9

LTC1569-7

Table 2. Wideband Noise vs Supply Voltage, Single 3V Supply

Bandwidth

Total Integrated Noise

105

125

155

Total Integrated Noise

Power Supply

3V

125

5V

135

±5V

145

Clock Feedthrough

Clock feedthrough is defined as the RMS value of the clock

frequency and its harmonics that are present at the filter’s

OUT pin (Pin 8). The clock feedthrough is measured with

PC board layout and the power supply decoupling. Table 4

shows the clock feedthrough (the RMS sum of the first 11

harmonics) when the LTC1569-7 is self-clocked with

clock feedthrough can be reduced with a simple RC post

filter.

Table 4. Clock Feedthrough

Power Supply

Feedthrough

3V

5V

±5V

DC Accuracy

DC accuracy is defined as the error in the output voltage

after DC offset and DC gain errors are removed. This is

similar to the definition of the integral nonlinearity in A/D

straight line that best fits the data. The DC accuracy

describes how much the actual data deviates from this

+ 0.00134V). In a 12-bit system with a full-scale value of

2V, the LSB is 488

µV. Therefore, if the DCERROR of the

filter is less than 488

µV over a 2V range, the filter has

12-bit DC accuracy. Figure 9 illustrates the typical DC

accuracy of the LTC1569-7 on a single 5V supply.

Figure 9

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

1569-7 F09

488

244

000

–244

–488

DC Offset

The output DC offset of the LTC1569-7 is trimmed to less

than

performance, appropriate PC layout techniques should be

used. The filter IC should be soldered to the PC board. The

power supplies should be well decoupled including a 1

µF

plane should be used. Noisy signals should be isolated

from the filter input pins.

When the power supply is 3V, the output DC offset should

not change more than

±2mV when the clock frequency

varies from 64kHz to 8192kHz. When the clock frequency

is fixed, the output DC offset will typically change by

±3mV

(

±15mV) when the power supply varies from 3V to 5V

(

±5V) in the divide-by-1 mode. In the divide-by-4 or

divide-by-16 modes, the output DC offset will typically

change – 9mV (– 27mV) when the power supply varies

from 3V to 5V (

±5V). The offset is measured with respect

to GND (Pin 3).

Aliasing

Aliasing is an inherent phenomenon of sampled data

filters. In lowpass filters significant aliasing only occurs

when the frequency of the input signal approaches the

sampling frequency or multiples of the sampling fre-

quency. The LTC1569-7 samples the input signal twice