Thursday, 26 November 2015

circuit analysis - How to bias a voltage for an ADC


I am part of a project that is implementing a power storage system. The storage device voltage must be monitored in order to direct power. The storage device voltage should remain between 12 and 36 V. The project uses a TI TMS320F28027 MCU which operates at 3.3 V. How can the 12 to 36 V be mapped onto 0 to 3.3 V for the MCU.


I posted a similar question here but it specified that the voltage would range from 0 to 60 V. The difference for this question is how to bias the voltage appropriately.



I also posted another question about biasing the output of a current sensor for an ADC, but it involved biasing a balanced voltage around 0 V. I am having trouble adapting the answer to this problem. Diagram from answer:


diagram from answer
Figure 1. Diagram from previous answer


An attempt to emulate the design methodology would be:


24 V maps to 1.65 V
38 V maps to 3.3 V
10 V maps to 0 V


The most basic circuit model:


enter image description here


How to design the bias?




Answer



Rearranging:



  • 38 V maps to 3.3 V

  • 24 V maps to 1.65 V

  • 10 V maps to 0 V


schematic


simulate this circuit – Schematic created using CircuitLab


Figure 1. An 11.5:1 potential divider.



The simplest solution is to use a potential divider with a ratio of 38:3.3 or 11.5:1. This would result in:



  • 38 V maps to 3.3 V

  • 24 V maps to 2.08 V

  • 10 V maps to 0.868 V


The 0.868 V offset can be removed in software. Again, you lose a little resolution with this approach.


If a negative voltage supply is available then the offset can be removed.


schematic


simulate this circuit



Figure 2. With a negative rail available the offset at minimum input voltage can be removed.


How:



  • The span is 38 - 10 = 28 V.

  • This has to be scaled to 3.3 V so a divider ratio of 28 / 3.3 = 8.5:1. Let's use 7.5k and 1k to give us the required ratio.


Now we need to figure out the negative reference voltage.



  • At 10 V in Vout will be 0 V. With the 8.5:1 ratio we will need to hold Vref at \$ - \frac {1}{8.5} 10 = -1.18 V \$.



So, R1 = 7.5k, R2 = 1k, Vref = -1.18 V should do the trick.


I'll leave it to you to work out how to create the reference voltage.


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