8

LTC1569-7

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 LTC1569-7 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 128kHz. For

other cutoff frequencies replace the 125k value with

125k • (128kHz/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 • (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.

APPLICATIONS INFORMATION

The oscillator is sensitive to transients on the positive

supply. The IC should be soldered to the PC board and the

PCB layout should include a 1

µF ceramic capacitor be-

tween V + (Pin 7) and V – (Pin 4) , as close as possible to

the IC to minimize inductance. Avoid parasitic capacitance

on RX and avoid routing noisy signals near RX (Pin 6). Use

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 Range

The input signal range includes the full power supply

range. The output voltage range is typically (V – + 50mV)

to (V + – 0.8V) when VS = 3V. 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 LTC1569-7 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

Figure 8

8

3

1

2

OUT

IN–

IN+

GND

1569-7 F08

125k

+

–

–

+

125k

i =

IN+ – GND

125k