LMF60 datasheet(11/20 Pages) NSC

11 / 20 page

10 LMF60 Application Hints (Continued)

If the LMF60-50 or the LMF60-100 were set up for a cutoff

frequency of 10 kHz the input impedance would be

RIN e

1 c 1010

10 kHz

1MX

In this example with a source impedance of 10k the overall

gain if the LMF60 had an ideal gain of 1 (0 dB) would be

AV e

1MX

10 kX a 1MX

099009 (b864 mdB)

Since the maximum overall gain error for the LMF60 is a01

dB b03 dB with a RS s 2kX the actual gain error for this

case would be a021 dB to b039 dB

15 CUTOFF FREQUENCY RANGE

The filter’s cutoff frequency (fC) has a lower limit caused by

leakage currents through the internal switches discharging

the stored charge on the capacitors At lower clock frequen-

cies these leakage currents can cause millivolts of error for

example

fCLK e 100 Hz ILEAKAGE e 1 pA C e 1pF

V e

1pA

1 pF (100 Hz)

10 mV

The propagation delay in the logic and the settling time re-

quired to acquire a new voltage level on the capacitors in-

creases as the LMF60 power supply voltage decreases

This causes a shift in the fCLK fC ratio which will become

noticeable when the clock frequency exceeds 500 kHz The

amplitude characteristic will stay within tolerance until fCLK

exceeds 750 kHz and will peak at about 04 dB at the cutoff

frequency with a 2 MHz clock The response of the LMF60

is still a reasonable approximation of the ideal Butterworth

lowpass characteristic as can be seen in

Figure 7

TLH9294 – 17

FIGURE 7a LMF60-100 g5V Supplies

Amplitude Response

TLH9294 – 18

FIGURE 7b LMF60-50 g5V Supplies

Amplitude Response

TLH9294 – 19

FIGURE 7c LMF60-100 g25V Supplies

Amplitude Response

TLH9294 – 20

FIGURE 7d LMF60-50 g25V Supplies

Amplitude Response

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