LMV821 datasheet(16/26 Pages) TI1

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Application Note (Continued)

To simplify the design process, certain components are set

equal to each other. Refer to Figure 10 and Figure 11. These

equal component values help to simplify the design equa-

tions as follows:

To illustrate the design process/implementation, a 3 kHz,

Butterworth response, low-pass filter DAAF (Figure 10)is

designed as follows:

1. Choose C

1 =C3 =C=1nF

2. Choose R

4 =R5 =1k

Ω

3. Calculate R

a and R2 for the desired Fc as follows:

4. Calculate R

3 for the desired Q. The desired Q for a

Butterworth (Maximally Flat) response is 0.707 (45 degrees

into the s-plane). R

3 calculates as follows:

Notice that R

3 could also be calculated as 0.707 of Ra or R2.

The circuit was implemented and its cutoff frequency mea-

sured. The cutoff frequency measured at 2.92 kHz.

The circuit also showed good repeatability. Ten different

LMV822 samples were placed in the circuit. The correspond-

ing change in the cutoff frequency was less than a percent.

TRI-LEVEL VOLTAGE DETECTOR

The tri-level voltage detector of Figure 13 provides a type of

window comparator function. It detects three different input

voltage ranges: Min-range, Mid-range, and Max-range. The

output voltage (V

O)isatVCC for the Min-range. VO is

clamped at GND for the Mid-range. For the Max-range, V

O is

at V

ee. Figure 14 shows a VO vs. VI oscilloscope photo per

the circuit of Figure 13.

Its operation is as follows: V

I deviating from GND, causes

the diode bridge to absorb I

IN to maintain a clamped condi-

tion (V

O= 0V). Eventually, IIN reaches the bias limit of the

diode bridge. When this limit is reached, the clamping effect

stops and the op amp responds open loop. The design

equation directly preceding Figure 14, shows how to deter-

mine the clamping range. The equation solves for the input

voltage band on each side GND. The mid-range is twice this

voltage band.

10012889

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