What is a good way to shift a voltage by say +1V? - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: November 11, 2016, 05:42:40 pm
content: I am asking this in the context of current monitoring. Say I have a low side shunt that produces small voltages from Gnd (0V) to say +0.1V across the shunt. I want to shift these by for example +1V (to the range 1.00V-1.01V) for use as an input into an op-amp circuit, so as not to touch or come close to the negative input rail of the amp (which has a 0V to +5V range, because it will be single supplied).I looked at op-amp summing circuits, but they do not isolate the low level input from the +1V being added very much (i.e., resistors between them that can cause the +1V being added to feed back into the small signal [being added]) and I want the low level input not to see (i.e., be affected) by the +1V being added to it.Any ideas? I hope my explanation was clear. If not, please ask questions.

author: danadak
author_rank: Super Contributor
timestamp: November 11, 2016, 06:09:19 pm
content: A summer, with the proper selection of OpAmp, operates intoa virtual ground, so there is virtually no coupling. This is truewhen the signals of interest is << GBW of the OpAmp.If I recall correctly the V- input of an inverting summmer, the V+being grounded, v- ~= Vin * ( G / Aol ), DC case. So if Vin is 1V, Aolof OpAmp is ~ 1,000,000, then V- ~= 1 uV for a G = 1 summer.That in turn means there is virtually no coupling between the inputsto the summer, no 'crosstalk". G is resistor ratio or closed loop gain,Aol is open loop gain of OpAmp at DC.Do the analysis, use LaPlace, single pole opamp, and you canlook at it out in frequency as well. Analysis pretty simple for asingle pole case.As you raise input freq or take more G in summer that virtual groundstarts increasing in voltage, loosing some of its "groundness". A goodway of looking at that is do a signal flow graph for the summer, andyou can walk yourself around the control loop and evaluate mentallyeffects of G changes on node voltage.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2016, 10:05:00 am
content: If you can shift it by some other close voltage, then a 1.25 volt bandgap shunt reference can be used.The method I like is to use a constant current through a resistor to create a stable level shift into the non-inverting input. Before rail-to-rail input operational amplifiers became ubiquitous, this was commonly done when common mode input range was a problem. It still has advantages when the relatively poor CMRR (common mode rejection ratio) of rail-to-rail input operational amplifiers becomes a problem.I do not quite understand your objection to using a single supply operational amplifier down to 0 volts.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 12, 2016, 10:38:00 am
content: I'm with @David,What's the problem with a summed opamp? , I use these all the time for current measurements with grounded shunts, just feed in Vref from somewhere with a high value resistor, it will only put microamps across the shunt. Even a sloppy circuit with a 1ohm shunt and 1k summing resistors shifts the measured current by 0.1% , negligible with 1% resistors in use.There is an interesting IC that can do almost what you want: the AD8420, check it out. It has two sets of inputs, it will drive the output until both input pairs have the same voltage, you can make all sorts of interesting circuits like Howland current pumps. And it does this neat trick of being able to add a voltage to a signal , while measuring that signal at high impedance.There are also "difference amps" , (Analog Devices make them) some will work below the negative rail, these devices typically have ~ 50k input resistance on both terminals, get one with a ref terminal.Instrumentation amplifiers can also shift a signal by 1v if you tie the reference pin to 1v, however you will need split supplies for an INA.

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2016, 11:06:11 am
content: For the record, my favorite way to implement current limit circuits is to use the constant current to create the reference or comparison voltage which follows the ground or supply current shunt. Then the single supply or rail-to-rail operational amplifier (or any operational amplifier with a bias supply) operates single ended and no instrumentation amplifier is needed. Note that the very old LM301A and some JFET operational amplifiers have an input common mode range which includes the positive supply making them useful for high side current monitoring. I have always wished for a series of single supply amplifiers that operate at the positive supply voltage but it never happened.So what I do is shift the reference instead of the signal from the current shunt. Many older bench power supplies use this technique for adjustable constant current limiting. I think modern designers have largely forgotten about it.I guess SharpEars wants to be able to monitor negative currents which would require the output of the amplifier to fall below the reference voltage. A single supply instrumentation amplifier may be the way to go for this as salbayeng suggests; the reference terminal is then used to add the offset.

author: rstofer
author_rank: Super Contributor
timestamp: November 12, 2016, 03:47:11 pm
content: All of this offset/scale stuff is discussed in section 4.3 of Op Amps For Everyonehttp://www.cypress.com/file/65366/downloadBasically, it is a summing amplifier biased at some convenient level. The examples use a single supply rail-to-rail op amp. Still, it's best to stay out of the corners.

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2016, 07:00:41 pm
content: Quote from: rstofer on November 12, 2016, 03:47:11 pmThis example is a 4 resistor instrumentation amplifier drawn slightly differently.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 20, 2016, 06:07:06 am
content: Quote from: David Hess on November 12, 2016, 07:00:41 pmI think you meant "differential" amplifier , the 4 resistor variant with two Rg and two Rf.With sufficiently low impedances and careful trimming, it functions like an INA.That's a good TI textbook reference, BTW.For a low budget, single ended, bidirectional ground rail sensing I've used BCV61 current mirrors with the shunt between the emitter legs: 20c (BCV61 + 10k +5k +2x100R) buys you a gain of 100 with 2.5v zero voltage, good enough for current limiting applications. (see schematic below) A small piece of PCB track 100mil x 800mil long is used as the shunt (orange in the schematic)The drift was awful with that circuit, switched to MCP6071, (and this circuit actually is offset by 1v just like the OP asked for) This is the standard circuit that most people use. (see below)For the latest model of that project, I don't do either of the above, but actually measure the on voltage of the MOSFET, which after all, is the part we are trying to protect.

