SDIY Building Blocks

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Passive Conditioning ​

What It Does

Conditioning circuits ensures that signals are properly prepared for further processing.

Examples:

  • Improve the accuracy of the measurement.
  • Reduce the noise in the signal.
  • Make the signal compatible with the next stage of processing.
  • Safeguard the subsequent processing stage from potential damage.

Remove negatives ​

For SDIY circuits you often need to consider unipolar or bipolar voltage levels. Some parts like to use +12V and -12V while other parts really don't want anything below 0V. The easiest way to get rid of negatives is to use a diode like the 1N4148.

With diodes ​

  • The diode will clip all voltages below 0V (isch) to ground.
  • The resistor limits the current through the diode.
  • Notice two versions. They can both be used, however I often use the left one, since the resistor to ground also acts as a pull-down resistor in one go.
  • Notice a slight offset at ground level. Diodes are not perfect, but are often good enough. See also Precision rectifier.

Warning

Use after a diode in series a pull-down resistor, else the circuit afterwards won't see a clear LOW.

TIP

When you doubt if a part can receive certain voltages first look at how it is powered. Most likely (with some exceptions) parts don't want to receive voltages outside their power supply.

Positive clamper ​

The positive clamper moves the whole signal up, and places the lowest peak at reference level.

Limit maximum ​

Always check the datasheets when combining parts. Often you discover parts cannot be mixed unconditionally. Here is how to limit the maximum voltage.

With diodes ​

  • The top diode clips all voltages above 5V (isch) to the 5V power supply.
  • Use a voltage divider if you don't have 5V but 12V in your circuit, for example.
  • Please play with different power levels.
  • The resistor protects the diode for too much current.

With zener diodes ​

A zener-diode is a diode that can open at an extra thresshold level. Look at the datasheet for the particular zener-diode you want to use.

  • Notice a zener-diode doesn't need an external voltage for it's second threshold level.
  • Notice it also removes any negative values near ground reference.
  • I have used a 5.6V Zener-diode here. Please play with this setting (edit part).
  • This configuration requires less parts, but is also less flexible, since you are depended on available zener diode values.

With TVS diodes ​

Use the datasheet

Some IC's have build in input protection called TVS-diodes (Transient-voltage-suppression diode), and the only thing you need to add is a current limiting resistor. You can use this as a lazy strategy for your conditioning. For this always look at the datasheet first !

  • CV40106b and HEF40106B are examples of IC's with build in clamping diodes.
  • Notice R1 (100k). This large resistor limits the current through the internal diodes.
  • R2 is arbitrary.
  • Although the protection is passive itself, it is part of an active component.

Max-Min limiting ​

With diodes ​

  • Notice this circuit is the combination of two above circuits. I wanted to give you the understanding that the lower limit also works with other voltages than GND / 0V.

With zener diodes ​

  • Notice the bottom zener-diode is the same as the top but connected upside down.

Remove positives ​

Negative clamper ​

The negative clamper moves the whole signal down, and places the highest peak at reference level.

Level shifting ​

With zener diode ​

This circuit lowers the output voltage by a quantity that is equal to the Zener diode's breakdown voltage. In this example I have used a 5.6V zener diode.

  • Notice the offset is equal to the zener diode value.
  • Notice changing the resistor value does not affect the offset.
  • Using GND instead of -10V will clip negative voltages.