OPAmps: How to calculate Vicr & Vid violations - Page 1
Forum section: electronics / beginners

author: hitech95
author_rank: Regular Contributor
timestamp: August 16, 2020, 11:07:00 am
content: Hi,Short Version:I have a design that I would create, on simulation works fine but with real op amps it have issues.After a lot of research i come up with the possible issue:Differential input voltageCommon-mode input voltage rangeAt school I only designed with ideal OPamps and I have no idea how to see if my design is in the limit of a real op amp.Any help on how to see if my circuit will destroy an OP AMP??Long Version:I've used to study op-amps back in school, it has been lot of years and some concepts are gone.I have done mostly math and ideal cases with them, real cases were quite simple:I used to just use dual 15V supply for simple 5Vpp circuits on LAB at school.I hate my electronic knowledge from school it is mostly useless on real world.Anyway, I'm tring to read voltages of 2 cells in series.The ground reference is the negative rail of the lower cell.To read the upper side cell I need an diff. configuration.My gain wold have to be less than 1. (around 0.725 due to the ADC range)So a straight not inverting amp is a no go. The op amps will be connected like so:My initial idea was to use the Vbat (series of two cells) to supply the op amps but I need a wide supply op amp.I would like to use something cheap in a sot23 package (like the LMV321). Most of thoose have an max 5V as supply voltage.So my plan ended with a rail to rail op amp supplied only by the bottom cell.Now the question is:Is there a limitation about the input voltages of thoose op amps? For example if the difference of V+ and V- and Vout is less than Vcc of the op amp will it work fine?What are the Common Mode voltage ratio and differential mode voltage? I've gried to understand thoose but I got nothing out.Does someone have a link about this or have an simple explaination?For reference this is the whole concept/idea:Thanks and have a nice day!EDIT, some more SPICE tests I did:1mA used by the op amps resitor network could be a problem... I have to reduce that someway.

author: hitech95
author_rank: Regular Contributor
timestamp: August 18, 2020, 04:48:39 pm
content: So, according to spice simulations looks like that a diff op amp configuration can be used for my needs, and this is good.But the issue with the inputs limits is still in my head, and I cannot figure it out.After some more reading I understand even less that before! Doh!What I've got is that v1 - v2 must below the VID, now its is a differential configuration with close loop so the two pins should be at the same potential.This means that at the beginning (on power on) the max differential voltage is 1.8V and it is below the VID. And then after some time with the feedback both input will be at the same potential.Where VID= (V+) -(V-) + 0.2 = 3.2-0+0.2=3.4VThen I get that (V1+V2)/2 must be below the VICR, this means that I still have 1.8V and it have to be below the VCC-0.8V value. My supply in worst case is 3.2V, so VICR= 3.2-0.8=2.4VThis is only for the case of the bottom side amplifier.Have I said something wrong, Am I completly off?The resistor network current is a bit too hi, so i would at least double the values.School told us to use resistor values around 10k and not too big (no idea why).What if I have to turn off the whole section of the circuit? (Save Power by enbling it only when needed)

author: hitech95
author_rank: Regular Contributor
timestamp: August 21, 2020, 11:02:43 am
content: Up?I'm tring my beast but I cannot find an pratical example of this online!

author: magic
author_rank: Super Contributor
timestamp: August 22, 2020, 07:22:31 am
content: That's quite long and I'm not sure if I have read everything, butCMIR - the range of ground-referenced voltages that the inputs are allowed to use. For an old opamp rated for dual ±15V supply it might be -14V to +13V or similar, for a modern rail to rail chip rated as 5V single supply it might be -0.1V to 5.1V. If this limit is exceeded the opamp may work incorrectly and if the absolute maximum rating is exceeded it may blow up.Differential input - the difference between IN+ and IN-. Generally it's zero (or a few mV - the offset voltage) when the opamp is operating, but it can be higher when the circuit doesn't function well. Some opamps will blow up if it's too much and then an absolute maximum rating is specified.

author: hitech95
author_rank: Regular Contributor
timestamp: August 22, 2020, 12:57:52 pm
content: Quote from: magic on August 22, 2020, 07:22:31 amOk but how can I see if my design is not violating thoose values?Just take a look to the schematic on first post, and consider that both op amps are powered by the low side generator.The voltage in the input pins of the upper op amp goes out the supply range of the op amp (0-3.2V considering that the two generators are batteries), but the difference of the two ingress is near zero.So I think that in my case CMIR is violated but VID is not.

author: magic
author_rank: Super Contributor
timestamp: August 23, 2020, 08:33:21 am
content: For differential voltage, I would say just forget it and get a chip which doesn't have such limit. It's mostly fancy, high performance opamps which have problems with Vid.IN- will be matched to IN+ by the opamp. So you need to find out if IN+ is within the allowed range. This is 3.6V on your fist schematic so you will need an opamp with more than 3.2V CMIR. Practically, rail to rail input would be best. Beware that additional problems may occur if the lower battery is discharged more than the upper one.I think TS27L2 / TLC27L2 would be okay here, powered from two cells in series. But it's a dual in SO8 rather than SOT23, dunno if that's a problem.BTW, those resistors are going to be discharging the batteries.

author: hitech95
author_rank: Regular Contributor
timestamp: August 23, 2020, 02:36:10 pm
content: Quote from: magic on August 23, 2020, 08:33:21 amYes the problem with this solution is that the second cell is not required, so I can have only a cell populated.So my idea was to use a zener to clamp the voltage of the second op amp to 4.5V and solve all my problems.Second op amp have senso only when second cell is connected, so it is powered by the cell in series.Quote from: magic on August 23, 2020, 08:33:21 amYes I know, I will have to increse the resistor values to decrease the current or find a way to disconnect the network when I don't need to read the cell voltage.