Convert 0-10 V to 0-3.3 V for ADC pin and protect it

Question

in a project I want to convert 0-10 DCV to 0-3.3 DCV and feed it to STM32 microcontroller ADC input pin. also I want some protection on ADC pin. I draw this schematic which I found here in Digi Key. please let me know if it will operate normally and if it has no problems!

• 6.8K and 3.3K resistors are voltage dividers to convert 0-10V to 0-3.3V
• SS14 works as a clamping diode and it's there to protect from less than zero volt spikes
• 1K resistors serves as a current limiting resistor for microcontroller pin.
• 3.9V zener diode makes us sure that the maximum voltage on the pin will stand in its range (max 4V)
• 100nF capacitors works as the bypass capacitor and also shape and low pass filter with the 1K resistor

Answer 1

The link that you provided shows protection for a digital input. The ADC's input resistance is much lower. The electrical operating specifications for the ADC usually provide a formula to calculate the maximum allowed source resistance. They often show preferred protection schemes that are appropriate for un-buffered inputs.

The 1k resistor is not required because the voltage divider provides the same function. The zener is not required as the Schottky internal diode in the micro will be sufficient. I suggest the following circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Make C1 small enough to track input changes. This circuit will usually require a slower sample rate and a longer sample time both that are programmable.

I prefer a buffer as shown below. It is not the only way and others may have alternate suggestions. This method isolates the ADC from the inputs and provides a very low source resistance thus removing all timing and loading restrictions on the ADC.

^{simulate this circuit}

Answer 2

The Digikey article appears to be talking about digital inputs. Your application is analog.

SS14 is a Schottky diode which is leaky, particularly at high temperatures. 10mA at rated voltage maximum at 100°C and 200uA at 25°C. A leakage of 0.5mA will cause an error of more than 1V. Probably it won't be that much at room temperature and 3.3V reverse voltage, but it will likely be measurable.

More seriously, a 0.5W 3.9V Zener diode will also conduct significantly at 3.3V. Here is data for the 500mW MELF diode series including 3.9V MLL5228A. Vz for the MLL5228A is specified at 20mA, so it appears to be the 4th curve from the left. At 3V it will typically conduct around 1.5mA. Some zeners are specified at much lower Vz and they will conduct less (at both 3.3V and at 3.9V).

So that circuit will affect the accuracy considerably and the effect will vary considerably with temperature.

Designing clamps that work without adding excessive error to precision analog signals is not altogether straightforward, and the first step is to determine your required accuracy and what maximum level of input voltage you are trying to protect against. You will also need to know what maximum input impedance the STM32 MCU can tolerate without undue ill effects. And how much current you are willing to allow the STM32 input protection networks to conduct.

Eg: Re comment, here is the general idea. It's also possible to use the emitter of a PNP BJT to clamp the voltage, provided there's a bit of current draw guaranteed from the 3.3V supply. Base to 3.3V, collector to ground.

^{simulate this circuit – Schematic created using CircuitLab}