Datasheet

LTC1069-7
8
10697fa
Wideband Noise
The wideband noise of the fi lter is the total RMS value
of the device’s noise spectral density and determines the
operating signal-to-noise ratio. Most of the wideband
noise frequency contents lie within the fi lter passband.
The wideband noise cannot be reduced by adding post
fi ltering. The total wideband noise is nearly independent
of the clock frequency and depends slightly on the power
supply voltage (see Table 3). The clock feedthrough
specifi cations are not part of the wideband noise.
Table 3. Wideband Noise
V
S
CLOCK FEEDTHROUGH
4.75V 125μV
RMS
±5V 140μV
RMS
Aliasing
Aliasing is an inherent phenomenon of sampled data
systems and it occurs for input frequencies approaching
the sampling frequency. The internal sampling frequency
of the LTC1069-7 is 50 times its f
CUTOFF
frequency. For
instance if a 48kHz, 100mV
RMS
signal is applied at the
APPLICATIONS INFORMATION
input of an LTC1069-7 operating with a 50% duty cycle
25kHz clock, a 2kHz, 741μV
RMS
alias signal will appear
at the fi lter output. Table 4 shows details.
Table 4. Aliasing
INPUT FREQUENCY
V
IN
= 1V
RMS
OUTPUT LEVEL
Relative to Input
OUTPUT FREQUENCY
Aliased Frequency
f
CLK
/f
C
= 25:1, f
CUTOFF
= 1kHz
40kHz (or 60kHz) –59.9dB 10kHz
47kHz (or 53kHz) –54.2dB 3kHz
48kHz (or 52kHz) –42.6dB 2kHz
48.5kHz (or 51.5kHz) –18.3dB 1.5kHz
49kHz (or 52kHz) –2.9dB 1.0kHz
49.5kHz (or 50.5kHz) –0.65dB 0.5kHz
Speed Limitations
To avoid op amp slew rate limiting, the signal amplitude
should be kept below a specifi ed level as shown in Table 5.
Table 5. Maximum V
IN
vs V
S
and Clock
V
S
MAXIMUM CLOCK MAXIMUM V
IN
5V ≥ 2.5MHz 340mV
RMS
(f
IN
≥ 200kHz)
±5V ≥ 4.5MHz 1.2V
RMS
(f
IN
≥ 400kHz)