Electronic – Voltage divider for battery monitor ADC – loading effect

By your values, I have to assume that you are measuring off of the 10kOhm resistor on the bottom. I would hazard a guess that your 10kOhm resistor is a little high and the 100kOhm resistor is a little low. This would mean if your 10kOhm resistor is taking more of the voltage in the voltage division than expected.

There are a few problems at work here.

1. The first is the resistor value error. Ohm's Law: V=I*R. So if there is 5% error in R, then for a constant I, there will be a %5 error in the expected value of V. The %error from resistor tolerances are the maximum +/- error. So in reality, there could be a resistor with +5% error and other with -5% error. So there is a much larger possible range in output voltages for a constant current.

   • This can be eliminated by either hard-coding the real resistances in software or adding a reference voltage that is more accurate than the resistor or A/D quantization error. This reference voltage would be used to get the real resistance values as the A/D sees them.

2. A/Ds can have an offset error. This really has to be calibrated out. Some A/Ds have this built in.

3. A/Ds also have quantization error. So if a voltage falls between two consecutive quantization levels, it will have to be rounded to one of those two, introducing error. There are ways of increasing the number of bits for an A/D by oversampling and averaging blocks of samples into one sample.

There are other errors that could be at work, but those three are the main ones I have run into. If the A/D is fairly linear, then measuring two accurate voltages with the A/D circuitry would let you build an affine linear equation to correct the data. This builds off of the problems in 1 and 2.

A/Ds can appear linear, but end up being very nonlinear at a particular range. I have heard of some implementations using a look-up table to correct the nonlinear behavior. But that is getting a little beyond your current problems.

Edit: One more item. It is a good idea to buffer your analog signals to the A/D. It does a few things, like add another device to protect the microcontroller, and limit any possible transient sampling behavior from the A/D go into the analog signal.

There is no one "usual" solution. Some are:

1. Use a high side switch. Use a fairly low impedance voltage divider, but only turn it on for a short time around each battery reading. Since it's on for only a very small part of the total time, the average current draw will be low. This is usually done with a P channel FET so as not to add a offset voltage.

2. Use as high a impedance divider as your A/D can tolerate. A/Ds are specified for some maximum source voltage impedance for two reasons, to charge up the sample and hold cap within the specified acquisition time, and so that leakage current causes a small enough offset to ignore. Too high impedance for the first reason can be overcome by a longer sampling time, which some A/Ds allow you to control. However, there is nothing you can do about the leakage. 100 nA will cause 100 mV error with 1 MΩ source impedance, for example.

3. Use a high impedance divider followed by a buffer amp. You still have to consider leakage, but good opamp input leakage is usually a lot less than microcontroller pin leakage.