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Choosing a diode for >1.8V protection

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I have an A/D board design that would take max 1.8V at its inputs and I would like to implement protection for these inputs so that whenever the user tries to input more than 1.8V, its shuts off the input to these pins. I am thinking of using a skotcky diode, any good suggestion? The lower the voltage drop across the diode the better it is.

thanks
 

ronsimpson

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What A/D board?
would take max 1.8V
Does it measure 0 to 1.8V? In most cases it can input more than 1.8V with out smoke. What is the absolute max voltage?

Problem: A diode does not have a sharp knee. You can not easy build a circuit that passes 1.8000V but blocks 1.801 volts. I worry that your circuit will cause 1.75volts to not read correctly.

So more information please.
 
Sure, here are the details. The A/D IC is AD9984A, the pins that I am trying to limit the input voltage is the analog input pins. Doesn't have to be exactly 1.8V, I am guessing its related to max power supply on VD which is 1.98V

thanks
 

ronsimpson

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upload_2016-6-30_21-46-31.png
I don't know what Vdd is in your case. The analog inputs should be limited to (Vd to 0V). I think the board has this already. I used an op-amp ahead of the ADC. The op-amp has its supply as Vd and 0V, so it can not output more than its supply.
 
View attachment 100231
I don't know what Vdd is in your case. The analog inputs should be limited to (Vd to 0V). I think the board has this already. I used an op-amp ahead of the ADC. The op-amp has its supply as Vd and 0V, so it can not output more than its supply.
VD is 1.98V in the above table you attached. I already have boards made for now, I am thinking of using video signal level shifters in my next design. But for now, I would like to add something to my current boards (10) to protect the input pins. Hence my question.
 

ronsimpson

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Most application have a 75 ohm resistor to ground. Then a 100 ohm resistor to the analog input. I can't find it in the data sheet but there must be internal clamping diodes. The internal diodes + your 100 ohm resistor will add a level of protection. Using this part, I have not done more.

Are you looking at video with the ADC?
 
Hi Ron,

I have a 75Ω resistor to ground at each of the analog inputs but not a 100Ω series. I followed their eval board schematics as a guide which doesn't have those 100Ωs. There is a 75Ω resistor to the ground and a 100nF capacitor in series to keep the input noise low at each analog input pin. At first I didn't notice that this would be a problem and started using the same input video signal I was using with AD9980 (where the VD for its is 3.3V). In a matter of minutes, it burned down the 1.8V LDO that I have for VD. This is only after connecting one analog input. I am not sure what the consequences would be if I have all three analog inputs active.

thanks
 
Maybe you should only run for 30 seconds until you find where the power is going. Maybe you need more heatsink on the LDO.
Only If I had any idea that it would burn down. Anyways since then I installed a heatsink on the LDO and making sure that my input is not going over 1.8V. That would be hard for someone else other than me using it to keep track of the input voltage.
 

ronsimpson

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That would be hard for someone else other than me using it to keep track of the input voltage.
Normally video voltage is known. There is no reason to apply 5 volts. Often the input is AC coupled so applying DC has no effect.
Probably two diodes in series will conduct at 1.2 to 1.4 volts.

page 40 left side said:
Alternatively, placing a 100 Ω to 120 Ω resistor
between the 75 Ω termination resistor and the input
coupling capacitor is beneficial.
If your input was above 1.8V and current flowed into the 1.8V supply it could latch up the input transistors and cause too much supply current. I don't have time to look at one of my boards but I don't remember adding input protection.
 

Roff

Well-Known Member
If you have to protect against overvoltage, you will probably also have to protect against undervoltage.
Most (all?) CMOS chips have diodes from each input to both rails, to provide electrostatic protection. These are silicon diodes. If you shunt them with low-capacitance Schottky diodes, essentially all of the current due to overvoltage or undervoltage will flow through the Schottky devices. The impedance of your source will probably limit the current to a safe level for the external diodes.
The problem with this type of overvoltage protection is that most regulators will not sink current. Repeated overvoltage may raise the supply pin(s) to a level which will damage either the ADC, or the regulator, or both. You need to provide a path to ground for current that flows through the protection diode. A low-value resistor (100 ohms?) will probably work, but it will waste some power. An active sinking supply such as a PNP emitter follower will work as a supply to clip to, but it takes up space.
Undervoltage protection can probably be accomplished with a low-capacitance Schottky diode to ground.
 
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