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I'm developing an analog input module based on TI's ADS8668 and want to protect the input channels from over-voltage. According to the datasheet, each analog input channel withstands voltages from -20V up to +20V.

Although my analog signals are expected to be in the range of 0-10V, wrongly connected inputs (e.g. miswiring) could lead to voltages up to +24V on the ADC input channel.

To protect my analog inputs, I tend to use Schottky diodes, as recommended by TI (Engineer It: How to Design Protection Circuits for Analog I/O Modules) and Analog Devices (Protecting ADC Inputs).

I have no supply rail in 10-20V range on my PCB, that I can clamp my diodes to. Therefore I would just place a Zener diode (D3) as shown below, to protect my analog inputs from voltages above Zener voltage.

Simulating this schematic in LT Spice using a 12V Zener results in a system behaviour according to my requirements (see below).

Is it sufficient (and good practice) to clamp my Schottky diode (D2) to the Zener diode without having any positive supply rail (unlike this solution, which uses a shunt regulator)?

Is there anything I need to beware of, especially the power dissipation of my components in an over-voltage case?

Thanks in advance!

schematic

simulate this circuit – Schematic created using CircuitLab

simulation output

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  • \$\begingroup\$ What is the value of R1? This is going to limit the current through D2 and D3 in the event of an overvoltage. \$\endgroup\$ – newothegreat Jun 27 at 13:25
  • \$\begingroup\$ R1 is supposed to be ~ 357 Ohms. \$\endgroup\$ – ser_u Jun 27 at 13:45
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Yes, that would work fine, but you can make this even more simple:

schematic

simulate this circuit – Schematic created using CircuitLab

Zener diodes behave more or less as a "normal" diode when in forward mode so we can use that to clamp negative voltages.

When using a 12V, 400 mW zener diode the maximum current through the zener diode can be 400mW / 12 V = 33 mA. There will then be 12 V across R1, R1 needs to limit the current to 33 mA so it needs to be: 12 V / 33 mA = 364 ohm or higher. I added some margin and suggest 470 ohms then the actual current will be 12 V / 470 ohm = 26 mA and R1 will dissipate 12 * 26 mA = 0.312 W so you might want to use a 0.5 W resistor (or use two 1 kohm 0.25 W resistors in parallel).

I would make the capacitor C1 10 nF, that should suffice to remove most pulses. Only if you need to measure high frequency signals C1 might need to get a smaller value or must be removed. Then the zener diode might be an issue as well as zeners aren't very fast. But if you stay below 100 kHz or so there should be no issue with this circuit.

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  • \$\begingroup\$ Sounds good. The datasheet of the ADC recommends adding external protection against ESD surges. Is it advisable to add a TVS diode in between of my RC Filter and the Zener to protect against ESD surges? Or is a combination of RC and Zener sufficient? Thanks \$\endgroup\$ – ser_u Jun 27 at 13:59
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    \$\begingroup\$ The ADC will have on-chip ESD protection as well, that in combination with the circuit above will provide sufficient protection. I do not think the TVS diode is needed but what you can do is reserve a space for it on the PCB but not place it yet. Place the TVS diode in parallel with the zener diode. Then if ESD robustness isn't sufficient you can easily add the TVS diode. \$\endgroup\$ – Bimpelrekkie Jun 27 at 14:07

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