Author Topic: SS OPA buffer, how to handle output voltage swing?  (Read 1210 times)

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Offline NalutoTopic starter

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SS OPA buffer, how to handle output voltage swing?
« on: July 25, 2018, 05:18:40 pm »
Hello,
I'd like to use a voltage divider in combination with an OpAmp as voltage follower / unity gain buffer turn an external 0...5V DC signal into a 0...2.5V signal and feed it into a 12 bit ADC (AD7927, Vref=2.5V). I planned on using a single 3.3V supply for the OpAmp and ADC. (5V USB > Buck > 3.3V)

5V       |
           R
           +----buffer-----adc_in
           R
GND    |

I don't know what OpAmp I will use yet, but after reading a few datasheets (OPA376 for example) I noticed:

"At TA = 25°C, RL = 10 kΩ connected to VS / 2, VCM = VS / 2, and VOUT = VS / 2, unless otherwise noted."

VCM Common-mode voltage range: V--0.1V ... V++0.1V
Voltage output swing from rail: depending on the load up to 50mV


IIRC the CM voltage is the voltage that is present at both inputs at the same time, so if I'd use it as a differential amplifier and had a DC offset on the signal.

Question:
Why does it say that the load is connected to VS/2 (sometimes called mid-supply)? Does that mean I have to give the input an offset voltage to make it operate around that mid-supply?

And I read that when you exceed the output swing of the OPA, that you either get "non-linearities" or that the signal can even just clip off at some value.
What does that look like in practice when I input a voltage of 10mV to the OPA? Would it always output at least the lower voltage swing, so +20...50mV? Or could it output 10mV, but if I increase the input to 11mV it could output 12mV for example? (non-linearity?)

How are buffer circuits built regulary in applications? Do they use dual supply OPAs for that so they can get down to 0V? Or do you just live with the "dead zone" near 0V?


Last question: Do you happen to know a good book in english or german where I could read about practical considerations when using OPAs in general / important circuits and stuff like the things I just asked above?

If something about the questions is unclear, please let me know. Would be nice if someone could try to make me understand that.
Thank you.  :)
« Last Edit: July 25, 2018, 06:59:58 pm by Naluto »
 

Offline HB9EVI

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Re: SS OPA buffer, how to handle output voltage swing?
« Reply #1 on: July 25, 2018, 06:27:49 pm »
if you are talking about a 'buffer', you mean a unity gain amp, so an amplification of +1 - that means nothing else than 10mV on the input will produce 10mV on the output.

there are opamps which get very close to the voltage rails - they are called rail-to-rail opamps; one example for your propose could be the MCP6281, it's a rail to rail input and output opamp; it can reach VSS down to a few mV above on the output
 


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