schematic and algorithm for auto-range voltmeter

Question

I'm creating Digital Voltmeter on ATmega16 with autorange to purpose. I just have a problem with designing autorange. Does anyone have a schematic of a voltage divider, and an algorithm for how auto-range should work?

I heard about FET selectable resistor ladder but currently I don't know how use it in my project. I mean schematic and how to control it from microcontroller.

The Voltmeter should be very simple - it is a project for my studies, therefore I don't want to use any digital potentiometers or sophisticated circuits. The ranges may be 2 levels for example.

Answer 1

A resistor ladder with FET switches is typically used to implement an analog-to-digital converter. For example, this is from patent US3755807:

However, the ATmega16 already has an ADC, so you don't need to use such a resistor ladder.

The easiest way to handle two ranges is to use two voltage dividers, and to use two different ADC inputs on your microcontroller for them (so the switching is done by software):

If you want to save the last bit of power and to increase the input impedance a little, put FET switches between the lower resistors and ground so that you can prevent current from flowing through any unused dividers. (Or simply use GPIO pins that you can switch between ground (output zero) and high impedance (input).)

Answer 2

Here's one simple way:

^{simulate this circuit – Schematic created using CircuitLab}

• When you drive D1, D2 and D3 to Z-state, there will be no voltage divider and you will be able to measure V_IN voltage in range from 0 to AREF

• When you drive D3 low, you'll get 1:1 voltage divider, and you will be able to measure V_IN voltage in range from 0 to 2*AREF

• In a similar way, driving D2 and D1 low will give you 3:1 and 7:1 dividers and measurement ranges of 4*AREF and 8*AREF respectively.

Algorithmically, you should keep driving D1 low when you connect the probe to V_IN, and progressively decrease divider ratio until the measured value increases to above 50% of your ADC range.

Shameless plug: I have a small project of mine which aims to be simple and educational, and I think I'll implement this auto-range feature next time I touch it.

Answer 3

Digital programmable gain using a CD4066: -

The "switches" inside the CD4066 are basically mosfets acting as either "on" or "off".

Digital programmable gain using MOSFETs: -

A mosfet as shown above can be simply presumed to be either open-circuit or short-circuit (just like a relay contact or switch). With the three mosfets open circuit, the gain is -(R2+R3)/R1. With one mosfet selected (by applying a proper gate voltage level), the feedback is reduced and the gain increases. You can choose values for Ra1, Ra2 and Ra3 to produce gains that change like this: -

Information taken from here (Digitally Programmable Amplifier Meets Sensor Gain, Ranging Needs).

Answer 4

Create a normal voltage divider that connects between the + and - terminals of your meter inputs.

The MCU measures the output of the divider with an A/D converter.

Normally a simple voltage divider consists of an upper and lower divider resistor. To make a multi-range divider, instead of using a single resistor on the lower side use several resistors in parallel. Put a MOSFET switch between each low-side resistor and ground. By turing on different combinations of MOSFETs you can select the divider ratio.

The IO/pin on most MCUs contain an ESD diode. The ESD diode will protect the IO pin from over-voltage provided the current is limited. It is usually permissible to conduct a small ammount of DC current into the diode (the amount will be specified in the datasheet). As long as your upper divider resistor is large enough to limit the current the MCU will not be damaged.