitor

filters and occurs when the frequency of the input signals

that produce the strongest aliased components have a

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

filter’s passband. For an LTC1068 device the sampling

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

band-limited, aliasing may occur.

Demonstration Circuit 104

DC104 is a surface mount printed circuit board for the

evaluation of Linear Technology’s LTC1068 product fam-

ily in a 28-lead SSOP package. The LTC1068 product

family consists of four monolithic clock-tunable filter

building blocks.

Demo Board 104 is available in four assembled versions:

Assembly 104-A features the low noise LTC1068CG (clock-

to-center frequency ratio = 100), assembly 104-B features

the low noise LTC1068CG-200 (clock-to-center frequency

ratio = 200), assembly 104-C features the high frequency

LTC1068CG-25 (clock-to-center frequency ratio = 25) and

assembly 104-D features the low power LTC1068CG-50

(clock-to-center frequency ratio = 50).

All DC104 boards are assembled with input, output and

power supply test terminals, a 28-lead SSOP filter device

(LTC1068CG Series), a dual op amp in an SO-8 for input

or output buffers and decoupling capacitors for the filter

and op amps. The filter and dual op amps share the power

Operating Limits

The Maximum Q vs Center Frequency (fO) graphs, under

Typical Performance Characteristics, define an upper limit

of operating Q for each LTC1068 device 2nd order section.

These graphs indicate the power supply, fO and Q value

conditions under which a filter implemented with an

LTC1068 device will remain stable when operated at

temperatures of 70

°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 7. 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 an LTC1068 device near the

limits defined by the Maximum Q vs Frequency graphs,

passband gain variations of 2dB or more should be expected.

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 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 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 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 is used to deter-

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