LTC1565-31
6
156531fa
APPLICATIONS INFORMATION
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
voltages 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 Diagram). 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 ±1%).
The output can also be AC coupled.
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 fi lter can accept a variety of common mode input volt-
ages. Figures 3 and 4 show the THD of the fi lter versus
common mode input voltage with a 2V
P-P
differential
input signal.
+
–
+IN
–IN
GND
V
–
+OUT
–OUT
V
+
SHDN
1
2
3
4
8
7
6
5
LTC1565-31
0.1μF
0.1μF
15645-31 F01
V
OUT
+
V
OUT
–
5V
V
IN
–
+
–
V
IN
+
V
IN
+
+ V
IN
–
2
DC COUPLED INPUT
V
IN
(COMMON MODE) =
V
OUT
+
+ V
OUT
–
2
V
OUT
(COMMON MODE) = =
V
+
2
+
–
+IN
–IN
GND
V
–
+OUT
–OUT
V
+
SHDN
1
2
3
4
8
7
6
5
LTC1565-31
1μF
0.1μF
0.1μF
100k100k
0.1μF
15645-31 F02
V
OUT
+
V
OUT
–
5V
V
IN
–
+
–
V
IN
+
AC COUPLED INPUT
V
IN
(COMMON MODE) = V
OUT
(COMMON MODE)
=
V
+
2
Figure 1
INPUT COMMON MODE VOLTAGE (V)
–90
THD (dB)
–70
–50
–30
–80
–60
–40
–3 –1 1 3
1565-31 F03
5–4–5 –2 0 2 4
V
IN
= 2V
P-P
f
IN
= 100kHz
V
S
= ±5V
INPUT COMMON MODE VOLTAGE (V)
0.5
–80
THD (dB)
–70
–60
–50
1.0
1.5
2.0 2.5
1565-31 F04
3.0
–40
–30
3.5
V
IN
= 2V
P-P
f
IN
= 100kHz
V
S
= 5V
Figure 3. THD vs Common Mode Input Voltage
Figure 4. THD vs Common Mode Input Voltage
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 fi lter. The
spurious free dynamic range, where the THD and S/N
ratio are equal, is 75dB to 76dB when the differential input
voltage level is 2V
P-P
; that is, for a single 5V supply, the
Figure 2
