Datasheet

LTC6602
19
6602fc
APPLICATIONS INFORMATION
Figure 10. Distortion vs Common Mode Output Voltage
output common mode voltages of the two channels are
not independent as they are both set by the V
OCM
pin.
Figure 10 illustrates the distortion versus output common
mode voltage for a 1.5V
P-P
differential input voltage and a
common mode input voltage that is equal to mid-supply.
Interfacing to the LTC6602
The input and output common mode voltages of the
LTC6602 are independent. The input common mode volt-
age is set by the signal source if DC coupled, as shown in
Figure 11. If the inputs are AC coupled, the input com-
mon mode voltage will pulled to ground by an equivalent
resistance of R
CM
, shown in Table 4. This does not affect
the filter’s performance as long as the input amplitude
is less than 0.5V
P-P
. At low filter gain settings, a larger
input voltage swing may be desired. Figure 12 shows two
circuits with AC coupled inputs. In a fixed lowpass cutoff
frequency, connecting resistors between each input and
V+
IN
will pull the input common mode voltage up, increas-
ing the input signal swing (Figure 12a). The resistance,
R
PULL-UP
, necessary to set the input common mode
voltage, V
ICM
, to any desired level can be calculated by
R
PULL −UP
= R
CM
V
SUPPLY
V
ICM
− 1






where
R
CM
= 20k • 90MHz/f
CLK
for LPFI = 0
R
CM
= 6.7k • 90MHz/f
CLK
for LPFI = 1
For example, if the lowpass cutoff frequency is set to
300kHz, 20k resistors connected between each input
and V+
IN
will set the input common mode voltage to
mid-supply.
Figure 11. DC Coupled Inputs
Figure 12. AC Coupled Inputs
COMMON MODE OUTPUT VOLTAGE (V)
DISTORTION (dBc)
6602 F10
–20
–60
–50
–40
–30
–70
–80
–90
0.5 2.51.5 2.01.0
T
A
= 25°C
f
IN
= 100kHz
V
IN
= 1.5V
P-P
R
BIAS
= 54.9k
45kHz-300kHz BPF
GAIN = 0dB
HD3
HD2
V+
IN
V+
A
V+
D
+INA
–INA
V
OCM
GND
LTC6602
0.1µF
DC COUPLED INPUT
V
IN
(COMMON MODE) = (V
IN
+ + V
IN
–)/2
V
OUT
(COMMON MODE) = (V
OUT
+ + V
OUT
–)/2 = V
SUPPLY
/2
6602 F11
+OUTA
–OUTA
V
SUPPLY
+
–
+
–
1µF
V
OUT
+
V
OUT
–
V
IN
+
V
IN
–
AC COUPLED INPUT
V
IN
(COMMON MODE) = V
SUPPLY
/2
6602 F12a
V
SUPPLY
+
–
+
–
0.1µF
0.1µF
R
PULL-UP
R
PULL-UP
V+
IN
V+
A
V+
D
+INA
–INA
V
OCM
GND
LTC6602
0.1µF
–OUTA
–OUTB
V
SUPPLY
1µF
V
IN
+
V
IN
–
(a) Fixed Lowpass Cutoff Frequency
6602 F12b
AC COUPLED INPUT COMMON MODE AT +INA AND –INA
V
SUPPLY
+
–
+
–
0.1µF
0.1µF
1.87k 1.87k1.87k
1.87k
V+
IN
V+
A
V+
D
+INA
–INA
V
OCM
GND
LTC6602
0.1µF
–OUTA
–OUTB
V
SUPPLY
1µF
V
IN
+
V
IN
–
RCM • V
SUPPLY
2 • RCM + 1.87k
= =
1.87k
(b) Variable Lowpass Cutoff Frequency