Amplifying millivolts to volts, getting oscillations - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: February 11, 2016, 01:16:17 pm
content: I have an oxygen sensor, which is essentially a lead-oxygen galvanic cell.The output is 0 to 50mV.I want to amplify this to something more usable, namely 0 to 3.3V (or 5V in the schematic below)Unfortunately, it does not work all that well.The output tend to oscillate in a random fashion. It's there some times, and it's rock steady the other times.At worst it's as much as +-50mV, which is +-1% O2. Not good enough.The only thing that had a result was putting a capacitor Cs across the feedback resistor. However, it only reduced the noise, not removed it.What am i doing wrong here?

author: Andy Watson
author_rank: Super Contributor
timestamp: February 11, 2016, 01:19:23 pm
content: The 1uF on the output of the op-amp is not going to help. What are the characteristics of the sensor - does it have a capacitive element to it?

author: dom0
author_rank: Super Contributor
timestamp: February 11, 2016, 01:21:46 pm
content: What's the op amp used?Do you need a high impedance input, or an input with defined impedance?If high impedance, why don't you use a non-inverting amplifier instead?What's the bandwidth requirement (=> I assume "practically DC"?)Offset? Drift?The 1 M? feedback resistance is not going to help with stability: together with a couple pF of stray and input capacitance will create a pole as low as a few dozen kHz.The 1 µF at the output are simply wrong.You have Av = 100, but 50 mV * 100 = 5 V, which is beyond the supply rail of the op amp (3.3 V).

author: Artlav
author_rank: Frequent Contributor
timestamp: February 11, 2016, 01:51:07 pm
content: Quote from: Andy Watson on February 11, 2016, 01:19:23 pmNone intentional, only a few resistors.It's essentially a battery.Quote from: dom0 on February 11, 2016, 01:21:46 pmAD820.The datasheet says that the amp being used should have input impedance higher than 1 megaOhm.The response time of the sensor is in low tens of seconds, so max bandwidth is under 1Hz.Not sure about the rest.Quote from: dom0 on February 11, 2016, 01:21:46 pmHm?I'm not sure what you are talking about.Quote from: dom0 on February 11, 2016, 01:21:46 pmNot a problem, i'm not expecting levels high enough for it to go over the rail, and even if it does, somehow, it's far out of valid range anyway.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 11, 2016, 05:42:42 pm
content: As Dom highlighted, 1M input resistances and 100M feedback are unusually high resistances for an op-amp and any extra capacitance (intentional or otherwise) around the inputs is going to add significantly to the phase. The op-amp should be able to cope with the high impedances provided you take care in the layout.The op-amp datasheet specifies the power supply voltage as 5 to 36V. Running it at 3.3V is a bit dodgy!The 1uF on the output should be removed. If you must load the output with 1uF, isolate it from the op-amp output and feedback loop with some resistance.

author: Artlav
author_rank: Frequent Contributor
timestamp: February 11, 2016, 10:46:44 pm
content: Ok, i removed the capacitor at the output, and the noise got much worse.But then i added the Cs across the 100M feedback, and it became rock steady.I can breath towards it, and it would curve down and up, smooth like a polished... uh... whatever.Thanks everyone for advice!

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2016, 11:17:17 am
content: Should never have a capacitor on the output, unless you are building an oscillator.You can terminate into a capacitor through a resistor, however.

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 14, 2016, 05:25:55 pm
content: Circuit impedance is too high.Use a non inverting op-amp configuration instead and you won't need a high z input resistor.Put 22 Ohms between the op-amp output and the capacitor.On the other hand, the output capacitor should not be needed.

author: German_EE
author_rank: Super Contributor
timestamp: February 14, 2016, 08:39:03 pm
content: Right now you have one chip trying to do two jobs, and it's failing at both of them. Use a two amplifier solution where:Amplifier #1 is a buffer with a high input impedance and unity gain. Maybe use an Instrumentation amplifier.Amplifier #2 is an amplifier converting your 50 mV to 5VTry and avoid the use of high value resistors as these will destroy your noise figure. Also remember to keep your power supply rails well bypassed and use balanced lines wherever possible.

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 08:49:54 pm
content: Quote from: German_EE on February 14, 2016, 08:39:03 pmHiThe only thing I would add to that is:Ground the low side of the sensor.If you *must* use a single op amp, ground the sensor the other way and re-wire the amp for a positive gain. That way you have rational gain resistors and a nice high input impedance.Also consider that the sensor may have issues with bias current. You may have to select your op amp carefully.Bob

author: bson
author_rank: Unknown
timestamp: February 15, 2016, 10:14:07 pm
content: I'd start over from a basic, very simple non-inverting amplifier. Something along the lines of:See where that gets you.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 15, 2016, 10:19:24 pm
content: Quote from: bson on February 15, 2016, 10:14:07 pm^ thisNot to mention using an op-amp that is actually designed to run off of a 3.3v supply...