Impact influences output of non-inverting op-amp LM324 - Page 1
Forum section: electronics / beginners

author: commerou
author_rank: Contributor
timestamp: September 20, 2015, 07:48:16 pm
content: Hi allI am currently working on a project were I use a LM324 as a non-inverting amplifier with gain 3000.It is amplifying a signal from a hall-effect sensor.Then comes the problem. When I hit the op-amp it makes a spike on the output. Why is this, and how can i prevent it?It is a big deal, because the final product is going to be mounted on a gokart. And there are a lot of vibrations on the gokart, and i dont want it to influence my final output of the opamp.Hope someone can help.Best regardsCommeroups. it is a Honeywell SS490 hall-effect sensor

author: Chris C
author_rank: Frequent Contributor
timestamp: September 20, 2015, 08:41:07 pm
content: That's a LOT of amplification, especially from a single op-amp.When you say "hit", do you mean tapping the IC directly with a finger or other instrument? That may not be a good test, because the object can act as a capacitor, and you have more than enough amplification that it could act as a proximity sensor.A direct, sharp impact on the IC may also induce some sort of piezoelectric effect, I'm not sure. But the physical effects of even strong vibration would not approach the sharpness of an impact.You might try testing this on a real gokart. If the issue persists, I would try using two LM324 in series, with the gain split between them (55 x 55 = 3025); perhaps adding some low-pass filtering as well.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 20, 2015, 08:57:42 pm
content: Are you using a ceramic capacitor anywhere in the circuit?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 20, 2015, 10:04:20 pm
content: Are you prototyping your circuit on a solderless breadboard?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2015, 11:26:36 pm
content: This sounds like many incorrect design decisions being made...- LM324 has large offset (max 3mV --> an output of 9V from your circuit), offset requirement unstated- LM324 has limited gain (min 25 V/mV, you're asking 3 V/mV), linearity requirement unstated- LM324 has high noise (~23nV/rtHz?), noise requirement unstated- LM324 has low bandwidth (GBW ~ 1MHz, a roll-off of 300Hz in your circuit), bandwidth requirement unstated- The SS495 has an amplifier internal, why do you need more gain?- Even the SS495A1 (best grade?) has 25 times more offset than the LM324 does, how will you trim or zero-offset this?- This is for a gokart, what function is it serving? If electrical current sense, you will be far better served with a proper DC current sensor. If motor or engine sensing, a stronger magnet, or closer proximity, or logic type Halle effect sensor, or an inductive or optical sensor, should be used instead.As for your observation, likely both components -- the sensor and the LM324 -- exhibit strain related drift. This occurs because silicon is sensitive to strain, changing the bandgap and therefore all voltage drops proportional to it (like the base-emitter voltage in the input differential amplifier). (Not exactly, but close enough.) The IC is built to minimize this type of error (and due to thermal influence as well), but amplified thousands of times, it doesn't quite cancel out and you observe the output changing rapidly with strain.This can be fixed by using a self-zeroing type amplifier, like the OPA2180, AD8628, or simply a better one, like OP07, OP37 or LT1012. But it'll be much easier to deal with the original problem, I think.Tim