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scaling and offsetting analog output

scaling and offsetting analog output

(OP)
I have a 2.5V DC output signal ( its range is 0.5V to 4.5V), it needs to be scaled from 25% of supply to 75% of supply voltage. The supply voltage is a variable.Due to constraints i cannot use microprocessors or other fancy signal processing parts, need to stick with basic parts like opamps , resistors bridges ect. 0.5V output needs to be scaled to 25% of supply voltage and 4.5V output output needs to be scaled to 75% of supply voltage. Essentially the output needs to be ratiometric to supply voltage. Is there a way to do it with basic circuit like opamp etc?. If easier to explain please draw it if needed.Thank you.

RE: scaling and offsetting analog output

What's the range of the variable supply voltage?

Mike Halloran
Pembroke Pines, FL, USA

RE: scaling and offsetting analog output

(OP)
The supply is +8V. I can scale output to this particular supply voltage with opam but the problem is user wants to be independent of the supply voltage. They want to choose a desired supply voltage and the out put sinal (0.5v to 4.5V) needs to scale to it in the following way - 25% of supply=0.5V and 75% of supply =4.5V. If the supply was fixed it would have been easier to choose particular scaling and offset values for an opamp circuit.

RE: scaling and offsetting analog output

I understood that the 'user' wants to select an arbitrary supply voltage.
My question had to do with the range within which that voltage lies, because that affects what sort of circuit, and what sort of components, you need to specify.
Either you need to figure out the maximum voltage that the user is likely to request, or just take a WAG and make the range part of your contract, and maybe marked on your product.

I.e., things are different inside the black box if
Vssmax = +10V or 100V or 1000V or whatever.
You probably need to also specify Vssmin and/or provide reverse polarity protection.





Mike Halloran
Pembroke Pines, FL, USA

RE: scaling and offsetting analog output

(OP)
Mike Yes, sorry misunderstood your question. The supply can vary from 5V to 20V DC. The sensor output will always be between 0.5V to 4.5V but the supply can vary from 5V to 20V.

RE: scaling and offsetting analog output

Is this a onesie? This would be easy to do with a UNO which is cheap. I was posting the procedure but my finger hit something and deleted it all.

RE: scaling and offsetting analog output

(OP)
This is NOT onesie.

RE: scaling and offsetting analog output

(OP)
I cannot use an Arduino Uno, just looking for an analog way to scale the sensor signal which varies from 0.5V to 4.5V to 25% of supply to 75% of supply respectively. The supply can be chosen between 5V to 20V DC

RE: scaling and offsetting analog output

Thinking aloud, this sounds like an analogue multiplier job. Haven't quite figured the detail, but will sketch some ideas and maybe post them if no one beats me to it.

A bigger problem might be actually finding an analogue multiplier today! See if the old AD633 and AD532 are still available, maybe there are some newer alternatives at lower cost.

RE: scaling and offsetting analog output

(OP)
ScottyUK , When i map 0.5 to 4.5V input range to current required output range which is 0-8V i get a transfer function Vo=sVin + b where Vo= desired output, Vin is the sensor signal (0.5 to 4.5V) and s and b are the span and offset. For s= 2 and b=-1. I get the desired output. If there was an analog way to implement this equation, that would solve the problem, am looking at your suggestion about AD633 and seeing if it can do this

RE: scaling and offsetting analog output

Y=mX+b

Offset and scale.

Two op-amp conceptual building blocks combined.

Google: op amp offset and scale

Nice PDF file from TI should be top hit.

RE: scaling and offsetting analog output

Hi VE1BLL,

That's trivially simple if 'm' and 'b' are constants - in this case they're supply-dependent variables. That was the reason for using a multiplier. Of course I could be mis-interpreting the requirement completely and making this a whole lot more difficult than it needs to be!



RE: scaling and offsetting analog output

(OP)
VE1BLL and ScottyUK 'm' and 'b' are depedent on maximum supply voltage. My supply is chosen by user it can be between 5-20V. The only thing that does not change is the sensor output which is 0.5V to 4.5V. That needs to be mapped to 0.5V=25% of suppy and 4.5V=75% of supply. If supply was fixed , one can calculate slope m= Vout(max)-Vout(min)/(Vin(max)-Vin(min), if your supply changes then you have to change the gain resistors values of the opamp to map to new max supply voltage, thats the issue

RE: scaling and offsetting analog output

Sorry, I must have read too fast. "My bad."

RE: scaling and offsetting analog output

Well, if it must be analog use a TL494 or similar. I saw a DC power meter in Electronic Design maybe 30 years ago I thought waqs pretty neat. I think I searched a couple years ago and found it. It used current to control the PWM 0-100%. The output was tied to a pullup to power being monitored. A RC filter averaged it out. A simple volts times amps. The Arduino design was based on this. No reason you can't pop the chip out and make hundreds. No pots and easier for those who don't know circuit design. Anyway. if this little board has to operate down to a 5V supply that certainly makes it a lot dicier.

