Datasheet

8
LTC1569-6
APPLICATIONS INFORMATION
WUU
U
Figure 6. Typical Divide Ratio in the
Divide-by-16 Mode, T
A
= 25°C
Figure 7. Filter Cutoff vs Temperature,
Divide-by-16 Mode, R
EXT
= 10k
TEMPERATURE (°C)
–50
NORMALIZED FILTER CUTOFF
1569-6 F07
1.010
1.008
1.006
1.004
1.002
1.000
0.998
0.996
0.994
0.992
0.990
–25
0 25 50 75 100
V
S
= 3V
V
S
= 5V
V
S
= 10V
V
SUPPLY
(V)
2
DIVIDE RATIO
1569-6 F06
16.32
16.16
16.00
15.84
4 6 810
R
EXT
= 5k
R
EXT
= 10k
R
EXT
= 20k
R
EXT
= 40k
a ground plane connected to V
–
(Pin 4) for single supply
applications. Connect a ground plane to GND (Pin 3) for
dual supply applications and connect V
–
(Pin 4) to a
copper trace with low thermal resistance.
Input and Output Voltage Range
The input signal range includes the full power supply
range. The output range is typically (V
–
+ 50mV) to (V
+
–
0.8V) when using a single 3V supply with the GND (Pin 3)
voltage set to 1.11V. In other words, the output range is
typically 2.1V
P-P
for a 3V supply. Similarly, the output
range is typically 3.9V
P-P
for a single 5V supply when the
GND (Pin 3) voltage is 2V. For ±5V supplies, the output
range is typically 8.5V
P-P
.
The LTC1569-6 can be driven with a single-ended or
differential signal. When driven differentially, the voltage
between IN
+
and IN
–
(Pin 1 and Pin 2) is filtered with a DC
gain of 1. The single-ended output voltage OUT (Pin 8) is
referenced to the voltage of the GND (Pin 3). The common
mode voltage of IN
+
and IN
–
can be any voltage that keeps
the input signals within the power supply range.
For noninverting single-ended applications, connect IN
–
to GND or to a quiet DC reference voltage and apply the
input signal to IN
+
. If the input is DC coupled then the DC
gain from IN
+
to OUT will be 1. This is true given IN
+
and
OUT are referenced to the same voltage, i.e., GND, V
–
or
some other DC reference. To achieve the distortion levels
shown in the Typical Performance Characteristics the
input signal at IN
+
should be centered around the DC
voltage at IN
–
. The input can also be AC coupled, as shown
in the Typical Applications section.
For inverting single-ended filtering, connect IN
+
to GND or
to quiet DC reference voltage. Apply the signal to IN
–
. The
DC gain from IN
–
to OUT is –1, assuming IN
–
is referenced
to IN
+
and OUT is reference to GND.
Refer to the Typical Performance Characteristics section
to estimate the THD for a given input level.
Dynamic Input Impedance
The unique input sampling structure of the LTC1569-6 has
a dynamic input impedance which depends on the con-
figuration, i.e., differential or single-ended, and the clock
frequency. The equivalent circuit in Figure 8 illustrates the
input impedance when the cutoff frequency is 64kHz. For
other cutoff frequencies replace the 125k value with
125k • (64kHz/f
CUTOFF
).
When driven with a single-ended signal into IN
–
with IN
+
tied to GND, the input impedance is very high (~10MΩ).
When driven with a single-ended signal into IN
+
with IN
–
tied to GND, the input impedance is a 125k resistor to GND.
When driven with a complementary signal whose com-
mon mode voltage is GND, the IN
+
input appears to have
125k to GND and the IN
–
input appears to have –125k to
GND. To make the effective IN
–
impedance 125k when
driven differentially, place a 62.5k resistor from IN
–
to
GND. For other cutoff frequencies use 62.5k • (64kHz/