8

LTC1569-6

APPLICATIONS INFORMATION

Figure 6. Typical Divide Ratio in the

Divide-by-16 Mode, TA = 25°C

Figure 7. Filter Cutoff vs Temperature,

Divide-by-16 Mode, REXT = 10k

TEMPERATURE (

°C)

–50

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

VS = 3V

VS = 5V

VS = 10V

VSUPPLY (V)

2

1569-6 F06

16.32

16.16

16.00

15.84

4

6

810

REXT = 5k

REXT = 10k

REXT = 20k

REXT = 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 2.1VP-P for a 3V supply. Similarly, the output

range is typically 3.9VP-P for a single 5V supply when the

GND (Pin 3) voltage is 2V. For

±5V supplies, the output

range is typically 8.5VP-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/fCUTOFF).

When driven with a single-ended signal into IN – with IN +

tied to GND, the input impedance is very high (~10M

Ω).

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/