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Topic: How to determine your amplification needs? (Read 999 times) previous topic - next topic

oscarcar

How should one go about trying to finding an op-amp & other amplification needs.

I know that's an awfully big question, but there's some simple questions that people can probably help me out with.

How does one calculate the "db" level needed to determine the gain needed for a sensor output to take advantage of the full range of an ADC?

For instance, let's say you have +-1mV, and you need to get it up to 0-5V.

You need x2500 gain. I think op-amps are typically rated as "db" gain, so how does that compare?

Is there a shorthand for such stuff?

I know in the example I gave that one would need to also need to center the voltage around 2.5V, and I believe there are chips to do that too.

westfw

op amps have extremely high inherent gain (an "ideal" op amp has infinite gain!); you add components to limit gain to where you want it.
It's the other parameters you have to look at:  power supply, whether the inputs are allowed to go to the limits that your signal does, whether the outputs go the limits of the A-D input, and so on.

So a "real" opamp might have an open-loop gain of 250000 and easily configured to a gain of 2500, but if powered from 0/5V the input signal  might need to be between 1.5 and 3.5 V, and the output may only travel between 1 and 4V...

Check out the wikipedia article.  It's a start: http://en.wikipedia.org/wiki/Operational_amplifier

Grumpy_Mike

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How does one calculate the "db" level

Take the ratio of input to output r = output voltage / input voltage
Then dBs = 20 Log(r)

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I think op-amps are typically rated as "db" gain,

No they are not.

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one would need to also need to center the voltage around 2.5V, and I believe there are chips to do that too.

Yes they are called op-amps.

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You need x2500 gain.

In practice this is very high and difficult to achieve due to the DC offset of the op amp. This is a small DC voltage that gets amplified at the same rate as the other signals and is dependent on the actual op-amp. It is normally in the mV range and so will swamp this signal.

oscarcar

Ok. I've learned a bit. Still a ways to go.

Along with wikipedia, I was reading this:

http://www2.electronicproducts.com/Selecting_op_amps-article-texas-may2004-html.aspx
& this:
http://www2.electronicproducts.com/Selecting_the_right_op_amp-article-facntexas_nov2008-html.aspx

It seems an instrumentation amplifier is a combined op-amp configuration that would be used for low level signals.

& the 20 x log(r). I assume that's natural log.

I'm sure I'll be back with more questions after I learn a bit more.


Grumpy_Mike

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I assume that's natural log

No to the base 10

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