LTC1566-1
6
15661fa
a common mode voltage that is equal to one-half of the total
supply voltage. Figure 5 illustrates the THD versus output
common mode voltage for a 0.5V
P-P
/2.0V
P-P
differential
input/output voltage and a common mode input voltage
that is equal to one-half the total supply voltage.
Output DC Offset Control
A unique feature of the LTC1566-1 is the ability to introduce
a differential offset voltage at the output of the filter. As
seen in the “Block Diagram”, if a DC voltage is applied to Pin
5 with respect to Pin 3, the same voltage will be added to
the differential voltage seen between Pins 8 and 7.
The output DC offset control pin can be used for sideband
suppression in differential modulators, calibration of A/D
converters, or simple signal summation. Since the voltage
summing occurs at the output of the filter, Pin 5 acts as a
unfiltered input. The response from Pin 5 to Pin 8 – Pin 7
with Pins 1,2 and 3 grounded is shown in Figure 7. The
range of voltages that can be applied to Pin 5 is shown in
Figure 6 where THD is plotted versus output offset. Pin 3 is
biased to mid supply.
Output Drive
Pins 7 and 8 can drive a 1kΩ or 300pF load connected to
AC ground with a ±0.5V signal (corresponding to a 2V
P-P
differential signal). For differential loads (loads connected
across Pins 7 and 8) the outputs can produce a 2V
P-P
differential signal across 2kΩ or 150pF. For smaller signal
amplitudes the outputs can drive correspondingly larger
loads.
–5 –4 –2 0 2 4
–30
–40
–50
–60
–70
–80
–90
–3 –1 1 3
1566-1 F03
5
INPUT COMMON MODE VOLTAGE (V)
THD (dB)
V
S
= 5V
V
S
= ±5V
DIFFERENTIAL OUTPUT (V
P-P
)
0.5
THD, SNR (dB)
–30
–40
–50
–60
–70
–80
–90
2.0 3.0
1566-1 F04
1.0 1.5
2.5 3.5 4.0
V
S
= 5V
V
S
= ±5V
S/N
OUTPUT COMMON MODE VOLTAGE (V)
–4
THD (dB)
–30
–40
–50
–60
–70
–80
–90
–1 2
1566-1 F05
–3 –2
1034
V
S
= 5V
V
S
= ±5V
Figure 3
Figure 4
Figure 5
APPLICATIO S I FOR ATIO
WUU
U
Input Common Mode and Differential Voltage Range
The range of voltage each input can support while operat-
ing in its linear region is typically 0.8V to 3.7V for a single
5V supply and –4.2V to 3.2V for a ±5V supply. Therefore,
the filter can accept a variety of common mode input
voltages. Figure 3 shows the total harmonic distortion of
the filter versus input common mode voltage with a
2V
P-P
differential output signal.
Figure 4 shows the total harmonic distortion and signal to
noise ratio versus differential output voltage level for both
a single 5V and a ±5V supply. The common mode voltage
of the input signal is one-half the total power supply voltage
of the filter. The spurious free dynamic range (SFDR), the
level where the THD and S/N ratio are equal, is 72dB.
For best performance, the inputs should be driven differen-
tially. For single-ended signals, connect the unused input
to Pin 3 or a common mode reference.
The filter DC differential swings listed in the “Electrical
Characteristics” are measured with input differential volt-
ages of 0.9V
P-P
and 1.5V
P-P
for 5V and ±5V supplies
respectively. Ideally the corresponding output levels would
be 3.6V
P-P
and 6V
P-P
. As seen in Figure 4, these levels are
above the range of linear operation. Input signals larger
than 0.9V
P-P
/1.5V
P-P
will result in phase inversion and
should be avoided.
Output Common Mode and Differential Voltage Range
The output is a fully differential signal with a common
mode level equal to the voltage at Pin 3 when Pin 5 is
shorted to Pin 3. The best performance is achieved using
