Unexpected LTspice Simulation Results for an Instrumentation Amplifier Circuit - Page 1
Forum section: electronics / beginners

author: Smauel
author_rank: Contributor
timestamp: October 17, 2020, 12:48:14 am
content: In designing an amplifier circuit from opamps based on the common "instrumentation amplifier circuit" I kept finding on google, I got quite strange nonlinear results from an LTspice simulation.The circuit is intended to amplify the output of a strain gauge half-Wheatstone bridge.The majority of the output of the amplifier is linear, but the extremes are inexplicably floored and ceilinged to roughly 0.7 and 7.5 volts. Changing the resistor values and gain changes at what magnitude the floor and ceiling occurs, and also at what input resistance in the Wheatstone bridge it occurs.It seems like the ceiling first appears at the outputs and inverting inputs of the first two opamps, and then the floor seems to appear at the the inverting input of the final opamp.The nonlinearity is not present in the Wheatstone bridge outputs, and seems to appear in the feedback sections of the opamp circuits.Why does this nonlinearity exist? I can't imagine the technical origin for it, and I want to know what a proper solution would be for it. Is it simply that I should be using a single instrumentation amplifier IC instead of making one from individual opamps?

author: Cerebus
author_rank: Super Contributor
timestamp: October 17, 2020, 01:05:04 am
content: Try taking a look at the Op Amp datasheet under "output voltage swing" (noting the supply voltages given), convert that to offsets from the rails, then look at the value of your power rails and actual observed output voltage swing. It should soon become obvious what the problem is.

author: Smauel
author_rank: Contributor
timestamp: October 17, 2020, 01:47:04 am
content: Wow, that was quick.I suppose I just assumed that the output of an opamp could swing as wide as its power rails. I put an ideal opamp into the sim and it worked, not levelling off until it reached 0 and 9V like I expected.So, I just need to watch the output voltage swing of whatever opamp I choose for the circuit and make the rest of the system work within those parameters.Unfortunately, voltage swings don't seem to be given for the exact supply rail values I'm using, but I think thats what you meant by "convert that to offsets from the rails," so I think I know how to account for that now.Thank you very much, and I appreciate that you encouraged me to figure it out myself but pointed me in the right direction

author: Cerebus
author_rank: Super Contributor
timestamp: October 17, 2020, 02:57:53 am
content: Looks like you got it.And yes, ideal op amp models can screw you up. I've had simulations where they have happily wondered off to gigavolt outputs or delivered kiloamps.How close the output can get to the rails varies from op amp to op amp. Rule of thumb for 'high voltage' op amps (±15V typical supply and typically older amps) is on the order of 1 to 1.5V. Some do better, some a little worse. Some are awkward and have asymmetric limits, being different for the different rails. Newer, low voltage op amps (e.g. total supply in single digits volts) work hard to get that figure lower, perhaps 0.7V. There are "rail to rail" op amps (they'll almost certainly have that in the title on the datasheet) which can actually get the output to the rails, just, with light loads.The next thing to look out for is the graph that you'll almost always find later in the datasheet that plots output voltage swing against output load, either as a current or resistance. The heavier the load, the lower the total output voltage swing will be. It's quite easy to mess up the linearity more subtlety than just crashing into the output limits as you did if you don't keep a weather eye on these bits of the specification.Depending on the exact op amp design you can't let theinputget too close to the rails (Again, there are "rail to rail" designs). So watch out in the data sheet for the maximum "input common mode range" or sometimes just "input range" (not the same thing, but they are often co-incident). Common mode here being the voltage that you're applyingin commonto both inputs simultaneously. Really old designs can even flip over to the opposite polarity output if you exceed the input common mode range (called "output phase inversion" or "output polarity inversion"). Just to round the set off, there's also a maximumdifferentialinput, what the voltage difference between the + and - inputs can be, which can be quite small - less of a bother as you'll normally keep it in the right area with feedback.