How to protect ADC inputs ?

I have an ADC with 8 multiplexed inputs (0-5v range) . now, how can I protect my circuit from inputs greater than 5 volts and inverted inputs without altering the level of the entrance signal ?

Thank you
leto.

Re: protect ADC inputs

For each input add a resistor in series. Then check the specification and circuit of your ADC. Many ADC's have clamping diodes built in. If not, then add 2 diodes to clamp to +5 and Gnd.

You'll need to analyze what effect the series resistor and diode leakage will have on your ADC accuracy. The resistor must limit the current to the maximum allowable current for your ADC given the maximum voltage expected.
This will limit the input voltage to about +5.7 and -0.7 v. If these values are within the maximum allowed by the ADC, you're done.

If not, you may need to use Schottky, hot carrier or other lower voltage diodes. These typically have low reverse voltage thresholds so you need to check that carefully.

Just as a point of interest:
Some older IC technologies are susceptable to latch-up which means that if an input exceeds the supply voltage and allows excessive current, the entire chip will latch-up, drawing lots of power and heat and possibly destroying itself. Even newer IC's can be destroyed with excessive overvoltage.

Re: protect ADC inputs

two methods

use cmos analogue mux/demux chips like 405x family

or use a current limit resistor usualy 150R - 330R

i think the adc wont sink much current so this insertion wont be much

if your worried just use the buffer chip
one like cd 4066 is ok then you can enable inputs or not etc...

Re: protect ADC inputs

Hello

You can use several configurations.

One is using an zenner with the adquate voltage.
Other is using two diodes.
The second solution you must be attention witch is the value of the input tension becouse can influence the alimentation of the ic and damage conected circuits.

The resistor in series must limit the courrent in case of overflow tension to ensure correct work of the zener.
You must calculate regarding the tension and max coureent of the zener or diode.
Normal an low value like 180 to 330 ohm



Regards

Re: protect ADC inputs

use esd cascade

Re: protect ADC inputs

Ave_Rapina said:

Hello

You can use several configurations.

One is using an zenner with the adquate voltage.
Other is using two diodes.
The second solution you must be attention witch is the value of the input tension becouse can influence the alimentation of the ic and damage conected circuits.

The resistor in series must limit the courrent in case of overflow tension to ensure correct work of the zener.
You must calculate regarding the tension and max coureent of the zener or diode.
Normal an low value like 180 to 330 ohm



Regards

Click to expand...

The schematic you show with a zener diode has nothing to do with limiting the input voltages. It is used here to set the reference voltage for the A-D converter. It has no effect on the AD inputs.

Re: protect ADC inputs

Yes, I noticed that, about zener.
Now, Im reading the datasheet of M74HC4051 multiplexer and it says:
"All inputs are equipped with protection circuits againt static discharge and transient excess voltage".
would I have to be calm? because there is no details about voltage limits, etc.

Re: protect ADC inputs

newelltech said:

Ave_Rapina said:

Hello

You can use several configurations.

One is using an zenner with the adquate voltage.
Other is using two diodes.
The second solution you must be attention witch is the value of the input tension becouse can influence the alimentation of the ic and damage conected circuits.

The resistor in series must limit the courrent in case of overflow tension to ensure correct work of the zener.
You must calculate regarding the tension and max coureent of the zener or diode.
Normal an low value like 180 to 330 ohm



Regards

Click to expand...

The schematic you show with a zener diode has nothing to do with limiting the input voltages. It is used here to set the reference voltage for the A-D converter. It has no effect on the AD inputs.

Click to expand...

Hello
You are right
It was my mistake
Just conect to the entrance obvius

Regards

Re: protect ADC inputs

leto said:

Yes, I noticed that, about zener.
Now, Im reading the datasheet of M74HC4051 multiplexer and it says:
"All inputs are equipped with protection circuits againt static discharge and transient excess voltage".
would I have to be calm? because there is no details about voltage limits, etc.

Click to expand...

Voltage limits are fully specified; you just have to know how to interprete the spec.
Find the section for Absolute Maximum Limits (or something like that)
There you will find the maximum current allowed into the input, sometimes called the clamp diode current. I think its about 20 mA. Since the inputs already have clamping diodes (for esd and transient protection as you read), you only need to add a series resistor. If you add a small resistor such as 200 Ohms as some have suggested, then your protection can be calculated as Vmax = 5 + 0.02*200 = 9 volts and Vmin = -0.02*200 = -4 volts. So you will be safe as long as no input exceeds 9 volts or -4 volts. If that's not enough protection, you need to increase the resistor until it protects you against the largest voltage expected.

Next look at the power rating which is probably about 500 mW. You will have to make some assumptions here. In the worst case, all 8 inputs might jump to the maximum voltage and you will be adding 8*0.02*0.7 = 112 mW dissipation in the clamping diodes which you will add to the normal dissipation of the chip in your circuit to get the total power. Next, you need to derate the 500 mW depending on the maximum temperature you expect. Finally, you must make sure that the total maximum power dissipated is less than the derated power. Remember this is the worst case so if you can stay within these limits you are very safe and can be calm. If not you either need to increase the value of the limiting resistors, make some less demanding assumptions, etc.
Also, you will need to calculate the offset due to the series resistor. If you're using a 1K and the leakage is 10 uA you will get a +/- 10mV error. If this is within your error budget (and I assume you have an error budget of course), then you can be calm. This would be a 2 bits of error for an 10-bit ADC meaning you might as well be using an 8-bit ADC.
This is probably more info than you wanted, but someone might get something from it.