LMP8602 datasheet(15/24 Pages) NSC

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ADDITIONAL SECOND ORDER LOW PASS FILTER

The LMP8602/LMP8602Q/LMP8603/LMP8603Q has a third

order Butterworth low-pass characteristic with a typical band-

width of 60 kHz integrated in the preamplifier stage of the part.

The bandwidth of the output buffer can be reduced by adding

a capacitor on the A1 pin to create a first order low pass filter

with a time constant determined by the 100 k

Ω internal resis-

tor and the external filter capacitor.

It is also possible to create an additional second order Sallen-

Key low pass filter as shown in Figure 2 by adding external

components R

2, C1 and C2. Together with the internal

100 k

Ω resistor R

1, this circuit creates a second order low-

pass filter characteristic.

30083455

K1 = 10, K2 = 5 for LMP8602, K2 = 10 for LMP8603

FIGURE 2. Second Order Low Pass Filter

When the corner frequency of the additional filter is much

lower than 60 kHz, the transfer function of the described am-

plifier van be written as:

Where K

1 equals the gain of the preamplifier and K2 that of

the buffer amplifier.

The above equation can be written in the normalized frequen-

cy response for a 2nd order low pass filter:

The Cutt-off frequency

o in rad/sec (divide by 2π to get the

cut-off frequency in Hz) is given by:

and the quality factor of the filter is given by:

For any filter gain K > 1x, the design procedure can be very

simple if the two capacitors are chosen to in a certain ratio.

Inserting this in the above equation for Q results in:

Which results in:

In this case, given the predetermined value of R1 = 100 k

Ω

( the internal resistor), the quality factor is set solely by the

value of the resistor R

LMP8602 Scheda tecnica(PDF) 15 Page - National Semiconductor (TI)