22 PROGRAMMING PINS

The LMF90 has five control pins that are used to program

the filter’s characteristics via a three-level logic scheme In

dual-supply applications these inputs are tied to either V

a

V

b

or GND in order to select a particular set of characteris-

tics For example the W input (pin 1) sets the filter’s pass-

connected to V

a

GND or V

b

respectively Applying V

b

and GND to the D input (pin 10) will set the notch depth to

40 dB or 30 dB respectively

The R input (pin 2) is another three-level logic input and it

sets the clock-to-center-frequency ratio to 33331 501 or

1001 for input voltages equal to V

a

GND or V

b

respec-

tively Note that the clock frequency referred to here is the

frequency at the CLK pin and at the frequency divider output

(if used) This is different from the frequency at the divider’s

input LD (pin 3) sets the frequency divider’s division factor

to either 716 596 or 2 for input voltages equal to V

a

GND

or V

b

respectively XLS (pin 7) enables and disables the

crystal oscillator and clock divider When XLS is connected

to the positive supply the oscillator and divider are enabled

and CLK is the output of the divider and can drive the clock

inputs of other LMF90s When XLS is connected to GND

the oscillator and divider are disabled and the CLK pin be-

comes a clock input for CMOS-level signals Connecting

XLS to the negative supply disables the oscillator and divid-

er and causes CLK to operate as a TTL-level clock input

Using an external 3579545 MHz color television crystal with

the internal oscillator and divider it is possible to build a

power line frequency notch for 50 Hz or 60 Hz line frequen-

cies or their second and third harmonics using the LMF90 A

60 Hz notch is shown in the Typical Application circuit on

the first page of this data sheet Connecting LD to V

a

changes the notch frequency to 50 Hz Changing the clock-

to-center-frequency ratio to 501 results in a second-har-

monic notch and a 331 ratio causes the LMF90 to notch

the third harmonic

Table I illustrates 18 different combinations of filter band-

width depth and clock-to-center-frequency ratio obtained

by choosing the appropriate W D and R programming volt-

ages

23 DIGITAL INPUTS AND OUTPUTS

As mentioned above the CLK pin can serve as either an

input or an output depending on the programming voltage

on XLS When CLK is operating as a TTL input it will oper-

ate properly in both dual-supply and single-supply applica-

tions because it has two logic thresholdsone referred to

V

b

and one referred to GND When operating as an output

CLK swings rail-to-rail (CMOS logic levels)

XTAL1 and XTAL2 are the input and output pins for the

internal crystal oscillator When using the internal oscillator

(XLS connected to V

a

) the crystal is connected between

these two pins When the internal oscillator is not used

XTAL2 should be left open XTAL1 can be used as an input

for an external CMOS-level clock signal swinging from V

b

to V

a

The frequency of the crystal or the external clock

applied to XTAL1 will be divided by the internal frequency

divider as determined by programming voltage on the LD

pin

24 SAMPLED-DATA SYSTEM CONSIDERATIONS

OUTPUT STEPS

Because the LMF90 uses switched-capacitor techniques its

performance differs in several ways from non-sampled (con-

tinuous) circuits The analog signal at the input to the inter-

nal bandpass filter (pin 12) is sampled during each clock

cycle and since the output voltage can change only once

every clock cycle the result is a discontinuous output signal

The bandpass output takes the form of a series of voltage

‘‘steps’’ as shown in

Figure 3 The steps are smaller when

the clock frequency is much greater than the signal frequen-

cy

Switched-capacitor techniques are used to set the summing

amplifier’s gain Its input and feedback ‘‘resistors’’ are actu-

ally made from switches and capacitors Two sets of these

‘‘resistors’’ are alternated during each clock cycle Each

time these gain-setting components are switched there will

be no feedback connected to the op amp for a short period

of time (about 50 ns) This generates very low-amplitude

The amplitude of each of these intermodulation compo-

nents will typically be at least 70 dB below the input signal

amplitude and well beyond the spectrum of interest

TABLE I Operation of LMF90 Programming Pins Values given are for nominal levels of attenuation

RV

b

V

a

DW

(dB)

(dB)

(dB)

V

b

b

30

012

0019

b

30

012

0019

b

30

012

0019

V

b

GND

b

30

026

0040

b

30

026

0040

b

30

026

0040

V

a

b

30

055

0082

b

30

055

0082

b

30

055

0082

V

b

b

35

012

0010

b

35

012

0010

b

35

012

0010

GND

GND

b

40

026

0024

b

40

026

0024

b

40

026

0024

V

a

b

40

055

0050

b

40

055

0050

b

40

055

0050

12