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LTC1164-6
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Passband Response
The passband response of the LTC1164-6 is optimized for
a f
CLK
/f
CUTOFF
ratio of 100:1. Minimum passband ripple
occurs from 1Hz to 80% of f
CUTOFF
. Athough the passband
of the LTC1164-6 is optimized for ratio f
CLK
/f
CUTOFF
of
100:1, if a ratio of 50:1 is desired, connect a single pole
lowpass RC (f
–3dB
= 2 f
CUTOFF
) 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% f
CUTOFF
.
Table 2. Typical Passband Ripple with Single 5V Supply
(f
CLK
/f
C
)
= 100:1, GND = 2V, 30kHz, Fixed Single Pole, Lowpass
RC Filter at Pin 9 (See Typical Applications)
PASSBAND PASSBAND GAIN
FREQUENCY (REFERENCED TO 0dB)
f
CUTOFF
= 1kHz f
CUTOFF
= 10kHz
T
A
= 25°CT
A
= 0°CT
A
= 25°CT
A
= 70°C
% of f
CUTOFF
(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
f
CUTOFF
– 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 f
CLK
at f
CLK
/f
C
= 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
V
S
50:1 100:1
±2.5V 60µV
RMS
60µV
RMS
±5V 100µV
RMS
200µV
RMS
±7.5V 150µV
RMS
500µV
RMS
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 100µV
RMS
,
90µV
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 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 f
CLK
±4%, will be aliased back into the filter’s passband. If, for
instance, an LTC1164-6 operating with a 100kHz clock
