LMP8602MM datasheet(16/24 Pages) NSC

16 / 24 page

2 can be calculated based on the desired value of Q as the

first step of the design procedure with the following equation:

For the gain of 5 for the LMP8602 this results in:

For the gain of 10 for the LMP8603 this is:

For instance, the value of Q can be set to 0.5

√2 to create a

Butterworth response, to 1/

√3 to create a Bessel response,

or a 0.5 to create a critically damped response. Once the

value of R

2 has been found, the second and last step of the

design procedure is to calculate the required value of C to give

the desired low-pass cut-off frequency using:

Which for the gain = 5 will give:

and for the gain = 10:

For C

2 the value is calculated with:

Or for a gain = 5:

and for a gain = 10:

Note that the frequency response achieved using this proce-

dure will only be accurate if the cut-off frequency of the second

order filter is much smaller than the intrinsic 60 kHz low-pass

filter. In other words, to have the frequency response of the

LMP8602/LMP8602Q/LMP8603/LMP8603Q circuit chosen

such that the internal poles do not affect the external second

order filter.

For a desired Q = 0.707 and a cut off frequency = 3 kHz, this

will result for the LMP8602 in rounded values for R2 = 51

Ω, C1 = 1.5 nF and C2 = 3.9 nF

And for the LMP8603 this will result in rounded values for R2

= 22 k

Ω, C1 = 3.3 nF and C2 = 0.39 nF

GAIN ADJUSTMENT

The gain of the LMP8602 is 50 and the gain of the LMP8603

is 100, however, this gain can be adjusted as the signal path

in between the two internal amplifiers is available on the ex-

ternal pins.

Reduce Gain

Figure 3 shows the configuration that can be used to reduce

the gain of the LMP8602 and the LMP8603.

30083456

FIGURE 3. Reduce Gain

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