A/D Protection Circuits
Protect an analog-to-digital converter from excessive or negative input voltages

You can protect an ATD converter from excessive or negative input voltages by using a Schottky diode and current limiting resistor. For example, you might be using an A/D to measure the output of a sensor that is specified to produce a 0-5V output signal. However, you find that under some fault conditions the sensor actually produces a negative output, or an output greater than 5V. In that case, a circuit protection device is needed to prevent damage.

Many A/D converter chips are sensitive to these out-of-specification input voltages and may be damaged. In general, A/D chips have internal parasitic substrate diodes that become forward biased if an input pin voltage exceeds the supply voltage to the chip by more than a diode drop, or has an excursion below ground by that much. If the current into the pin is sufficiently limited that usually doesn't cause damage. But to maintain the A/D's accuracy you usually can not use too much series resistance, so the input current may not be limited. Too much current flowing into the input pin of an A/D under these fault conditions can cause internal heating, destruction, or latch-up of the part. If latch-up occurs, it is generally remedied only by powering off the chip.

The classic way to protect the downstream A/D from the negative voltage is with a resistor-diode protection circuit like the following:

The circuit shows a current-limiting series resistor and a Schottky diode to ground used to protect A/D inputs from a "Power sensor". The "Power sensor" is meant to typify any device which may output an out-of-range signal that must be measured. This circuit is shown protecting an A/D converter input on Mosaic's Analog I/O Wildcard.

You can protect against positive excursions of the input voltage beyond the supply rail also, by including another diode, as shown in this figure:

Choose the resistor and diode values as follows:

• The resistor should be large enough to limit the current into the diode when a negative input voltage is applied, but small enough so that its voltage drop under normal conditions doesn't adversely affect the A/D converter readings. The 16-bit resolution A/D used on the Analog I/O Wildcard has a typical input leakage current of ±1 μA. That and the leakage current of the diode flowing through the resistor creates a small offset in the A/D readings. For a resistance of 100 Ω or less the error is negligible; for a resistance of 1 KΩ the error may be as much as 0.05% of full scale – still negligible for most applications. See the discussion of source resistance limitations for A/Ds for more info.
• The diode used should be small signal (hence low leakage) Schottky diode, not a rectifier diode. For example, BAT54S is a surface mount packaged dual diode. Although surface mount, you can solder leads onto it. Or, discrete diodes may be used, like the BAT41, BAT42 or BAT43.

You may mount the resistor/diode externally to the A/D board, or in some cases there may be room reserved on the A/D board itself. In the case of Mosaic's Analog I/O Wildcard, there are land patterns reserved for resistor-capacitor filters or resistor-diode protection.

The following figure shows an RC filter mounted directly on the A/D board. Each input channel has reserved space for a series resistor, and a component (either capacitor or diode) to ground:

Only one channel is shown, AD16_CH0. The component locations use designators R29 and C40. As manufactured, R29 is populated with a zero ohm resistor (that is, a short), and nothing is placed on the capacitor pads at C40. You may replace the zero ohm resistor with any value of your choosing, and mount a capacitor for filtering.

You may also use these locations to mount circuit protection components, as shown here:

The series resistor should be in the range of 100 Ω to 1 KΩ. The error caused by a resistance of 100 Ω will be smaller than you can measure, while a 1 KΩ resistor will cause an error of less than 0.05% (assuming a 2 μA leakage current into the A/D and diodes).

The figure shows the resistor replaced with a 100 Ω or 1 KΩ surface mount resistor (in a 0603 package), and a schottky diode (also in a 0603 package) placed at location C40. A good diode to use would be CDBU00340 available from Digikey.

The Analog I/O Wildcard's circuit schematic, found here, shows the designators used for the filtering/protection pads for each of the input channels.

Resources:

Figures source file