10 LMF60 Application Hints

The LMF60 is comprised of a non-inverting unity gain low-

pass sixth-order Butterworth switched capacitor filter sec-

tion and two undedicated CMOS Op-Amps The switched-

capacitor topology makes the cutoff frequency (where the

gain drops 301 dB below the DC gain) a direct ratio (1001

or 501) of the clock frequency supplied to the lowpass filter

Internal integrator time constants set the filter’s cutoff fre-

quency The resistive element of these integrators is actual-

ly a capacitor which is ‘‘switched’’ at the clock frequency

(for a detailed discussion see Input Impedance section)

Varying the clock frequency changes the value of this resis-

tive element and thus the time constant of the integrators

ratio of the input and feedback capacitors in the integrators

The higher the clock to cutoff frequency ratio (or the sam-

pling rate) the closer the approximation is to the theoretical

ratios of 501 (LMF60-50) or 1001 (LMF60-100)

11 CLOCK INPUTS

The LMF60 has a Schmitt-trigger inverting buffer which can

be used to construct a simple RC oscillator The oscillator

frequency is dependent on the buffer’s threshold levels as

well as on the resistorcapacitor tolerance (See

Figure 1 )

Schmitt-trigger threshold voltage levels can vary significant-

ly causing the RC oscillator’s frequency to vary greatly

from part to part

used to drive the CLK R input of the LMF60 This input is

TTL logic level compatible and also presents a very light

load to the external clock source (E2 mA) with split sup-

plies and LSh tied to system ground The logic level is pro-

grammed by the voltage applied to level shift (LSh) pin (See

the Pin Description for LSh pin)

12 POWER SUPPLY BIASING

The LMF60 can be biased from a single supply or dual split

supplies The split supply mode shown in

Figures 2 and 3 is

the most flexible and easiest to implement As discussed

TTL or CMOS clock logic levels

Figure 4 shows two

schemes for single supply biasing In this mode only CMOS

clock logic levels can be used

TLH9294 – 8

1

RC In

(

1

137 RC

FIGURE 1 Schmitt Trigger RC Oscillator

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