How to properly voltage divide an AC/differential signals

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.

• The "resistors in the voltage divider" presumably refer to Resistor 6 and Resistor 7 in the schematic, providing the biased signal voltage to one of the inputs of the op-amp.
  • These dividers would be selected such that their combined resistance in parallel is significantly smaller than the input impedance of the op-amp. Otherwise, the current into the op-amp input will change the bias voltage - like putting an additional resistor of comparable value in parallel with one leg of the divider.
  • Also, they should be of high enough value that the current through them does not draw an unreasonable amount of current from DC Voltage Source 1.
  • Finally, the resistors should be of sufficient power rating to tolerate the power dissipation P = I^2 x R for the current drawn through them, I = V(DC1) / (R1 + R2)
• The op-amp gain resistors would depend on the gain desired. Since the schematic does not indicate inverting and non-inverting inputs, it is not clear (at least to me) whether this is intended as an inverting or non-inverting amplifier, and what the intention of that part of the design is.
• Dynamic voltage dividers can be implemented in a variety of ways, which include using a digital potentiometer IC, an resistor array with switched legs, etc.
  • If one assumes that the question envisages only 3 possible input signal voltages, a dynamic voltage divider may not be needed. One could structure 3 voltage dividers, and switch the appropriate one in using a very small R_ds(on) MOSFET, such that the MOSFET's resistance does not discernibly impact circuit behavior.
  • Just 2 voltage dividers would suffice, if the intention is merely to provide a counter-bias of -1 Volts for the 0-2 Volt signal when that is the source.
• Overvoltage protection (presumably referring to the input signal voltage) can be achieved by using a voltage limiting zener diode circuit, at the simplest level.
  • Depending on the currents involved and the behavior desired, other solutions such as voltage foldback or SCR-based crowbar circuits may be called for - apparently not applicable in this case.