LIMIT ADC INPUT WITH ZENER DIODE ??
LIMIT ADC INPUT WITH ZENER DIODE ??
(OP)
Hi.
I have a project involving a load cell.
The actual load to be measured is about 20kg, however in order to provide some mechanical protection (i.e. to prevent the load cell getting loaded past its elastic limit, due to operator error), I decided to install a 50kg load cell instead.
The load cell has a 10V excitation voltage, and 2mV per V output.
Since I have already sacrificed some resolution by opting for a 50kg rather than a 20kg device, I want to have the input to my 5V ADC making the most of what resolution I have left. (i.e. I DON'T want to amplify the signal very slightly to keep the output within a safe value for my 5V ADC, and thereby losing any of the ADC resolution available).
To cut a long story short, I want to amplify my signal so that with a full load of 20kg applied, I have 5V on my 5V ADC input pin.
I want to amplify my input signal of ((20kg / 50kg) * 2mV per V) of 0.008V by 625 = 5V at full 20kg load
Since in reality the load cell has a safe overload of 150% and a ultimate overload of 200%, in the worst possible case the ADC would see 25V with the suggested amplifier arrangement! (if the amplifier was capable of outputting 25V, which it isn't, but yet it can easily still output more than 5V).
Nevertheless, my question is, can I simply include a 5V zener diode on the input to the ADC, so that any spikes above 5V would be clipped off?
It seems too simple a solution, so I'm guessing I can't do it?
Would it mess up the linear behaviour of the load cell output amplified signal?
If so, is there an easy way to provide protection for the ADC input?
It can only tolerate 5.5V
Thanks
I have a project involving a load cell.
The actual load to be measured is about 20kg, however in order to provide some mechanical protection (i.e. to prevent the load cell getting loaded past its elastic limit, due to operator error), I decided to install a 50kg load cell instead.
The load cell has a 10V excitation voltage, and 2mV per V output.
Since I have already sacrificed some resolution by opting for a 50kg rather than a 20kg device, I want to have the input to my 5V ADC making the most of what resolution I have left. (i.e. I DON'T want to amplify the signal very slightly to keep the output within a safe value for my 5V ADC, and thereby losing any of the ADC resolution available).
To cut a long story short, I want to amplify my signal so that with a full load of 20kg applied, I have 5V on my 5V ADC input pin.
I want to amplify my input signal of ((20kg / 50kg) * 2mV per V) of 0.008V by 625 = 5V at full 20kg load
Since in reality the load cell has a safe overload of 150% and a ultimate overload of 200%, in the worst possible case the ADC would see 25V with the suggested amplifier arrangement! (if the amplifier was capable of outputting 25V, which it isn't, but yet it can easily still output more than 5V).
Nevertheless, my question is, can I simply include a 5V zener diode on the input to the ADC, so that any spikes above 5V would be clipped off?
It seems too simple a solution, so I'm guessing I can't do it?
Would it mess up the linear behaviour of the load cell output amplified signal?
If so, is there an easy way to provide protection for the ADC input?
It can only tolerate 5.5V
Thanks





RE: LIMIT ADC INPUT WITH ZENER DIODE ??
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
I will have a look at that.
Could you please point to a link showing an image of such, if such is readily available?
I will have a look also, but just in case I misinterpret what you mean.
Thanks
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
But, I certainly agree with using a diode from the input signal to the 5V rail. This will limit the input voltage to 5.7V which will likely not actually hurt the input. If you're chicken to do that then use a Schottky diode as they have only ~0.3V forward limiting the input to 5.3V. They can be slightly problematic since they leak reverse current which varies greatly with temperature.
This is sort what we're describing:
Keith Cress
kcress - http://www.flaminsystems.com
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
I will give that a try later.
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
Well, given that the OP was very selective about describing the circuit, instead of giving complete information (like the Burr-Brown opamp/reference), the answers are correspondingly useless.
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
Link
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
I fixed the issue by incorporating two precision voltage references, operated by some spare microcontroller outputs. The high reference voltage was commanded "on", and fed to the ADC reference voltage pin when necessary to make the high readings. Once the readings were down to the lower range, the ADC reference was switched to a lower value allowing the same ADC to provide higher resolution measurements.
Dunno if that helps, Mrs. Spock. But logical it is...
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
It also doesn't have an amplifier. Instead it has a carefully programmed statistical sampling software on-board.
STF
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
Dan - Owner
http://www.Hi-TecDesigns.com
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
I'm only getting back to this now.
Regarding D2 in the image below, am I correct to presume it is an option?
I mean, am I correct to presume that D1 on its own, (along with the resistor), is adequate to protect against a positive voltage spike?
If I am wrong to think that, could someone please explain the function of D2.
I don't understand it.
Thanks
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
Most integrated semiconductor devices have protection diodes like the ones in the above circuit. Check the data sheets. If they are present then you don't need external ones as long as R1 (above) is large enough to limit the current to below what the internal diodes are rated for.
Keith Cress
kcress - http://www.flaminsystems.com
RE: LIMIT ADC INPUT WITH ZENER DIODE ??
Thanks for the clarification.
Since the positive spike I am predicting will happen, will in reality be as a result of excess load on the load cell, (beyond its planned window of operation), I suppose I am probably using the word "spike" incorrectly, as like you said, it suggests a negative spike might also be in the vicinity. I'm quite confident there won't be a negative spike since this signal is the output from my INA125 amplifier, and as I have it wired, it can't output a negative.
Your solution is a nice simple modification for me to add on, since the circuit is already finished.
It may well be that the excess load never happens, but its good to just make provision and forget about it.
Thanks again.