8

LTC1164-6

11646fa

S

APPLICATI

I FOR ATIO

Passband Response

The passband response of the LTC1164-6 is optimized for

a fCLK/fCUTOFF ratio of 100:1. Minimum passband ripple

occurs from 1Hz to 80% of fCUTOFF. Athough the passband

of the LTC1164-6 is optimized for ratio fCLK/fCUTOFF of

100:1, if a ratio of 50:1 is desired, connect a single pole

lowpass RC (f –3dB = 2 fCUTOFF) at the output of the filter.

The RC will make the passband gain response as flat as the

100:1 case. If the RC is omitted, and clock frequencies are

below 500kHz the passband gain will peak by 0.4dB at

90% fCUTOFF.

Table 2. Typical Passband Ripple with Single 5V Supply

(fCLK/fC) = 100:1, GND = 2V, 30kHz, Fixed Single Pole, Lowpass

RC Filter at Pin 9 (See Typical Applications)

PASSBAND

PASSBAND GAIN

FREQUENCY

(REFERENCED TO 0dB)

fCUTOFF = 1kHz

fCUTOFF = 10kHz

TA = 25°CTA = 0°CTA = 25°CTA = 70°C

% of fCUTOFF

(dB)

(dB)

(dB)

(dB)

10

0.00

0.00

0.00

0.00

20

– 0.02

0.00

0.01

0.01

30

– 0.05

– 0.01

– 0.01

0.01

40

– 0.10

– 0.02

– 0.02

0.02

50

– 0.13

– 0.03

– 0.01

0.03

60

– 0.15

– 0.01

0.01

0.05

70

– 0.18

– 0.01

0.01

0.07

80

– 0.25

– 0.08

– 0.05

0.02

90

– 0.39

– 0.23

– 0.18

– 0.05

fCUTOFF

– 2.68

– 2.79

– 2.74

– 2.68

The gain peaking can approximate a sin

χ/χ correction for

some applications. (See Typical Performance Characteristics

curve, Passband vs Frequency and fCLK at fCLK/fC = 50:1.)

When the LTC1164-6 operates with a single 5V supply and its

cutoff frequency is clock-tuned to 10kHz, an output single

pole RC filter can also help maintain outstanding passband

flatness from 0

°C to 70°C. Table 2 shows details.

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 (Pin 9). The clock feedthrough is tested with

the input (Pin 2) grounded and, it depends on PC board

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

layout techniques the values of the clock feedthrough are

shown in Table 3.

Table 3. Clock Feedthrough

VS

50:1

100:1

±2.5V

60

µVRMS

60

µVRMS

±5V

100

µVRMS

200

µVRMS

±7.5V

150

µVRMS

500

µVRMS

Note: The clock feedthrough at

±2.5V supplies is imbedded in the wideband noise of the filter. (The

clock signal is a square wave.)

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 R/C lowpass network at the output of

the filter (Pin 9). This R/C will completely eliminate any

switching transient.

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. For instance, the

LTC1164-6 wideband noise at

±2.5V supply is 100µVRMS,

90

µVRMS of which have frequency contents from DC up to

the filter’s cutoff frequency. The total wideband noise

(

µVRMS) is nearly independent of the value of the clock.

The clock feedthrough specifications are not part of the

wideband noise.

Speed Limitations

The LTC1164-6 optimizes AC performance versus power

consumption. To avoid op amp slew rate limiting at

maximum clock frequencies, the signal amplitude should

be kept below a specified level as shown on Table 4.

Aliasing

Aliasing is an inherent phenomenon of sampled data

systems and it occurs when input frequencies close to the

sampling frequency are applied. For the LTC1164-6 case,

an input signal whose frequency is in the range of fCLK

±4%, will be aliased back into the filter’s passband. If, for

instance, an LTC1164-6 operating with a 100kHz clock