Lowpass filter oscillations
Lowpass filter oscillations
(OP)
Hello;
I have designed and built the attached amplifier/ lowpass filter. The circuit works pr say, however the low pass filter starts to have a small oscillation when its output approaches 2 volts. the period of the oscillation is about 14 us. U4 power supply pins are bypassed with 0.1 uf caps. The +5 power supply has no measureable ripple, and the -5 power supply has about 30 mv of ripple. The oscillations are about 0.1 volt amplitude.
Can some one give me some suggestions?
I have designed and built the attached amplifier/ lowpass filter. The circuit works pr say, however the low pass filter starts to have a small oscillation when its output approaches 2 volts. the period of the oscillation is about 14 us. U4 power supply pins are bypassed with 0.1 uf caps. The +5 power supply has no measureable ripple, and the -5 power supply has about 30 mv of ripple. The oscillations are about 0.1 volt amplitude.
Can some one give me some suggestions?





RE: Lowpass filter oscillations
Have you tried other amplifiers? Have you tried to remove the protection?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Lowpass filter oscillations
Another tale from the crypt. More than 40 years ago at my first job they had this transistor/FET transducer amp that they built. It had this $45 FET that they were so proud of (I bought a good used car for about that price). I looked at the schematic and it had this big cap on the output to ground, like about .1uF. They told me it would oscillate without that. Thinking of tube circuits I disconnected the gate lead of the FET and put a 100 ohm resistor in series with it. That was enough dampening to stop the oscillations.
RE: Lowpass filter oscillations
The diodes are there for surge protection. I live in west central Florida, which is the lighting capital of the US!
I am attaching the data sheet for the LMC6064.
On further checking it looks like the ripple maybe caused by oscillations in other amps with int the package. I am continuing to work on it. Thanks for all of the replies and keep them comming.
RE: Lowpass filter oscillations
There are good application notes in that data sheet. I think you should study them and also read the foot notes.
lightning does not hit instrumentation dirctly - not even in Florida. There are far better methods to protect the input, if you think it is necessary.
Do not take offence if I point out that the phrase is 'per se' and the meaning is 'in itself' or 'as such'.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Lowpass filter oscillations
The good news is that these switched transients do not affect the overall performance of the circuit. I have checked linearity throughout the range of the circuit and it is to witnin two or three ADC counts for the 12 bit ADC.
RE: Lowpass filter oscillations
Please change to a working topology, I can suggest MFB as a possibiliy.
TI has excellent application otes on filter design, listing the limitations of different types. Example:
http://
Cheers,
Benta.
RE: Lowpass filter oscillations
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RE: Lowpass filter oscillations