Opamps and offsets

Sorry forgot:

ore/less

In between 2 voltage followers. The resistors cannot affect each other. The offset can however.

I was looking at ELFA for better offsets but for the price LM324 is ok. I need to pay ~10 times more for something better, which might not be good enough.

The resistors I have used are measured 10.00-10.01K, still it gets a differential input of 122mV to 124mV. Only 2mV but also 2%

Sonnich

I'm not sure I completely understand what you're trying to do, but if you need an inexpensive op amp with better offset than the LM324, the LM224A costs only a bit more and has 2mV max offset guaranteed.

Beware that some panel displays can not use it's (-) supply tied to the common of the input. If you have a display that is giving you a COM input and (-) pin for supply, chances are it will not work if you want to use the same power for the rest of your circuit..

Some displays are designed to operate from a isolated power source like a battery. They use Vground in them which divides the power source.

And then again, there are others that do work just fine and have a single common for input and power.

I have in the past used cheap displays that were configed that way by using a 555 timer in astable mode and generate + and - output via cap coupling (charge pump) at a high freq rate to drive both the + a - leads of the display. These of course were LCD types. The common was then able to be tied to the main source. of course, you can get pre-made DC-DC units that are small and cheap.

Jamie

more/less

Next to impossible to understand what you're talking about...you need to find out how to post a schematic.

s o

Excatly. I will go shopping soon for the "real one" I need, the current one is from my drawer. However, it can be useful in the future.

Yep. Still I like the challenge in not doing that

My schematic:

depening on the need, 2 resistors as a divider directly on the panel meter (mine is prepared for that) is an option. The idea is to increase the voltage over R2 (from 1/100 to 1/10) by that minimising the affect of the offset. And using an LM224. Next the differntial amplifier was the biggist of my problems. I will soon see how it works.

WBR Sonnich

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Looks like you have pins 8, 9 and 10 on U13 miswired, and it would be helpful if you put the ohm value of all those resistors on the schematic. The panel meter may already have a high input impedance instrumentation amplifier on its front end, which means you are ruining everything with your circuit. To get a 1:10 divider use a R:9R resistor chain to GND, something like 10K in series with 91K should be adequate. You can make a 91K by paralleling a 100K and 1Meg.

I maybe wrong but it looks like U13 inputs are backwards? That there looks like a nice hysteresis switch! Or you have the network in backwards? Or both!

Jamie

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I corrected the schematic, there were some mistakes.

Anyway, the idea I had was to change the R10-12 from 1/100 to 1/10 in order to avoid the offset problem.

Then divide by 100 just before the panel meter if needed by use of the final op-amp.

The opamp not in use is grounded, it is just not so on the schematic as it is not important for the functionality of the rest.

WBR Sonnich

Have you seen the self powered 8 to 30 VDC panel meters:

--
You can't fix stupid. You can't even put a Band-Aid? on it, because it's
Teflon coated.

Maybe so, but you still have U13 inverting the signal as Jamie says.

What voltage do you want at "Hi in" for the conditions (voltage) shown on your schematic?

John

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Have you used these meters? Any good?

Grant.

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Either 1/10 or better 1/100 of the power.

I just changed it so the input resistors are 1/10, hence my differential input is 1/10 of power. When testing @15.000V I got

1.505V before the voltage followers and 1.507 after. This is ok.

For testing I took the meter off and used the last opamp to give me a virtual analog ground (2 4K7 resistors, voltage follower and a 10 nF condensator on the output to 0V). Using this analog ground I still get

1.468V at the differential output, even that my input is 1.507. That is 2.7% off, which I am not sure off - seems a bit too much, but 2 resistors of 1% =3D 2% error.

I wonder whether I should accept that and add a variable resistor for Rf and Rg (they have to be the same)

Sonnich

According to your schematic, the voltage at U1.3 pin 5 is 10.05V when the +12 V supply is at 15V.

The voltage at U1.2 pin 8 is 9.9V when the +12V supply is at 15V.

This is a difference of 0.15 V.

The difference is 1/100 of the +12V (actually +15V) supply voltage.

Not 1/10 of +12V (+15V), but 1/100 of +12V (+15V).

Please revise your schematic show the actual circuit.

Also, I must assume that R1B is connected to U1.2 pin 8 and R2B is connected to U1.3 pin 7. Otherwise, your data cannot be reconciled with the schematic.

Don't forget the offset error of U1.1.

That may not help your situation. You must reconcile schematic vs data first.

John

Yep, that was so, but I wrote here that I would change it. So I did. The schematic was subject to change as it did not work. I found the problems today:

1) got a new LM224 to replace my "from the drawer" LM324 from 1989. That got me back to 1,492V when I have 15V supply. 2) I tried without the panel meter, but using 2 resistors and the last opamp to give me ground - I get 1,5V. With 3 digits there will be no problem.

The new schematic is here:

As I wrote, I tried it that way.

Now done The new schematic is here:

You are right. It was a mistake on the schematic.

The new LM224 fixed that, also I have now 1,5V instead of 150mV, by that 10 times less sensible.

Final comment:

It is working now, a new LM224 was the main problem. Now I added the thermometer too, and I have what I want. I havent got the LM335 yet as it is in backorder, but I expect that to work :)

Also, replacing my panel meter with a new one will be a good thing, mine is as old as the LM324 ;) As you may have figured, I havent done this kind of work for some time...

WBR & thanks Sonnich