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Can't get a TLC272IP OP-AMP to work. what am I doing wrong?

Hello

I received this week a few TLC272IP op-amp and I wanted to test them.

I created a circuit on a breadboard which looks like the attached image

From what I read on the net, this is a  typical non inverting amplifier schema where gain should be (1 + R2/R1)

I connected PIN 3 of the TLC272 to pin 3 of my arduino uno and using PWM I set the output to roughly 1V ("analogWrite(3, 51);")


I used my voltmeter to read the voltage between ground and pin 3 and I could really see 1V 
given I used R1 = R2 = 47k, I was expecting to have a gain of x2 and thus read voltage of 2V between pin 1 of the op-amp and ground 

I get something that is very unstable, below 1V.

I tried with 4 different TLC272IP I just got and get the same behavior.

am I doing something wrong there? what do I miss? or do I need to review how op-amp are really working?

(I purchased the TLC272IP because it's supposedly single power supply)

thanks for any hint.

JM

PS. in some doc i read that I should not let the second op-amp in the air, so i tried as well by connecting pin 5 to ground and shorting 6 and 7 with a wire or a small 100k resistor but this did not change anything.

Picture of Can't get a TLC272IP OP-AMP to work. what am I doing wrong?
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verence2 years ago

a) check your circuit again. That is not a non inverting amplifier what you have.

b) ceck against a data sheet like this

c) you can not set a PWM output to 1V. You may set it to 20% duty ratio which - for 5V and a slow multimeter - will read like 1V. The OPAMP will amplify the digital, fast switching signal.

d) if you really want to convert the PWM into an analog voltage, you need an R-C filter

e) it doesn't make sense to 'amplify' a 5V PWM signal with a amplifier powered with 5V. Unless you use a unity-gain amplifier to drive a low resistance load. You can't get a voltage higher than 5V out of the amplifier In fact, depending on the OPAMP you may get only some volts below the supply voltages out. So, it may work for your 1V example, but then you could just multiply the PWM-setting inside the µC by 2.

jm-l (author)  verence2 years ago

Hi thanks for taking the time to comment on my question.

for point a), I'll double check against the literature, to be honest I'm just getting started in electronics and it is not my domain of expertise (nor is english my first language) so I had a quick look in wikipedia and I thought I had used the proper wiring (cf image attached from wikipedia http://en.wikipedia.org/wiki/Operational_amplifier... )

as the three of you answering made that note, I guess I'll dig a bit more into this.

b) yes I did check the data sheet but not in great details, mainly just to have confirmation of the pinout, max voltage and slew rate (if I got that concept right) and the part related to operations at 5V. To be frank tons of the terms in there are confusing to me - I still have tons to learn.

c) I do get that part and the duty ratio. My hypothesis (probably wrong) about this was that if my slow multimeter was seeing 1V as input, and if my basic understanding of op-amp were right that the output of the op-amp wired as such would mimic the input, then I should see a stable output with a gain compared to my input... guess I'm wrong on that one...

d) OK indeed now I get this part. my PWM does send 5V into the circuit so indeed this is probably very stupid of me to expect something different in the output if I power the op-amp with 5V.... :-)

I'll have a deeper look at this tonight, try to find what is wrong in my wiring (I'm still not sure I get that) and try with powering my op-amp with 12V and sending in a true 1V signal from another source than my PWM or try the RC filter thing.

I'd appreciate if you could point me to why my wiring is wrong - but I like the idea of not being given the answer and having to investigate - appreciate the time you took to answer.

Op-Amp_Non-Inverting_Amplifier.jpg
jm-l (author) 2 years ago

I think I see my mistake with the wiring. wikipedia description are for split supply (Vdd+ and Vdd- going into the op-amp) and I'm using a single-supply one.

I'm reading http://www.ti.com/lit/an/sloa030a/sloa030a.pdf to learn more about this..

did I get that right?

The problem is your trying to use a PWM signal from the Arduino to give you a 1V output. Have you read the data sheet on this and walked through the supplied schematics and suggestions there for testing the op-amps?

jm-l (author)  mpilchfamily2 years ago

thanks for taking the time to answer.

Assuming you actually wire it as a non-inverting amp, which as Verence points out, it isn't, try to put an RC filter between the "PWM"pin and pin 3 on the opamp. Make the R about 100K, , make the C about 2uF. This will give you a BUFFERED output voltage, with a low output impedance, which is great for driving external circuits.

jm-l (author)  steveastrouk2 years ago

thanks for the answer. I'll double check what I did wrong on the wiring of a non-inverting amp.