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2016, 07:11:56 am
content: Quote from: salbayeng on November 20, 2016, 06:07:06 amNope, I mean instrumentation amplifier (or a difference) amplifier. It lacks differential outputs and if it is called a "differential amplifier", then what would the differential amplifier made up of 2 operational amplifier and 3 resistors with differential outputs be called? Figure A-17 shows this if you remove the instrumentation amplifier made from the third operational amplifier.Instrumentation or difference amplifiers make fine level shifters within their limitations.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 20, 2016, 09:07:46 am
content: Some confusion here,All the figures in "section 4.3" (that Rstofer refers to) are basic single op-ampsHowever in "section A.4.3" (not the one that Rstofer refers to) you can see figure A-26 is a messy summing circuit,Start looking around here and you will find figure A-17And yes figure A-17 is definately an INA with the 3 opamp / single gain resistor configuration.I guess you landed here.The link is actually two books in one, the appendices make up 2/3 of the book!!!?it jumps around a lot!But a very good book, it explains things much better than many textbooks on my shelves.

author: Brutte
author_rank: Frequent Contributor
timestamp: November 20, 2016, 11:41:13 am
content: Quote from: SharpEars on November 11, 2016, 05:42:40 pmUse a raw differential configuration amplifier (four identical resistors). Just tie reference to your Uref=+1V and the output would be:Uout= (Ub-Ua) + Urefwhere Ub and Ua are voltages on both sides of the shunt.

author: Kalvin
author_rank: Super Contributor
timestamp: November 20, 2016, 01:46:47 pm
content: Take one op amp stage and use it as a unity gain buffer, so the Vout = Vin. Take another op amp or whatever you prefer and make a 100uA current source. Add a 10kohm resistor between the unity gain op amp output Vout and the output of the current source. You will get Vin+1V from the positive side of the 10kohm resistor as the 100uA constant current will create a constant voltage of 1V across the resistor, and the resistor will follow the Vout.

author: void_error
author_rank: Frequent Contributor
timestamp: November 20, 2016, 01:55:10 pm
content: You might find this useful.http://earmark.net/gesr/opamp/gain_offset.htm

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 22, 2016, 10:03:52 am
content: Wow someone went to the trouble to make that calculator. Soon no-one will be able to read a text book.I find myself typing things like "50hm attenuator calculator" or "dBm to voltage" as I can get an answer in less time than taking two steps to the bookshelf!

author: void_error
author_rank: Frequent Contributor
timestamp: November 22, 2016, 03:27:32 pm
content: It's based onthisapp note. Took me a while to find the calculator when I first needed it but it's much quicker than doing it by hand. If I don't find an online calculator I make a spreadsheet.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 27, 2016, 10:15:58 pm
content: Yup spreadsheets are good. I use them to figure out which pair of standard values give the desired ratio (e.g. for op-amp gain, regulator feedback, adding in offsets, dividers). I try to use just 1, 2.2 and 4.7 for resistors , and multiples of 1 for capacitors on the first pass, then pairs of these on second pass, then throw in E6 series on the third pass, and using E12, E24 or E96 as a last resort .With a spreadsheet, you try 3 or more possible combinations, and check them at max and min operating levels.I also get aggressive with Muntzing as well, one circuit I designed had 5 different sorts of diodes and some 2n7002's, I replaced with 3 instances of a fast, 1000v rectifier and a dozen 2n7002's half of these were used as diodes (grounded gate), I made an active rectifier with a 2n7002 to replace a schottky diode, and it worked much better: 6cent item replaces a 20c item , and removes a line from the BoM!(Confused my assembly house, as they were loading transistors onto pads with ref des beginning with "D").

author: Lukas
author_rank: Frequent Contributor
timestamp: November 28, 2016, 01:27:00 pm
content: One PNP emitter follower does the job. If you need more than .7V, just use two of them in series.

author: Zero999
author_rank: Super Contributor
timestamp: November 28, 2016, 07:11:21 pm
content: Quote from: SharpEars on November 11, 2016, 05:42:40 pmWhich op-amp are you using?I doubt there's any need for level shifting. If the op-amp is specified to work down to 0V, then it should be able to amplify 0.1V from a shunt with no problem. There are even op-amps which will work with their inputs slightly below the negative rail, such as the MCP601, see link below. The main issue you may have is the offset voltage, but that can be easily compensated for if you're using an MCU.http://ww1.microchip.com/downloads/en/DeviceDoc/21314g.pdf

author: MarkL
author_rank: Unknown
timestamp: November 28, 2016, 09:22:59 pm
content: Why not look for an amp that's designed for current sensing? Here's one option that's 5V, input goes (way) beyond the rails, and also provides level shifting so you can do bi-directional sensing:https://datasheets.maximintegrated.com/en/ds/MAX9918-MAX9920.pdfThis particular amp has significant input bias which you'll need to determine is ok or not for your application.If you don't like this one, there's plenty of sense amp choices out there. This is not a unique problem.

author: salbayeng
author_rank: Frequent Contributor
timestamp: November 29, 2016, 04:33:33 am
content: Nice find MarkL,I'd seen a nearly identical item from AD , with a nearly identical H Bridge sensing application. An accessible ref input is helpful.