LTC1264CS Linear Technology, LTC1264CS Datasheet - Page 10

• Manufacturer Part Number: LTC1264CS
• Description: High Speed/ Quad Universal Filter Building Block
• Manufacturer: Linear Technology

Operating Limits
The Typical Maximum Q vs Clock Frequency and Band-
pass Gain Error graphs, under Typical Performance Char-
acteristics, define an upper limit of operating Q for each
LTC1264 2nd order section. These graphs indicate the
power supply, f
filter implemented with an LTC1264 will remain stable
when operated at temperatures of 85 C or less. For a 2nd
order section, a bandpass gain error of 3dB or less is
arbitrarily defined as a condition for stability.
When the passband gain error begins to exceed 1dB, the
use of capacitor C
is connected from the lowpass node to the inverting node
of a 2nd order section). Please refer to Figures 4 through
9. The value of C
and as a guide it should be about 5pF for each 1dB of gain
error and not to exceed 15pF. When operating LTC1264
very near the limits defined by the Typical Performance
Characteristics graphs, passband gain variations of 2dB
or more should be expected.
Speed Limitations
To avoid op amp slew rate limiting, the signal amplitude
should be kept below a specified level as shown in Table 2.
Table 2. Maximum V
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 pins. The clock feedthrough is tested with the
filter’s input grounded and it depends on PC board layout
and on the value of the power supplies. With proper layout
techniques, the typical values of clock feedthrough are
listed under Electrical Characteristics.
Any parasitic switching transients during the rise and fall
edges of the incoming clock are not part of the clock
LTC1264
A
10
V
Single 5V
PPLICATI
S
7.5V
5V
CLK
MAXIMUM CLOCK
C
O
3MHz to 4MHz
1MHz to 2MHz
C
4MHz to 5MHz
can be best determined experimentally,
IN
will reduce the gain error (capacitor C
and Q value conditions under which a
U
vs V
S
S
and Clock
I FOR ATIO
U
W
0.35V
0.5V
0.5V
MAXIMUM V
RMS
RMS
RMS
f
f
IN
IN
f
IN
U
400kHz
250kHz
160kHz
IN
C
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 RC lowpass network at the final filter
output. This RC 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 passband and it
cannot be reduced with post filtering.
The total wideband noise ( V
the value of the clock. The clock feedthrough specifica-
tions are not part of the wideband noise.
For a specific filter design, the total noise depends on the
Q of each section and the cascade sequence. Table 3
shows typical 2nd order section noise (gain = 1) for Q
values and supplies operating at 25 C. Noise increases by
20% at the highest operating temperatures.
Table 3. 2nd Order Section Noise ( V
2 or 3 (R2 = R4)
Aliasing
Aliasing is an inherent phenomenon of switched-capacitor
filters and it occurs when the frequency of input signals
approaches the sampling frequency. The input signals
that produce the strongest aliased components have a
frequency, f
filter’s passband. For the LTC1264 the sampling fre-
quency is twice f
band-limited, aliasing may occur.
Q
1
2
3
4
5
V
40 V
50 V
60 V
75 V
90 V
S
IN
= 2.5V
, such as (f
RMS
RMS
RMS
RMS
RMS
CLK
. If the input signal spectrum is not
SAMPLING
RMS
V
S
110
= 5V
50
60
75
90
) is nearly independent of
RMS
) for Modes 1, 1b,
– f
IN
) falls into the
V
S
= 7.5V
115
135
60
75
95