Where to "inject" dcv in op amp buffer to elevate/offset/bias output 2.5v - Page 1
Forum section: electronics / beginners

author: cape zoloh
author_rank: Contributor
timestamp: July 12, 2020, 03:10:03 pm
content: First off, this is a single ended input 24 bit resolution ADC, so the solution must be very accurate/low drift.Doing so as accurate as possible, described below. If there are solutions providing better precision without being super complex, I'd like to hear it.So,The ADC can take 0-5vdc at its input. I would like it to take -2.5 to +2.5v.I'm having a op amp buffer to drive the ADC input. The reference voltage is 5.0vdcCan I somehow elevate the voltage at the ADC input to +2.5v. This way the signal can swing +/-2.5v (0-5v with elevation) without any damage to the adc.Divide the reference voltage and use that should be natural. I just don't know where or how to do it.In short, I would like a circuit that works like this:voltage out = voltage + vref/2Hope this makes sense.Attached a thing (2 different circuits, not that the inverting and non-inverting inputs are different on the two circuits) I was playing with (link to interact with it here:http://tinyurl.com/y7fmft2mBoth circuits gives inverted results. I want input=0 to give output=2.5v and input=+2.5v to give output=5.0 and input=-2.5v to give output=0 with the circuits gives the opposite.Don't even know if these circuits are good for the job.

author: JesseLackey
author_rank: Newbie
timestamp: July 12, 2020, 03:29:33 pm
content: Hi, this is a common problem ... look for examples from precision A/D chip datasheets and appnotes, such as:https://statics.cirrus.com/pubs/appNote/an241-1.pdfsection 3.3https://statics.cirrus.com/pubs/proDatasheet/CS5381_F2.pdfsection 3.4 is simplerNote that you may need a lowpass filter as well, and some of these circuits provide that as well.Good luck!J

author: cape zoloh
author_rank: Contributor
timestamp: July 12, 2020, 05:51:13 pm
content: I know everything I need, its just this small and "simple" basic part. I've seen it done in circuits before. The circuit I provided is a very simplified one, to take errellevant stuff out. Excuse me bad spelling.Now I can't find them. Your links didn't give me much more info regarding my specific problem IMHO.Maybe the LTC2400 paper, but that one requires a LTC1043, which I would rather ignore til later (it might be the best solution, who knows) (I've actually read this paper over and over again three too many times)Now I just want to know how to bias a 1vdc circuit into 2vdc for example. And from there I'll come up with something that satisfies all needs in the circuit.If you think I missed something, by all means - please let me know

author: Bud
author_rank: Super Contributor
timestamp: July 12, 2020, 06:12:29 pm
content: Quote from: cape zoloh on July 12, 2020, 03:10:03 pmWhat ?

author: cape zoloh
author_rank: Contributor
timestamp: July 12, 2020, 06:15:37 pm
content: Quote from: Bud on July 12, 2020, 06:12:29 pmIts wrong!What I meant was I want a offset of +2.5.Which means: Input=+2.5v gives output=+5.0vAnd the other: Input= -2.5v gives output=0vThanks for noticing, I'll have to change the other post...Maybe I've made this simple matter into a mess. Let me know if I have.I saw an elegant solution a couple of weeks ago where they took the half vref voltage and connected into the feedback loop of the op amp somehow.

author: Ian.M
author_rank: Super Contributor
timestamp: July 12, 2020, 07:47:54 pm
content: Precision, non-inverting and positive reference makes it difficult (but not impossible) to do in a single stage. Start with the classic differential amplifier circuit. Make it unit gain (all resistors equal). Ground its negative input. To sum in the offset replace the resistor between OPAMP in+ and Gnd with a potential divider between Vref and Gnd using four resistors identical to the ones round the OPAMP to get Vref/2 with a Thévenin equivalent resistance equal to the value of the other resistors.For accurate results you need a true RRIO OPAMP and extremely well matched resistors. Also note that the Vref supply impedance must be negligable (less than the resistor tolerance in absolute ohms) and that the signal source 'sees' the input as a pullup to Vref/2 with an impedance of 2x Rin.LTspice sim attached.

author: Bud
author_rank: Super Contributor
timestamp: July 12, 2020, 08:00:14 pm
content: Is the goal to use standard/jelly bean ICs? There are ADC driver chips with a dedicated Common Mode pin which you can bias at Vref/2.

author: cape zoloh
author_rank: Contributor
timestamp: July 14, 2020, 03:34:30 am
content: These are for differential input ADC right? The one I use is single ended. But please, if here are such devices for single ended input ADCs, I would like to know what they are. Thanks!Quote from: Bud on July 12, 2020, 08:00:14 pm