Timeline for How would I design a protection clipper circuit for ADC input?
Current License: CC BY-SA 3.0
21 events
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| Feb 11, 2019 at 9:04 | review | Suggested edits | |||
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| Jul 17, 2012 at 18:35 | comment | added | The Photon | @stevenvh, You can call it "wildly varying", I can call it "leakage current". Driving an ADC, there are other good reasons to have low source impedance besides just needing to provide for leakage current here. One solution has essentially no leakage current below the clipping point but requires high source impedance to avoid uncontrolled circuit operation; the other solution is independent of other supply loads but requires a low source impedance. Classic engineering trade-off. | |
| Jul 17, 2012 at 17:46 | comment | added | stevenvh | @ThePhoton - like you say the 17 ohms is at 20 mA. Not a bad figure, even when they always get better the closer you are to about 6.5 V. But at 0.25 mA it's 1600 ohm, and depending on the source impedance (I'm thinking of a resistor divider) you'll be much closer to that. At 5 uA this 5.1 V zener becomes a 2 V zener! That's what I meant by "wildly varying". All zeners have this for very low currents. | |
| Jul 16, 2012 at 17:44 | vote | accept | waspinator | ||
| Jul 16, 2012 at 17:36 | comment | added | The Photon | @m.Alin, typically SC-70 or SOT-23. See On Semi BAT54SW, for example. | |
| Jul 16, 2012 at 17:28 | comment | added | m.Alin | "You can buy the two diodes in a dual package for exactly this purpose" Interesting. What's the package called? | |
| Jul 16, 2012 at 17:26 | comment | added | The Photon | @waspinator, You use the same supply that's supplying your ADC. The current coming from the overvoltage condition needs to be sunk through the supply pins of the ADC and other parts in your circuit. | |
| Jul 16, 2012 at 17:20 | comment | added | The Photon | MMSZ52xxxT1G. I just picked it randomly as their lowest-power (500 mW) zener family. 17 Ohms for the 5.1 V device at 20 mA. | |
| Jul 16, 2012 at 17:09 | comment | added | stevenvh | That OnSemi devices indeed don't look bad. Do you have a part number? I'd like to see their differential resistances. | |
| Jul 16, 2012 at 16:59 | history | edited | The Photon | CC BY-SA 3.0 |
zener voltage variation with current.
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| Jul 16, 2012 at 16:40 | history | edited | The Photon | CC BY-SA 3.0 |
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| Jul 16, 2012 at 14:15 | comment | added | waspinator | ok so I should use regular/schottky diodes for better operation. What should I use for the 5V supply? would a microcontrollers 5V output or a USB port be safe? | |
| Jul 16, 2012 at 12:55 | comment | added | stevenvh | Note that a zener's voltage varies wildly with current, see the example in my answer. | |
| Jul 16, 2012 at 4:55 | comment | added | The Photon | For the second circuit, the diode must be able to handle 5 V reverse bias (should be okay with almost any diode) and must be able to handle however much current might be present in an overvoltage condition (depends on R1 and what kind of overvoltage you're expecting). | |
| Jul 16, 2012 at 4:52 | history | edited | The Photon | CC BY-SA 3.0 |
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| Jul 16, 2012 at 4:47 | comment | added | The Photon | And finally, I did the schematics at circuitlab.com. Good for simple circuits but frustrating when you want a component that's not in their library (like an ADC). | |
| Jul 16, 2012 at 4:45 | comment | added | The Photon | Also, the zener is not hooked up to 5 V. It's hooked up the way it's shown in the schematic (cathode to the signal line, anode to ground). | |
| Jul 16, 2012 at 4:44 | comment | added | The Photon | The resistor value will depend on the input impedance (or current input needs) of your adc, the sampling rate, and the required bandwidth of the input signal you're measuring. 100 was just the default value in the circuitlab editor. | |
| Jul 16, 2012 at 3:26 | comment | added | waspinator | thanks, I'll try it. is there a reason behind the resistor value? The zener diode is hooked up to 5V right? Will any old diode work for the second option? What happens if I cannot sink enough current for the 5V supply? Also what software did you use to make the diagrams? They look great. | |
| Jul 16, 2012 at 3:20 | vote | accept | waspinator | ||
| Jul 16, 2012 at 3:22 | |||||
| Jul 16, 2012 at 3:05 | history | answered | The Photon | CC BY-SA 3.0 |