I am looking for a way to remap a voltage between 0..24 volts to 0..5 volts in order to read this voltage in a microcontroller ADC.For that ,I am intending to use an operational amplifier as an attenuator.Assuming that i have read several posts that couldn't solve my problem i have to ask:

  1. Are they any IC that can convert 24 volts to -24 volts to provide negative supply to the Op amp?if so can you suggest an op amp reference that could stand to a 48 volts?
  2. Is it possible to provide a non symetric voltage to an Op amp in the positive and negative supply like for example 24 volt and -5 volts
  • 3
    \$\begingroup\$ Is there a reason why you can't just use a voltage divider? \$\endgroup\$ Jul 28 '15 at 10:47
  • \$\begingroup\$ It is possible to provide non symmetric voltage to an op amp. Just go through the datasheet and remain within the limits specified. You will just get non symmetric saturation voltages but you would be fine unless you are making something like a Schmidt Trigger. \$\endgroup\$ Jul 28 '15 at 10:47
  • \$\begingroup\$ If you really wanted to buffer a 24V signal directly you might actually need supplies more like -5V and +30V. That's the only way you can get exactly to the signal levels and provide a really high impedance input (G\$\Omega\$ / nA or fA input current). \$\endgroup\$ Jul 28 '15 at 13:06

Why you need -ve supply, since your voltage range is in between 0 to 24 volt, you can connect the -ve supply of the op-amp to GND. -ve supply is only required if the input signal to op-amp is -ve.

Why can't you proceed with a voltage divider solution. Here you can set the resistor in such a way that the output of voltage divider sets to 5V or little lesser(4.75V) when the voltage is 24V as you mentioned.

Something similar to shown belowenter image description here

  • \$\begingroup\$ I learned from the forum that if i have a value close to -Vcc which is ground in your case the circuit will lack precision \$\endgroup\$ Jul 28 '15 at 10:57
  • \$\begingroup\$ You can go for -ve supply if the lower voltage in your application is really closed to GND potential. But the best solution is to proceed with voltage divider. Here no need to worry as such, whether the lower voltage is close to opamp -ve supply :-) \$\endgroup\$ Jul 28 '15 at 11:02
  • \$\begingroup\$ And you dont have to generate negative supply \$\endgroup\$ Jul 28 '15 at 11:03
  • \$\begingroup\$ Why did you put a capacitor parrallel to R2? \$\endgroup\$ Jul 28 '15 at 11:14
  • \$\begingroup\$ It acts as a noise Suppressor/Filter. R1 and C1 acts as a low Pass filter. \$\endgroup\$ Jul 28 '15 at 11:26

All you need to convert 0-24 V to 0-5 volts is two resistors. The first resistor (R1) is in series with the input, and the second (R2) connects after that to ground. This is such a common construct it has it's own name, which is "voltage divider". Look that up, even here on this site, and you should find lots of info.

If the first resistor has 4x the resistance of the second, then the divider will divide by 5 (has a gain of 0.2). That allows the 0-5 V output to represent 0-25 V input.

The divide ratio defines the ratio between the two resistors. Since there are two resistor, you still have one degree of freedom left. Two possibilities for nailing this down is to consider the input impedance or the output impedance. Let's say the A/D wants to be driven by no more than 10 kΩ. The impedance driving the A/D input is the parallel combination of R1 and R2. A easy way to get that is to make R2 10 kΩ, then R1 in parallel with it will make the output impedance it bit lower, giving you some margin.

With R2 = 10 kΩ, R1 would be 40 kΩ, so the output impedance would be 8 kΩ

  • \$\begingroup\$ This circuit works only if we suppose that the output has an infinite resistance.I have also to make a generic board for differenct controllers. \$\endgroup\$ Jul 28 '15 at 11:00
  • \$\begingroup\$ @Med: Huh? It's not clear what you mean by "output", but the output impedance of a voltage divider is the parallel combination of the two resistors, as I said in the answer. You said this needs to drive a microcontroller A/D, but otherwise provided no specs. Most such A/Ds have very high input impedances, but require the signal to not have more than some maximum impedance. This requirement can be met with a voltage divider, as I also explained and even showed with example, in the answer. \$\endgroup\$ Jul 28 '15 at 12:31

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