LTC1569-6_1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1569-6_1 Datasheet PDF : 12 Pages
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LTC1569-6
APPLICATIONS INFORMATION
it reaches the typical limits VMAX and VMIN. The above
voltage swings are for RLOAD = 10k for VS = 3V and 5V.
RLOAD = 20k for VS = ±5V.
To maximize the undistorted peak-to-peak signal swing of
the filter, the GND (Pin 3) voltage should be set to 2V
(1.11V) in single 5V (3V) supply applications.
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/fCUTOFF).
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 • (128kHz/
fCUTOFF), as shown in the Typical Applications section. The
typical variation in dynamic input impedance for a given
clock frequency is ±10%.
Wideband Noise
The wideband noise of the filter is the RMS value of the
device’s output noise spectral density. The wideband
noise data is used to determine the operating signal-to-
noise at a given distortion level. The wideband noise is
nearly independent of the value of the clock frequency and
excludes the clock feedthrough. Most of the wideband
noise is concentrated in the filter passband and cannot be
removed with post filtering (Table 2). Table 3 lists the
typical wideband noise for each supply.
Table 2. Wideband Noise vs Supply Voltage, Single 3V Supply
Bandwidth
Total Integrated Noise
DC to fCUTOFF
DC to 2 • fCUTOFF
DC to fCLK
80µVRMS
95µVRMS
110µVRMS
Table 3. Wideband Noise vs Supply Voltage, fCUTOFF = 64kHz
Total Integrated Noise
Power Supply
DC to 2 • fCUTOFF
3V
95µVRMS
5V
100µVRMS
±5V
105µVRMS
Clock Feedthrough
Clock feedthrough is defined as the RMS value of the clock
frequency and its harmonics that are present at the filter’s
OUT pin (Pin 8). The clock feedthrough is measured with
IN + and IN – (Pins 1 and 2) grounded and depends on the
PC board layout and the power supply decoupling. Table␣ 4
shows the clock feedthrough (the RMS sum of the first 11
harmonics) when the LTC1569-7 is self-clocked with
REXT = 10k, DIV/CLK (Pin 5) open (divide-by-4 mode). The
clock feedthrough can be reduced with a simple RC post
filter.
8
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