10
LTC1065
1065fb
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Table 3. CMR Data, f
CLK
= 100kHz
25°C
POWER SUPPLY ∆V
IN
–40°C25°C85°C(V
OS
Nulled)
±2.5V ±1.8V 84dB 83dB 80dB 83dB
±5V ±4V 82dB 78dB 77dB 78dB
±7.5V ±6V 80dB 77dB 76dB 80dB
5mV/DIV
2µs/DIV
1065F08
f
CLK
= 100kHz, f
C
= 1kHz, V
S
= ±5V, 1MHz SCOPE BW
Figure 8. LTC1065 Output Clock Feedthrough + Noise
0.5mV/DIV
2µs/DIV
1063 F09
f
CLK
= 100kHz, f
C
= 1kHz, V
S
= ±5V, 1MHz SCOPE BW
Figure 9. LTC1065 Output Clock Feedthrough + Noise
The above data is valid for clock frequencies up to 800kHz, 900kHz, 1MHz, for
V
S
= ±2.5V, ±5V, ±7.5V respectively.
Clock Feedthrough
Clock feedthrough is defined as the RMS value of the clock
frequency and its harmonics which are present at the
filter’s output pin. The clock feedthrough is tested with the
filter input grounded and it depends on the quality of the
PC board layout and power supply decoupling. Any para-
sitic switching transients during the rise and fall of the
incoming clock, are not part of the clock feedthrough
specifications; their amplitude strongly depends on scope
probing techniques as well as ground quality and power
supply bypassing. For a power supply V
S
= ±5V, the clock
feedthrough of the LTC1065 is 50µV
RMS
; for V
S
= ±7.5V,
the clock feedthrough approaches 75µV
RMS
. Figures 8
and 9 show a typical scope photo of the LTC1065 output
pin when the input pin is grounded. The filter cutoff
frequency was 1kHz, while scope bandwidth was chosen
to be 1MHz so that switching transients above the 100kHz
clock frequency would show.
Wideband Noise
The wideband noise data is used to determine the operat-
ing signal-to-noise ratio at a given distortion level. The
wideband noise (µV
RMS
) is nearly independent of the value
of the clock frequency and excludes the clock feedthrough.
The LTC1065’s typical wideband noise is 80µV
RMS
. Figure
9 shows the same scope photo as Figure 8 but with a more
sensitive vertical scale. The clock feedthrough is imbed-
ded in the filter’s wideband noise. The peak-to-peak wide-
band noise of the filter can be clearly seen; it is approxi-
mately 420µV
P-P
. Note that 420µV
P-P
equals the 80µV
RMS
wideband noise of the part multiplied by a crest factor
of 5.25.
Aliasing
Aliasing is an inherent phenomenon of sampled data
filters. It primarily occurs when the frequency of an input
signal approaches the sampling frequency. For the
LTC1065, an input signal whose frequency is in the range
of f
CLK
±6% will generate an alias signal into the filter’s
passband and stopband. Table 4 shows details.
Example: LTC1065, f
CLK
= 20kHz, f
C
= 200kHz,
f
IN
= (19.6kHz, 100mV
RMS
)
f
ALIAS
= (400Hz, 3.16mV
RMS
)
