Hi all

Ive been scratching my head over this for a while. I have an analog input circuit, using an ADC1208 ADC. There is some filtering on the input, and the first part of the circuit has a simple voltage follower amp, ie signal into + of op amp, - connected to Vout. +/-12V supply to op amp.

My analog signal in is obviosuly the range 0 - 5V, where im using the full 12 bit resolution of the ADC. My signal can go very low, right down to below 100mV which i need to detect.

What i need is some protection on the input to avoid the condition of +24V being inadvertantly applied to my analog input. And have it not interferring with the normal operation of the ADC at low voltages.. I tried a transistor and zener diode circuit but it obviously wont pass low voltages. I need the protection because the device is being installed by muppets in industrial situations and they are good at making cock ups...

any simple diode arrangement that may work?

 

I can't find a datasheet for ADC1208. Can you post a link?

 

I like the idea of an op amp buffer. Use a rail to rail type. Power it from the max input to the adc. Amplify and level shift as necessary. If you think you can not get it to drive all the way to the positive rail use a slightly lower voltage for the adc reference voltage, this normally will not change the max, min input voltage. The whole thing provides the low impedance most adc's require.

 

sorry my bad, ADC128SO22 is the full device part name of the ADC.

russ, do you think using this buffer will give me the protection i need as the output cannot swing above the op amp supply, yet the op amp input will handle large voltage on its input?? or do i misunderstand?

 

oh and the existing supply to the op amps is +12V, i like the idea of trying to find a rail to rail device so i only need to supply +5V...

 

sorry my bad, ADC128SO22 is the full device part name of the ADC.

A Google search for that finds nothing. As I suggested before, you should provide a link to the datasheet.

 

oh and the existing supply to the op amps is +12V, i like the idea of trying to find a rail to rail device so i only need to supply +5V...

A rail-to-rail op amp is the simplest solution, and they are available with R-R input and output, but you will lose quite a few LSBs on each end of the range.
You can use active clamps, but a datasheet will tell us how much protection we need to provide.

 

Jim.
Most op amps cannot take voltage above the rails, use a voltage divider to get in range. Add reversed diodes to the rails ( after some resistance ) as further protection. Or in an inverting circuit the op amp does not even "see" the input voltage. You should do some research on op amp circuits google or: http://www.opencircuits.com/OpAmp_Links

Loss at the rails may happen, generally more of a problem at the positive side, that is why you may want to change the reference voltage so the op amp need not drive to the positive rail.

 

hopefully the link below works for the ADC

http://www.farnell.com/datasheets/86060.pdf

thanks for the pointers so far, i will go hunting. As a test on my bench i tried using an old LM358 with +5V supply, set up in non inverting fashion. Output seems to follow +Vin well until about 3.5V. I can then wind +Vin right up to +24V with no effect at the output and seemingly no damage to the amp... The spec for this particular amp is a maximum Vin of 30V. So maybe you a right, drop the Vref at the ADC and work this way...

 

What is the bandwidth of your input data? What clock rate are you using?
The choice of op amp depends on the answers to these questions.

 

im gonna be running at the lower sampling rate, we do not require high speed. something like 50ksps or little above this.

input signal full range 0 -5V

is this the correct information?