Voltage divider for a voltage range spanning negative voltages

Question

We have a sensor giving out a signal between -10 V and +10 V, and a signal acqusition device that only accepts voltages between -2 V and +2 V. So we need to scale down the signal into the smaller range. As possible solutions I have identified:

A. Use a voltage divider

Following the information in this page: https://learn.sparkfun.com/tutorials/voltage-dividers, I could use two resistance, R1 = 5000 Ohm, and R2 = 1200 Ohm.

• I am not sure what am I allowed to use a Ground. Both devices (sensor and acqusition) have a ground connection, which are connected to each other. Can I use this common ground as for the ground of the voltage divider ?

• The impedance of the signal acquisition device is specified as 1 MegaOhm. Why is this value given at all? Should I include it somehow in my voltage divider calculations ?

B. Buy some off the shell device

I have seen some voltage down regulators, but they appear to only address power applications. I assume they are too noisy to be used for sensor read-out. Furthermore, there appears to be no device offering downscaling negative voltages.

• Are there commercial voltage downscaling devices made for sensor reading ? I am not familiar in this area and I might be just ignorant of a family of devices that already does this in a plug an play manner.

Answer 1

I am not sure what am I allowed to use a Ground. Both devices (sensor and acqusition) have a ground connection, which are connected to each other. Can I use this common ground as for the ground of the voltage divider ?

If your circuit has a ground, then that's the one you need to use here.

The impedance of the signal acquisition device is specified as 1 MegaOhm. Why is this value given at all? Should I include it somehow in my voltage divider calculations ?

The impedance is in parallel with R2. If it is much greater than R2, you can ignore it. If it isn't, then you need to ensure that your voltage divider gives the correct scaling using R2 in parallel with the input impedance, rather than just R2.

Are there commercial voltage downscaling devices made for sensor reading ? I am not familiar in this area and I might be just ignorant of a family of devices that already does this in a plug an play manner.

I'm not aware of any, but that doesn't mean they don't exist. A high-impedance op-amp set up to give the desired gain would make an alternative to a simple resistive divider.

Answer 2

You can use a voltage divider. The grounds must be common, and any difference in potential will lead to errors. With your resistor values, the source must be able to drive a 6200 ohm load.

The ratio of the resistors is 1200/6200 so a +/-10V input will give you (nominally) about +/-1.935V. The resistor tolerances will affect what you actually get. 1% tolerance values (E96 series) that are close are 4.99K and 1.21K and even if the resistors are worst-case off it will still be less than 2V for 10V in.

If you were looking for an exact ratio you would take the 1M\$\Omega\$ input resistance into account, but it's around 0.1% effect.

For example, if you used a perfect 4.99K\$\Omega\$ resistor, the exact value of the other resistor for 10V->2V would be 1247.5\$\Omega\$ not considering the 1M\$\Omega\$ and 1249.06\$\Omega\$ taking the input loading into effect, or about -0.12% less.

To adjust the resistor for the loading the equation is:

Rnew = 1/(1/Rold - 1/Rload)