LM3886
SNAS091C –MAY 1999–REVISED MARCH 2013
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NOTE
CCIR/ARM: A Practical Noise Measurement Method; by Ray Dolby, David Robinson and
Kenneth Gundry, AES Preprint No. 1353 (F-3).
In addition to noise filtering, differing meter types give different noise readings. Meter responses include:
1. RMS reading,
2. average responding,
3. peak reading, and
4. quasi peak reading.
Although theoretical noise analysis is derived using true RMS based calculations, most actual measurements are
taken with ARM (Average Responding Meter) test equipment.
Typical signal-to-noise figures are listed for an A-weighted filter which is commonly used in the measurement of
noise. The shape of all weighting filters is similar, with the peak of the curve usually occurring in the 3 kHz–7 kHz
region as shown below.
Figure 51.
SUPPLY BYPASSING
The LM3886 has excellent power supply rejection and does not require a regulated supply. However, to eliminate
possible oscillations all op amps and power op amps should have their supply leads bypassed with low-
inductance capacitors having short leads and located close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency
instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or
electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic
capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines.
If adequate bypassing is not provided the current in the supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal causes low distortion at high frequencies requiring that
the supplies be bypassed at the package terminals with an electrolytic capacitor of 470 μF or more.
LEAD INDUCTANCE
Power op amps are sensitive to inductance in the output lead, particularly with heavy capacitive loading.
Feedback to the input should be taken directly from the output terminal, minimizing common inductance with the
load.
Lead inductance can also cause voltage surges on the supplies. With long leads to the power supply, energy is
stored in the lead inductance when the output is shorted. This energy can be dumped back into the supply
bypass capacitors when the short is removed. The magnitude of this transient is reduced by increasing the size
of the bypass capacitor near the IC. With at least a 20 μF local bypass, these voltage surges are important only if
the lead length exceeds a couple feet (>1 μH lead inductance). Twisting together the supply and ground leads
minimizes the effect.
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