Electronic – Auto-Ranging Meter Design

Auto ranging would typically be handled by adding a PGA (programmable gain amplifier) in front of your A/D converter. In some instances to make maximum utility of the range of your A/D converter you would also add a programmable offset into the PGA as well. The output of the PGA would get clamped via some diodes to the A/D converter reference voltage range.

Another scheme is to design multiple amplifiers, each with its own gain and offset, that sample the input signal and condition it in a particular way. Each of these amplifiers could be setup with trimpots to permit calibrating to a precise operating range. The outputs of these amplifiers would be gated to the A/D converter through a many to one Analog Multiplexor.

It is still possible to "calibrate" the scheme with the PGA amplifier with programmable offset by feeding known signal levels through the circuit and then noting the programmable settings it takes to get an aimed for output from the A/D converter. These would get stored into some non-volatile memory of the MCU to be used for adjusting/scaling the A/D readings to the proper values when taking readings of actual signals.

The typical way to protect ADC inputs (and many other high impedance inputs) is to have a current limiting resistor and clamping diodes at the pin. Many chips already have clamping diodes inside the chip, but it is often useful to have some outside as well (especially when the integrated clamping diodes do not have proper documentation on their specs, which is most of the time).

Your resistor-divider is already going to function as a current limiting resistor. As a general rule, you want your resistor values to be as high as is practical-- with something in the neighborhood of 10K to 500K being reasonable.

As the resistors go higher in value you start to get weird effects from the parasitic capacitance (lowers your frequency response) and the leakage current of the ADC and diodes (messes up your accuracy). If you are unsure about these things, then pick something on the low side of 10K to 500K. If you have already figured out the math or are willing to just try it and see if it works then go with something higher.

There would be one diode between GND and the input. And another diode between the input and VCC. If you have a signal that goes + and -, then connect one diode to the V+ rail and the other to the V- rail. Orient the diodes in such a way that it works (turn the diodes around and nothing will work). Put the diodes between your ADC and the resistor divider.

It works like this... If you get a voltage spike coming in, the diodes will redirect it to a power rail and not directly into the chip. The current limiting resistor will, duh, limit the current to something reasonable. You want to current to be limited for several reasons: 1. You don't want so much current that the diodes blow up. 2. You don't want so much current going into the power rails that it effects the voltage regulation of that rail.

With this setup, the maximum voltage on the input pin will be VCC + the forward voltage drop of the diode. And the minimum voltage on the input will be GND - the forward voltage drop.

There are hundreds or thousands of appropriate diodes for this. The BAV99 is just one of many, but is a good one to start looking at.