Hi Trevor
If you insert an OpAmp configured as a voltage follower after your variable divider you will negate any effect it has on what you are trying to trouble shoot by giving you a low impedance source (about 75Ω). This will enable you to have a varying voltage for testing purposes without having to worry about what effect it may have.
Use a “rail to rail” OpAmp here such as the LMC6482 which is quite happy at 5V or 3.3V supply (or 15V if required).
Note: an “ordinary” audio type OPAmp WILL NOT DO THIS JOB as the output of such an amp will only swing from about 1 something V to about VCC minus about 1.5V.
In fact the LMC6482 is a dual unit so for an experiment you could insert the second one configured the same between your 2k/5k divider and the ADC pin to see if that improves accuracy and try to isolate what is causing this problem.
Cheers Bob
Note: The data sheet for this IC is pretty impressive. Normally a voltage follower configuration presents a high input impedance because of the 100% feed back. The LMC6482 (or the quad version the LMC6484) is listed as having an input resistance of 10TeraΩ which is way up there with the gods.
OK… looks like the LM358’s I bought for this project will be limited to my CT sensor… which we chatted about some 6 months ago! All that has been on the back burner whilst sorting out more pressing issues… like, trying to ensure we don’t run our bore dry in this bloody awful drought we are in! Which is where my pressure sensor comes into play…
I’m not that invested in milking the mouse on this… as I said, it works “well enough” as-is… provided I don’t have any serious misconfiguration that might cause problems elsewhere,
I know we did have some discussion a while back. If you are using LM358s as voltage follower configuration the LMC6482 would be a better choice anyway.
Understood. This must be the only country on the planet where you can have drought with bush fires, floods and snow all at the same time. Who would live anywhere else with such variety at hand.
Cheers Bob
Hi Trevor
Jaycar.
LMC6482 Stock code ZL3482
LMC6484 Stock code ZL3484
I don’t know where you live but I just checked around me and all between Mona Vale and Newcastle have them in stock.
Cheers Bob
Oh and just to head off any stability problems ground all unused INPUTS. As the inputs on this chip are extremely high resistance.
INPUTS only DO NOT ground any OUTPUTS.
EDIT:
Using these chips as voltage followers you should be able to return to your original divider resistor values. This will save you a bit of divider current.
Insert the voltage follower between the divider output and the ADC input.
Thanks. Meanwhile, I found some on AliExpress… 5 x quads for $6, free delivery, bundled with a few other odds 'n ends I need anyway. Jaycar want $6 for 1…
I live in Balhannah… Adelaide Hills. The local Mitre 10 is also a Jaycar agent… but would not have these in stock. My last couple orders from AliExpress have only taken a little over a week to arrive… they now have a warehouse in Chullora, NSW (?), so delivery times are quite reasonable.
Hi Trevor
Yes, Chullora is a Sydney suburb. But according to Ali Express web site this chip is not in Aus. Sold by SanFa Store whoever that is. This one is made by a company called Yxpc Electronic Co Ltd. Never heard of them but that is not surprising. The Chinese postage system has improved over the last year or so so postage might not be too bad.
Jaycar is a bit thin on the ground in SA isn’t it. The chips they sell might be TI. don’t remember now. I think through hole is getting to be a thing of the past these days. I notice TI now manufacture quite a range of this type of chip. All surface mount though and a couple I looked at had a max operating voltage of 7.5 and recommended V of 5.5 max.Good rail to rail performance though and an input resistance of 10^13Ω and input cap of 2pF.
Anyway as long as your chips are what they say they are I think you will find them very useful indeed.
Cheers Bob
FInally got around to hooking up the LMC6484, very simple circuit… voltage divider output going to V+, Vout linked to V- for a voltage follower config. Vcc is 3.3V on pin 4, pin 11 is ground (ie no -ve supply) 0.1 pF across power rails. All other V+ and V- inputs grounded as per your advice.
No matter what my input is (ranging from about 0.5V up to 2V) - I get ZERO on pin 1 - Vout.
Tried a second circuit (on the same chip) - same result.
I fear I may further reveal my great ignorrace re op amps…I also referred to this site, which shows something a little simpler than in the LMC6484 datasheet examples. “A little simpler” is an understatement … apart from linking the output to inverting input, there is nothing else!
Seems I have something basically very wrong… or maybe the IC is dead ??
That is a bit confusing. Vout should be linked to - or inverting input. It could be your terminology but if you did that you might have bricked the OC although I think it will tolerate shorts to ground.
There is nothing else required.
Very unusual. I would say EXTREMELY unusual.
Does not matter. The input current is extremely low at 0.02pA which is almost non existent so it will not affect operation. The inputs have (or should have, I did not find a schematic of the actual amplifier) diodes to clamp the inputs to 1 diode voltage drop from the + and - rails. This resistor would be to protect these diodes in the case the input went too far above or below the supply.
For this reason if this has any chance of happening it is not a bad idea to have about 10kΩ in series with the non inverting input. It will not affect anything as there is no current in this resistor unless the input voltage is exceeded.
Good you got it sorted. Might be an idea to quickly check the rest of those Chinese chips in case there are more causing confusion.
Cheers Bob
Just terminology… I did not connect Vout to the supply -ve! Vout to invertting Input Probably should have written that as Vin- rather than V-.
.
As for DOA, I guess the other possibility is I destroyed it, either by static or whatever. Anyway… all working fine now.
Thanks again for all your advice as I’ve worked through this project. It has grown from a fairly simple auto on/off switch into something far more sophisticated. I now have about 3,000 lines of Python code running it all, and I’m not done yet!
Today’s job is to recalibrate the pressure sensor with the op amp in place,
PS Just out of curiosity, I hooked up an LM358. On the bench, it looked like it was working OK, even dropping input voltage quite low. I did not do a thorough study, as it was by then about 1:AM, Anyway… I trust your advice, and reverted to the LMC6484 to be sure, to be sure!
Yes it will work OK within its limits which could well be short of the rails by a volt or so. So will not get to zero. You can get there by using a split supply. Like pos volts and neg supply which in your case would be inconvenient as it would probably be the only bit that requires a negative voltage supply.
Anyway you seem to be all going now. Good.
Cheers Bob
