LTC6603 Linear Technology, LTC6603 Datasheet - Page 19

• Manufacturer Part Number: LTC6603
• Description: Dual Adjustable Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
not independent as they are both set by the V
Figure 10 illustrates the distortion versus output common
mode voltage for a 2V
common mode input voltage that is equal to mid-supply.
Interfacing to the LTC6603
The input and output common mode voltages of the LTC6603
are independent. The input common mode voltage is set
by the signal source if DC coupled, as shown in Figure 11.
If the inputs are AC coupled, as shown in Figure 12
(Circuit A), the input common mode voltage will be pulled to
ground by an equivalent resistance of R
This does not affect the fi lter’s performance as long as
the input amplitude is less than 0.5V
settings, a larger input voltage swing may be desired.
V
IN
+
Figure 10. Distortion vs Common Mode Output Voltage
+
–
V
IN
–60
–65
–70
–75
–80
–
0.8
+
–
0.1μF
DC COUPLED INPUT
V
V
R
GAIN = 24dB, T
SIGNAL FREQUENCY = 200kHz
HD2, LPF1 = 1
IN
OUT
BIAS
Figure 11. DC Coupled Inputs
HD3, LPF1 = 1
(COMMON MODE) = (V
COMMON MODE OUTPUT VOLTAGE (V)
(COMMON MODE) = (V
= 30.9k, V
1.0
1μF
V
SUPPLY
HD3, LPF1 = 0, LPF0 = 1
P-P
A
S
HD2, LPF1 = 0,
LPF0 = 1
1.2
= 25°C
= 3V,
differential input voltage and a
IN
1.4
V+
V+
V+
+INA
–INA
V
GND
OCM
OUT
+ + V
IN
A
D
+ + V
IN
LTC6603
P-P
–)/2
1.6
CM
OUT
. At low fi lter gain
, shown in Table 5.
–)/2 = V
6603 F10
+OUTA
–OUTA
1.8
SUPPLY
OCM
/2
V
V
OUT
OUT
6603 F11
+
–
pin.
Connecting resistors between each input and V+
pull the input common mode voltage up, increasing the
input signal swing. The resistance, R
set the input common mode voltage, V
level can be calculated by
where
For example, if the lowpass cutoff frequency is set to
2.5MHz, 5k resistors connected between each input and
V+
supply.
Circuit A of Figure 12 is for a fi xed CLK and LPF0, LPF1
setting. If the clock varies or the LPF0, LPF1 setting changes
then Circuit B of Figure 12 should be used.
Due to the sampled data nature of the fi lter, an anti-aliasing
fi lter at the inputs is recommended.
The output common mode voltage is equal to the voltage
of the V
supply voltage by an internal resistive divider (see Block
Diagram). To alter the common mode output voltage, V
can be driven with an external voltage source or resistor
network. If external resistors are used, it is important to
note that the internal 2k resistors can vary ±30% (their
ratio varies only ±1%). The fi lter outputs can also be AC
coupled.
The LTC6603 can be interfaced to an A/D converter by pull-
ing CLKCNTL (Pin 5) to V+
as a clock output, which can be used to drive the clock
input of the A/D converter. This allows the A/D converter
to be synchronized with the fi lter sampling clock, avoiding
“beat frequencies” and simplifying the board layout. Any
routing attached to the CLKIO pin should be as short as
possible, in order to minimize refl ections.
Similarly, the LTC6603 can be interfaced to another LTC6603
in a master/slave confi guration as shown in Figure 13. This
R
R
R
IN
R
CM
CM
CM
PULL UP
will set the input common mode voltage to mid-
= 20k•80MHz/f
= 5k•80MHz/f
= 40k•80MHz/f
OCM
pin. The V
= R
CM
CLK
OCM
V
CLK
CLK
SUPPLY
V
D
for LPF1=1
ICM
. This confi gures CLKIO (Pin 15)
for LPF1=0, LPF0=1
pin is biased to one half of the
for LPF1=0, LPF0=0
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1
PULL-UP
ICM
LTC6603
, to any desired
, necessary to
19
IN
OCM
will
6603f