LTC1565-31 Linear Technology, LTC1565-31 Datasheet - Page 5

• Manufacturer Part Number: LTC1565-31
• Description: 650kHz Continuous Time/ Linear Phase Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS
Interfacing to the LTC1565-31
The difference between the voltages at Pin 1 and Pin 2 is
the “differential input voltage.” The average of the voltages
at Pin 1 and Pin 2 is the “common mode input voltage.”
The difference between the voltages at Pin 7 and Pin 8 is
the “differential output voltage.” The average of the volt-
ages at Pin 7 and Pin 8 is the “common mode output
voltage.” The input and output common mode voltages
are independent. The input common mode voltage is set
by the signal source, if DC coupled, or by the biasing
network if AC coupled (Figures 1 and 2). The output
common mode voltage is equal to the voltage of Pin 3, the
GND pin. The GND pin is biased to one-half of the supply
voltage by an internal resistive divider (see Block Dia-
gram). To alter the common mode output voltage, Pin 3
can be driven with an external voltage source or resistor
network. If external resistors are used, it is important to
note that the internal 5k resistors can vary 20% (their
ratio only varies 2%). The output can also be AC coupled.
+
–
V
IN
+
+
–
+
–
V
V
DC COUPLED INPUT
V
V
IN
IN
IN
OUT
–
+
(COMMON MODE) =
0.1 F
+
–
(COMMON MODE) =
0.1 F
V
0.1 F
100k
IN
U
AC COUPLED INPUT
V
–
IN
(COMMON MODE) = V
1
2
3
4
Figure 1
Figure 2
1 F
INFORMATION
U
+IN
–IN
GND
V
100k
LTC1565-31
–
V
IN
V
OUT
+
+ V
2
+
1
2
3
4
SHDN
+OUT
–OUT
IN
+ V
2
=
–
+IN
–IN
GND
V
V
OUT
LTC1565-31
–
+
W
V
2
OUT
+
8
7
6
5
–
=
(COMMON MODE)
SHDN
+OUT
–OUT
V
2
+
5V
V
V
V
+
OUT
OUT
15645-31 F01
0.1 F
8
7
6
5
+
–
U
5V
V
V
OUT
OUT
15645-31 F02
0.1 F
+
–
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 3.5V 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. Figures 3 and 4 show the THD of the filter versus
common mode input voltage with a 2V
signal.
Figure 5 shows the THD and S/N ratio versus differential
input voltage level for both a single 5V supply 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, where the THD and S/N ratio
are equal, is 75dB to 77dB when the differential input
voltage level is 2V
Figure 3. THD vs Common Mode Input Voltage
Figure 4. THD vs Common Mode Input Voltage
–30
–40
–50
–60
–70
–80
–90
–30
–40
–50
–60
–70
–80
–5
0.5
V
V
f
V
V
f
IN
IN
IN
S
IN
S
–4
= 5V
= 100kHz
= 5V
= 100kHz
INPUT COMMON MODE VOLTAGE (V)
INPUT COMMON MODE VOLTAGE (V)
= 2V
= 2V
1.0
–3
P-P
P-P
P-P
; that is, for a single 5V supply, the
–2
1.5
–1
2.0
0
1
2.5
2
LTC1565-31
3
P-P
3.0
1565-31 F03
1565-31 F04
4
differential input
3.5
5
5