1. Home
  2. Electronics Design
  3. Question about opa350 datasheet

Question about opa350 datasheet

I am looking at an analog opamp amplifier for a digital scope and I was wondering if someone knows how this opamp circuit works. I have captured the section in question, of course there are more stages, but this is the one which is the most interesting for me.

The circuit goes like this:

there is an op amp with a negative feedback loop through a resistor Rf

then, the minus input is also connected to a resistor divider which is connected from the positive 5v supply to ground and both resistors are 1.8k, so the divider is 2.5v.

the input goes into the plus input and the output is the output of the opamp

Thanks

Reply to
joakent
Loading thread data ...

Trivial - centres any amplifiction around 2.5V. Repost on sci.electronics.basics nd see if you can find anybody with the patience and spare time to provide chapter and verse.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

(5-V)/R - V/R + (Vo-V)/R =0 this simplifies to Vo = 3V - 5 I can't see that this is amplifying around 2.5Volts ? ??

Reply to
joakent

on

Pity about that. What you are describing is a follower-with-gain.

If you apply 2.5V to the non-inverting input of the op amp, you get

2.5V out - independent of the gain. As you move non-inverting input away from 2.5V the output moves further away from 2.5V in the same sense, to an extent that depends on the gain.

Happy?

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

In addition to what Bill already told you, consider that the divider between +5 and gnd, using two equal-valued resistors, is equivalent to a 5V/2 (=2.5V) supply connected through an R/2 resistance. Then the gain, just like other non-inverting op amp configurations, is (practically) 1+Rf/(R/2). If R=Rf, then the gain is 1+2Rf/Rf = 3. The

2.5V Thevenin equivalent of the divider just sets a bias, and it should be clear that if the input to the (+) terminal is 2.5V, and feedback causes the (-) input to follow the (+) input so it's at (practically) 2.5V also. Thus there is no current in the Thevenin-equivalent R/2 resistance, and no current in Rf, and Vout = 2.5. So, Vout = (1+2*Rf/R)*(Vin-2.5V)+2.5V. Sound like what Bill wrote? Hope so. Sound like what you wrote for the Rf=R case: Vo = 3*Vin - 5? Hope so. Note that yours can be re-written as (Vo-2.5) = 3*(Vin-2.5). Maybe then the "amplifies around 2.5V" will be more obvious.

Cheers, Tom

snipped-for-privacy@bt> > snipped-for-privacy@bt> > > I am looking at an analog opamp amplifier for a digital

on

Reply to
Tom Bruhns

Is it also an adder? i.e. if instead of 2.5V from the divider we apply any voltage, how will that affect the formula?

...and what if after the divider (or thevenine equivalent) we have a 60k resistor (which goes to the opamp minus) ?

Tom Bruhns wrote:

Repost on

Reply to
joakent

Don't top post.

Ask your teacher.

The 60k resistor should only be carrying the bias current into the input, so won't actually do anything to your signal.

If the input capacitance of the op amp you are using the usual couple of picofrarads to ground, the 60k will insert a couple of hundred nanoseconds of RC lag into the feeback loop, which might impair the stability of the feedback loop.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

See another poster arrived at (1+2*Rf/R)*(Vin-2.5V)+2.5V

Is this a follower? Is this an amplifeir?

I can't imagine why that would be, considering I am describing to you an amplifier from a real commercial product, (of prefessional quality)

Reply to
joakent

It is quite possible that I'm not understanding where your 60k resistor is, but real commercial audio products of "professional" quality aren't always well-designed - my first boss at Cambridge Instruments had a pair of Quad electrostatic speakersm which were a difficult load to drive, and his hi-fi shop let him try out a number of up-market audio amplifiers.

In general, the quality of the electronic design went down as the price went up.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

If you believe that you must be some sort of moron, you get what you pay for, unless someone is ripping you off. Would you like to see the diagram, and then answer my question, you were a little rude before, so I wonder if I should trust you.

Reply to
joakent

Amplifiers that are produced in high volume get designed and redesigned until they are as good as possible and as cheap as possible. As you go up-market, the production volume goes down, and the the design costs get amortised over progressively fewer systems.

People like NAD and Sony do good design. As you move up-market from there you rapidly move into the region where the total sales can't support good design, and the business is devoted to separating the gullible from their money. I had a subscription to HIFi News and Record Review for about twenty years - they printed a couple of my letters to the editor in the 1980's - but I stopped reading them when they stopped paying for technical evaluations of the equipment they were reviewing, sometime after Peter Walker of Quad stopped sending them equipment to review because they made such as hash of the job.

Send me the diagram - my e-mail address is real (bill dot sloman at ieee dot org) - and I'll send you a comment.

You can use Google Groups to find out whether you can trust me or not - a search on "Bill Sloman" finds some 15,800 messages, going back to the

23rd July 1996.

You aren't alone in finding me rude - John Larkin complains about my rudeness whenever he loses an argument. You'll have to put up with it if you want the advice.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.