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LTC1065

DC Accurate, Clock-Tunable Linear Phase 5th Order Bessel Lowpass Filter

Linear Technology 

LTC1065

APPLICATI S I FOR ATIO

Table 3. CMR Data, fCLK = 100kHz

POWER SUPPLY ∆VIN – 40°C 25°C

±2.5V

±1.8V 84dB 83dB

±5V

±4V 82dB 78dB

±7.5V

±6V 80dB 77dB

85°C

80dB

77dB

76dB

25°C

(VOS Nulled)

83dB

78dB

80dB

The above data is valid for clock frequencies up to 800kHz, 900kHz, 1MHz, for

VS = ±2.5V, ±5V, ±7.5V respectively.

Clock Feedthrough

Clock feedthrough is defined as the RMS value of the clock

frequency and its harmonics which are present at the

filter’s output pin. The clock feedthrough is tested with the

filter input grounded and it depends on the quality of the

PC board layout and power supply decoupling. Any para-

sitic switching transients during the rise and fall of the

incoming clock, are not part of the clock feedthrough

specifications; their amplitude strongly depends on scope

probing techniques as well as ground quality and power

supply bypassing. For a power supply VS = ±5V, the clock

feedthrough of the LTC1065 is 50µVRMS; for VS = ±7.5V,

the clock feedthrough approaches 75µVRMS. Figures 8

and 9 show a typical scope photo of the LTC1065 output

pin when the input pin is grounded. The filter cutoff

frequency was 1kHz, while scope bandwidth was chosen

to be 1MHz so that switching transients above the 100kHz

clock frequency would show.

Wideband Noise

The wideband noise data is used to determine the operat-

ing signal-to-noise ratio at a given distortion level. The

wideband noise (µVRMS) is nearly independent of the value

of the clock frequency and excludes the clock feedthrough.

The LTC1065’s typical wideband noise is 80µVRMS. Figure

9 shows the same scope photo as Figure 8 but with a more

sensitive vertical scale. The clock feedthrough is imbed-

ded in the filter’s wideband noise. The peak-to-peak wide-

band noise of the filter can be clearly seen; it is approxi-

mately 420µVP-P. Note that 420µVP-P equals the 80µVRMS

wideband noise of the part multiplied by a crest factor

of 5.25.

2µs/DIV

fCLK = 100kHz, fC = 1kHz, VS = ±5V, 1MHz SCOPE BW

1065F08

Figure 8. LTC1065 Output Clock Feedthrough + Noise

2µs/DIV

fCLK = 100kHz, fC = 1kHz, VS = ±5V, 1MHz SCOPE BW

1063 F09

Figure 9. LTC1065 Output Clock Feedthrough + Noise

Aliasing

Aliasing is an inherent phenomenon of sampled data

filters. It primarily occurs when the frequency of an input

signal approaches the sampling frequency. For the

LTC1065, an input signal whose frequency is in the range

of fCLK ±6% will generate an alias signal into the filter’s

passband and stopband. Table 4 shows details.

Example: LTC1065, fCLK = 20kHz, fC = 200kHz,

fIN = (19.6kHz, 100mVRMS)

fALIAS = (400Hz, 3.16mVRMS)

1065fb

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