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Design >> Mixed-Signal Design >> 3rd Year College Project (Need Help Please)
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Message started by james007on on Apr 3^rd, 2011, 4:05pm

Title: 3rd Year College Project (Need Help Please)
Post by james007on on Apr 3^rd, 2011, 4:05pm

I have been working away at this for practically a month and still have not gotten anywhere. My teachers cant be bothered to sit down and help me figure this out. It is itching at my skin. I am simply trying to bring in an Alternating Signal that is symmetrical about ground that could range anywhere between 30mVpp and 1Vpp and I would like to amplify this signal using an LM358n single supply opamp. I dont know what it is but, the inverting configuration I am using attenuates the signal, and the non-inverting configuration amplifies DC only. By DC of course I mean the DC offset I have applied to the op-amp input signal via the non-inverting input of the op amp.

Title: Re: 3rd Year College Project (Need Help Please)
Post by aaron_do on Apr 3^rd, 2011, 6:23pm

Hi,

What kind of resistor values are you using? Basically your op-amp must be able to drive R1 + R2. Otherwise your loop gain will be insufficient. You may consider using larger resistors. One more thing, your output DC level will be different from your input DC level unless you AC couple the input, which is to say add a large capacitor between your input source and the input resistor. Alternatively, you could set your input signal source to apply the same DC level as the non-inverting pin so that there is no DC input across the input pins.

Apart from that, its not easy to help without actually seeing your setup.

cheers,
Aaron

Title: Re: 3rd Year College Project (Need Help Please)
Post by james007on on Apr 3^rd, 2011, 7:39pm

R1 = R_input = 5.1K
R2 = R_feedback = 500K_{potentiometer}
Input is coupled with a small capacitor.

Also my circuit is very similar to that found in the Texas Instrument Application Note SLOA058- November 2000. Page 6 i believe and I have also created the VCC/2 buffer mentioned on more previous pages within the same application note.

Thank you for your response.

Title: Re: 3rd Year College Project (Need Help Please)
Post by aaron_do on Apr 3^rd, 2011, 10:39pm

Hi,

I'm not sure about the driving capability of the opamp, but the resistors seem large enough. I assume you took into account the frequency response? i.e. how small is your capacitor, and what frequency are you operating at? Perhaps it is not an issue with the design (could be a faulty connection etc.). Try probing all of your connections and check for shorts etc. As I said, its not easy to help without being there. You should probably ask a TA or something like that.

BTW if the input is AC coupled, then your output DC should equal to the non-inverting input DC, but your gain is pretty high (100 times) so make sure the DC conditions are correct before adding an AC source.

cheers,
Aaron

Title: Re: 3rd Year College Project (Need Help Please)
Post by james007on on Apr 4^th, 2011, 6:21am

When I was using the non-inverting configuration I was getting DC gain. And when I used the inverting configuration I got AC attenuation. The cap I am using at the input is a 4 pF cap and my frequency is anywhere from 100kHz~120kHz. I believe the datasheet for the LM358 says its good up to like 1MHz, but please feel free to double check this.

Once again I appreciate your timely responses.

Title: Re: 3rd Year College Project (Need Help Please)
Post by RobG on Apr 4^th, 2011, 6:56am

Can you draw the circuit? What frequency is the input?

Title: Re: 3rd Year College Project (Need Help Please)
Post by aaron_do on Apr 4^th, 2011, 7:28am

Hi,

So I might be misunderstanding your circuit a bit.

For the non-inverting configuration, you need to DC bias your input. Just make sure your input source is giving you a DC value (say 1/2 VCC) on top of the input AC signal. The feedback loop will take care of DC biasing your inverting input pin.

For the inverting configuration, you should apply the DC (1/2 VCC) to the non-inverting pin, and you could add a capacitor in series at the input (the inverting pin). Your cap value of 4 pF appears to be a bit small as you are looking at around 400 kohm at 100 kHz, so your AC gain is no more than 500k/400k = 1.2 (and that's only if you set the potentiometer to the max). If you want more gain, consider using a much larger capacitor (but not too large).

good luck,
Aaron

Title: Re: 3rd Year College Project (Need Help Please)
Post by RobG on Apr 4^th, 2011, 8:07am

james007on wrote on Apr 3^rd, 2011, 7:39pm:

You should link to the article. It is http://focus.ti.com/lit/an/sloa058/sloa058.pdf
I assume you are using the circuit on the top. The circuit is fine so you either didn't hook it up properly or one of the components is not working or sized properly. Are your opamp supplies hooked up correctly? Are the inputs to the correct pins? Check the DC biases at the opamp output and input and see if they make sense. Find the ones that don't make sense, find out why they don't make sense, and fix them ;D. I'm not trying to sound snarky, but you really need to learn how to do this. Your professors probably aren't too busy... this is something you need to figure out on your own -- and I bet you will remember where you screwed up! My favorite was when I didn't have the scope connected up.... I sure did have low noise, however.

rg

Title: Re: 3rd Year College Project (Need Help Please)
Post by james007on on Apr 5^th, 2011, 9:56am

I believe it must have something to do with that input capacitor because if your calculations are correct that appears to be roughly the gain I was getting. However My professors are all a lazy bunch and cannot be bothered to help me with this and have instructed me to 'just use a split supply op amp'. This frustrates me very much as this means I will require 2 power sources on what is supposed to be a portable pocket/key size 'FOB'. I hate my school. We really do need a fresh set of faculty... Dont you just hate 10year. However if I get a moment of free time over the next few days I will swap that cap out and put in a 100pF cap and let you's know how it goes.

Thank you aaron_do,

Title: Re: 3rd Year College Project (Need Help Please)
Post by loose-electron on Apr 8^th, 2011, 2:11pm

Hang on there!