11
LTC1164-7
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Table 8. Clock Feedthrough
V
S
50:1 100:1
Single 5V 70µV
RMS
70µV
RMS
±5V 100µV
RMS
200µV
RMS
±7.5V 120µV
RMS
500µV
RMS
Note: The clock feedthrough at Single 5V is imbedded in the wideband
noise of the filter. The clock waveform is a square wave.
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 8.
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 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’s passband and
cannot be reduced with post filtering. For instance, the
LTC1164-7 wideband noise at ±5V supply is 105µV
RMS
,
95µV
RMS
of which have frequency contents from DC up to
the filter’s cutoff frequency. The total wideband noise
(µV
RMS
) is nearly independent of the value of the clock.
The clock feedthrough specifications are not part of the
wideband noise.
Speed Limitations
The LT1164-7 optimizes AC performance vs power con-
sumption. To avoid op amp slew rate limiting at maximum
clock frequencies, the signal amplitude should be kept
below a specified level as shown in Table 9.
Table 9. Maximum V
IN
vs V
S
and Clock
POWER SUPPLY MAXIMUM f
CLK
MAXIMUM V
IN
±7.5V 1MHz 2.0V
RMS
(f
IN
> 20kHz)
0.7V
RMS
(f
IN
> 250kHz)
±5V 1MHz 1.4V
RMS
(f
IN
> 20kHz)
0.5V
RMS
(f
IN
> 100kHz)
Single 5V 1MHz 0.5V
RMS
(f
IN
> 100kHz)
Table 10. Transient Response of LTC Lowpass Filters
DELAY RISE SETTLING OVER-
TIME* TIME** TIME*** SHOOT
LOWPASS FILTER (SEC) (SEC) (SEC) (%)
LTC1064-3 Bessel 0.50/f
C
0.34/f
C
0.80/f
C
0.5
LTC1164-5 Bessel 0.43/f
C
0.34/f
C
0.85/f
C
0
LTC1164-6 Bessel 0.43/f
C
0.34/f
C
1.15/f
C
1
LTC1264-7 Linear Phase 1.15/f
C
0.36/f
C
2.05/f
C
5
LTC1164-7 Linear Phase 1.20/f
C
0.39/f
C
2.20/f
C
5
LTC1064-7 Linear Phase 1.20/f
C
0.39/f
C
2.20/f
C
5
LTC1164-5 Butterworth 0.80/f
C
0.48/f
C
2.40/f
C
11
LTC1164-6 Elliptic 0.85/f
C
0.54/f
C
4.30/f
C
18
LTC1064-4 Elliptic 0.90/f
C
0.54/f
C
4.50/f
C
20
LTC1064-1 Elliptic 0.85/f
C
0.54/f
C
6.50/f
C
20
* To 50% ±5%, ** 10% to 90% ±5%, *** To 1% ±0.5%
