Page 264 - Op Amps Design, Applications, and Troubleshooting

246 ACTIVE FILTERS

TABLE 5.4

Design Goal Measured Performance
Resonant frequency 5500 hertz 5693 hertz
Minimum Z [N lOkilohms 10.65 kilohms
Q Not specified 22

struct the circuit. More precise values would yield performance correspondiiigly
closer to the design goals. The Q of the circuit can go as high as 40 or 50 with care-
ful selection of components.

5.6 TROUBLESHOOTING UPS FOR ACTIVE FILTERS

We can generally classify potential troubles in active filters into two broad classes:

1. DC problems
2. AC problems

Our first measurements will quickly tell us which type of problem exists. We can
then focus our efforts on areas that might cause this type of problem.
DC Problems. Problems that cause the output of the op amp to be at some
abnormal DC level are generally located in the same manner as described in pre-
vious chapters. Basically, you need to ensure that the proper V* and V~ are present
directly on the appropriate pins of the op amp. If these voltages are correct, com-
pare the polarity of the differential input voltage (v d) with the polarity on the out-
put pin. If the polarities contradict normal op amp behavior, then suspect the op
amp; if they are correct for a normal op amp, then measure the DC level on the
input to the op amp circuit. A prior stage may be sending an abnormal DC level
into this stage, which makes it appear to be defective.
If all of these appear normal, then verify the integrity of the feedback circuit.
If it is open, the output of the op amp will be at one of the two extremes.
AC Problems. If the DC voltages are correct in the filter circuit, but the filter
does not correctly discriminate against certain frequencies, then suspect the fre-
quency determining components. If the resonant, or cutoff, frequencies have sim-
ply shifted slightly, you might suspect a change in component values. On the
other hand, if the AC operation of the circuit has been altered dramatically, then
suspect an open component. Normal DC values with abnormal AC values often
point to an open capacitor or to a resistor that is isolated from DC (e.g., R 3 in Fig-
ure 5.18).
As with all op amp troubleshooting tasks, it is essential that you understand
the proper operation of the circuit and continuously contrast the actual perfor-
mance with the known expected performance.