Application Note (Continued)
Notice that R3 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.
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:
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 (VO) is at VCC for the Min-range. VO is
clamped at GND for the Mid-range. For the Max-range, VO is
at Vee. Figure 14 shows a VO vs. VI oscilloscope photo per
the circuit of Figure 13.
Its operation is as follows: VI deviating from GND, causes
the diode bridge to absorb IIN to maintain a clamped condi-
tion (VO= 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.
To illustrate the design process/implementation, a 3 kHz,
Butterworth response, low-pass filter DAAF (Figure 10) is
designed as follows:
1. Choose C1 = C3 = C = 1 nF
2. Choose R4 = R5 = 1 kΩ
3. Calculate Ra and R2 for the desired Fc as follows:
10012889
4. Calculate R3 for the desired Q. The desired Q for a
Butterworth (Maximally Flat) response is 0.707 (45 degrees
into the s-plane). R3 calculates as follows:
15
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