RE: scaling and offsetting analog output

I agree with ScottyUK; this really seems to involve multiplication. Log amps are one possible approach.

RE: scaling and offsetting analog output

Depending on the necessary bandwidth the approach that OperaHouse might work. Feed the signal in the PWM modulator with the low at 25% modulation and the high end at 75%. Buffer the output of the PWM to each supply rail, low pass and you have your output.

RE: scaling and offsetting analog output

Everyone says...... I want it simple, just a board with 30 or so parts.
I've designed analog my whole career. I used to love the idea of tweaking pots. I've grown up. Life is easier with a with a $2 microcontroller and about six parts.

RE: scaling and offsetting analog output

Another idea to throw into the mix: voltage to frequency conversion followed by frequency to voltage conversion. Similar to the PWM idea in practice I suppose, but one less parameter to deal with (only frequency, not duty). Maybe check out the LM231 datasheet for ideas. I haven't thought it through, but I figure you sensor input could drive a V-F converter directly, while the output would drive a NPN pulled up to your desired output voltage. Then you just need to do a F-V conversion, tune for offset, and presto!

RE: scaling and offsetting analog output

I think I've seen something like this in a load cell resistive bridge. Though in my case I took great pains to make the supply voltage as close to a steady 5.0 Volts as possible. Allowing the supply voltage to fluctuate makes the instrument amp and the analog-digital conversion more difficult to accomplish.

Would you be able to install a high-accuracy voltage regulator in the sensor, that allows it to accept input voltages up to 20V, while regulating the voltage across the sensor's element a constant 5V? It would look a little like the attachment below...

As for the other separate requirement, it took me a while to tease the two of them apart, given how they are related to one another. Does the sensor have to be referenced to the same ground as the supply voltage? Could you create a false ground for the purpose of this sensor? The reason I ask is that the upper and lower limits you stated for output (0.5v and 4.5v) aren't starting out a 25% and 75% of the 5V base supply. So the slope of the Y=Mx+B lines going from the supply voltage's 25% and 75% ratios aren't going to be the same, so you add a lot of complexity. Maybe you can fool the sensor by making it think that ground is some other voltage, causing the output to be the sum of the "high ground" voltage plus the lower scale sensor output voltage. And you choose that to equal 25% of the supply voltage.

Then all you need for the scaling with respect to the upper limit of the supply voltage is a resistive voltage divider with points at 25% and 75% to pick from. I think I'm over my head now, but maybe you can see the other side of the river from here.

STF

RE: scaling and offsetting analog output

Assuming that the sensor's output ALWAYS varies between 10% and 90% of the supply voltage, a basic resistance divider can be built like this:


o -- +Vss (in)
|
[] R1
|
o-----------------o Sensor Vss
|................ |
[] R2 `````````` [ ]o----------------o Sensor output
|```````````````` |
o-----------------o Sensor Gnd
|
[] R3
|
o -- Gnd (input)


(ASCII art is such fun, especially when you can't use more than one space at a time).

So pick R1, R2 and R3. R1 has to offset the Vss down so that the sensor's Vo can't be higher than 75% and R3 has to offset the GND up so that the sensor's Vo can't be lower than 25%. Since the natural range of Vo is between 10% and 90% of Vss, then R1 needs to offset Vss down enough that the Vo output ends up at 75% of Vss, and it seems like R3 needs the same value to do the reverse to Gnd. R2 needs a value to drop the intervening fraction of the Vss between them.

Example:
Vss = 15.0 Volts
Vo (max) = 75% * 15V = 11.25V
Vo (min) = 25% * 15V = 3.75V
Full swing = Vo (max) - Vo (min) = 11.25v - 3.75v = 7.5v


R1 = 187 ohm (Voltage drop is 2.8125 volts)
R2 = 750 ohm (Voltage drop is 9.375 volts)
R3 = 187 ohm (Voltage drop is 2.8125 volts)
total 1000 ohm (current through divider is 15 mA)

Sensor supply voltage is 9.375V
90% of 9.375V = 8.4375V
10% of 9.375V = 0.9375V

Full swing = 8.4375v - 0.9375v = 7.500v (confirmed)

I doubt that the sensor can be driven with only 15mA, I was just starting with even numbers. You must scale the resistance values down as necessary to allow a suitable power supply. Then a further adjustment will be needed to account for the sensor's OWN power drain from R1 to R2, which will throw off the scale calculations above if it is a significant fraction of the current through the divider.

STF

RE: scaling and offsetting analog output

Maybe I'm missing something, but the only viable solution is a microcontroller. It is cheap simple and easy. It may not even need a buffer if the load is light. With the multiple AD inputs you can also monitor the output and correct. An additional PWM output can add an extra bit and be used to adjust the output for load. No calibration needed. It is a foregone conclusion that an analog multiplier is not realistic from a cost and accuracy standpoint.

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