# Converting [-24V=>24V] to [0=>1.8V]

Discussion in 'General Electronics Chat' started by Oussama Zaidi, May 26, 2016.

1. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
Hi
I use the circuit in the picture below to convert a DC signal from [-24V=>24V] range to [0=>1.8V] range
I have two questions:
1)When I simulated it in ISIS I found that it gives me the right results only if Vee=-Vdd that is mean if I connect Vee to GND it gives me wrong results I can't understand why can anyone give me an explanation?
2) I need to use an adjustable resistance (timmer resistors) to make the output in the right range after realisation and it is generally the one which has the lowest value which means 1K but in this case I have 4 resistances unless of 2 so did I need to replace both of 1k resistances with adjustable ones or replacing just one is sufficient and if it is just one what is better replacing R8 or R10 and why?
Thank you.

2. ### Kjeldgaard Member

Apr 7, 2016
295
110
It is a common mode problem you have. When the input is negative, there will be a negative voltage on the inputs of the operational amplifier, which then require a negative supply.
I would both scale and offset adjust at the input voltage divider.
Something with a resistor of (approximately, I think) 4400 Ohm from the junction of R9 and R10, and to + 5V. The operational amplifier must, depending on the requirement of the following circuit, be removed or just be a buffer.

3. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
I am sorry but I didn't understand the last part the 4.4K resistor is it fixed or variable value and in place of which resistor exactly I need to use it? I was thinking of using a 1K variable in place of the 1K fixed.

4. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,687
1,579
Without the op-amp and keeping the input resistor the same value I make the values:

Then you could make R2 adjustable to set the zero output and R3 to set the 1.8V.

Roderick Young likes this.
5. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
An active circuit is better because it is more stable and consume less, also the 0 to 1.8V is made to be the input of the BeagleBones ADC pins, in this case the pins are not protected they can easily be damaged I think.

6. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,687
1,579
You can use the op-amp as a unity gain buffer and the current consumption will not be much affected.

7. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
The voltage divider will consume especially R1.

8. ### Papabravo Expert

Feb 24, 2006
11,760
2,487
Your argument would be stronger with some numbers, or maybe the numbers would reveal your intuition to be off base.

9. ### crutschow Expert

Mar 14, 2008
20,269
5,738
The voltage divider will consume no more current than your op amp circuit.
If you want less current drawn from the input then increase all the resistor values proportionally.

10. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
There is no advantages of using the opamp instead of a simple voltage divider ?

11. ### crutschow Expert

Mar 14, 2008
20,269
5,738
Only if you need a higher impedance input and a low impedance output.

12. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
How about the stability and the noise elimination I think an active circuit is better in that a voltage divider can be a source of noise, no?

13. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,687
1,579
Much the same resistive divider is built around the op-amp in your original circuit so the noise from the resistors is present in both versions. The op-amp would add to that noise not reduce it. How can some resistors plus an op-amp be more stable than the resistors alone? For instance the amplifier will also add input offset voltage and input currents which will change with temperature.

Apr 5, 2008
18,700
3,640
Hello,

A pure resistive solution will depend on the input impedance of the ADC.
Resistors can make noise, metalfilm resistors will introduce less noise as carbon resistors.

Bertus

15. ### Oussama Zaidi Thread Starter Member

Mar 1, 2016
53
0
Can you give me the equation of the voltage divider that you did Vout=f(Vin) because I can't figure it from your voltage divider schematic.

16. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,687
1,579
Start from this:
Vo/R3 = (5 - Vo)/R2 + (Vi -Vo)/R1

Mar 1, 2016
53
0
Than you.