What kind of diode to use with ADC inputs

Question

I am looking for diodes to protect my ADC from over voltage conditions. From what I have researched so far, I know that I need schottky diodes hooked up from the input to the ground and the input to the VCC line, but I am confused of what properties of the diodes that I need to consider.

My ADC has a max input of 3V, and has some internal protection, but I have read that it is not good to rely on the internal protection circuitry. What kind of diodes properties would I need to protect my circuit, if someone accidently hooks up a 5V or a 12V input?

The diode properties that I am confused about are things like forward voltage, reverse voltage, etc.

Answer 1

The function you describe is the use of "Clamping Diodes" to protect the ADC input from voltage swings too far above the positive rail, or too far below the negative (typically ground) rail.

See the the Voltage Clamp section in this nice Diodes and Transistors guide, and specifically the schematic provided in it:

The diode parameters of interest:

• Forward voltage: Less than or equal to the ADC's upper tolerance limit above the nominal positive full-range voltage of the ADC (often Vcc, and specified as 3 Volts per your question) and the ADC's lower tolerance limit below ground voltage.

  • So if your 0-3 volt ADC is designed to not get damaged by inputs up to +4 volts and down to -1 volts, then maximum forward voltage required of D1 and D2 is 1 volt each, so the 0.7 volt silicon diodes shown in the schematic would be good enough
  • Schottky diodes are usually recommended not for low Vf, but for fast switching, since modern ADCs typically can tolerate voltages of at least a volt or two above Vcc, and a volt or two below ground, without harm.
  • In many cases a standard silicon diode might be better suited than a similarly rated Schottky due to its lower reverse leakage current.

• Reverse breakdown voltage: Greater than the maximum envisaged voltage likely at the ADC input, even with erroneous connections. This is rarely a constraint with commonly used diodes.

• Diode forward current rating: Sufficient to be able to serve as a short for the incoming signal difference after passing through the resistor R.

  • Thus, with a 12 Volt worst-case input, a 3 volt positive rail, 0.7 Volts Vf for D1, and R=100 Ohms, the diode should be able to handle 83 mA without magic smoke coming out. There is no dearth of suitable diodes meeting this parameter.

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Notice that the diodes in the schematic are connected "upside down" compared to typical diode uses:

The idea is, when Vin is within the range {Vmax+Vf .. Vmin-Vf}, both the diodes are reverse biased, and only the marginal reverse leakage current flows through them.

When Vin goes above Vmax + Vf, the diode D1 conducts, and shorts the signal to the upper voltage line.

When Vin goes below Vmin - Vf, D2 conducts and shorts the signal to the lower voltage (or ground) line.

It is assumed that the upper and lower (ground) voltage rails are regulated with a low enough impedance that they can shunt the excess current from the clamping diode without perturbation of regulated voltage. Hence the ADC would never see those out-of-range voltages at Vout.

I hope this answered your questions.

Answer 2

Diode protection of inputs is well covered by answers here plus many web references.

Less obvious is that the spec sheet will have "absolute maximum" specs and "operating specs". The latter may be termed "Recommended operating conditions". This should be understood to mean "recommended if you want your system to work OK conditions".

Specs which allow the pins to be taken outside the supply rails are almost always in the "absolute maximum" specs and NOT in the "operating" specs. "Operating" specs almost always specify voltages that NEVER are taken outside the supply rails.

If you want your design to ALWAYS work then you must NEVER allow it's input pins to be driven outside operating specification range when in use.

Maximum specs are system destruction limits. There is a good possibility that if Vin exceeds operating specs but not absolute maximum specs then the device may functionally misbehave. This can be a one-off problem at the time, or a problem which starts when input is out of spec and continues while input remains out of spec, or may start when Vin is out of spec and continue until the system is fully powered down. An obvious example of the latter is a full IC "crash" where the IC goes gaga and will not recover until powered off.

Out of spec input on an ADC pin may affect readings on other ADC channels - perhaps an electrically adjacent one or all ADC inputs. This may occur while Vin is out of spec but (less commonly but possible) remain in this mode until repowered.

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Worst case, an input that meets Abs max spec but fails operating spec MAY result in IC malfunction even after the IC is powered completely down, then again powered up and reset. I have seen this happen. The IC may remain non compus mentis for many minutes after being wholly powered down. This is caused by charge being injected via the "protetcion" diodes into nodes that are not normally driven. I have seen this happen in ICs used in commercial designs (including mine :-( ) such that special anti-glitching circuitry is needed to prevent or recover the condition.

Note: Many people will tell you that taking pins to maximum limits during normal operations is acceptable. This is not true if you never want uncontrollable outcomes. If it does not bother you if your system faults, locks up or walks funny then by all means follow the advice of people who say its OK to violate operating specs.

Answer 3

• Always follow the Absolute Maximum recommendations with some margin for input protection.
• Perhaps look for advice in their App Notes.
• Generally ADC inputs have a series resistance and a switched hold Cap. so that over voltage will be current limited to some value but adding too large a series resistance may affect RC sample time constant and cause measurement error.
• Due to CMOS latchup effects, never exceed the supply rails (+/-) as indicated in spec.
• If you want additional input protection then add sufficient resistance that can be clipped by external resistor and TVS diode of suitable value.

There are other factors to consider such as common mode rejection and balanced differential inputs for noise immunity. Ensure your protection is balanced if you have a differential input by protecting both sides equally.

These are just general guidelines. Without details, I can only guess. it may only need a 1K~10KOhm in series with internal/external clamp diode to provide more input protection.

Side notes.

Keep in mind that the best noise immunity comes when the source impedance is low using CM choke filters on twisted pair or coax inputs with good filters on supply, good isolation from noisy sources and good separation of analog and digital ground. SAR type ADC's tend to be more noise sensitive in embedded controllers more than dual-slope integration type ADC's which are slower.