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 filter can accept a variety of common mode input volt-

ages. Figures 3 and 4 show the THD of the filter versus

input signal.

+

–

+IN

–IN

GND

+OUT

–OUT

SHDN

1

2

3

4

8

7

6

5

LTC1565-31

0.1μF

0.1μF

15645-31 F01

+

–

5V

–

+

+

2

DC COUPLED INPUT

2

=

2

+

–

+IN

–IN

GND

+OUT

–OUT

SHDN

1

2

3

4

8

7

6

5

LTC1565-31

1μF

0.1μF

0.1μF

100k

100k

0.1μF

15645-31 F02

+

–

5V

–

+

+

AC COUPLED INPUT

=

2

Figure 1

INPUT COMMON MODE VOLTAGE (V)

–90

–70

–50

–30

–80

–60

–40

–3

–1

1

3

1565-31 F03

5

–4

–5

–2

0

2

4

INPUT COMMON MODE VOLTAGE (V)

0.5

–80

–70

–60

–50

1.0

1.5

2.0

2.5

1565-31 F04

3.0

–40

–30

3.5

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 filter. The

spurious free dynamic range, where the THD and S/N

ratio are equal, is 75dB to 76dB when the differential input

Figure 2