Hello,
I plan to work with the ADS114s08 as a remote temperature monitor, using thermistors which are spread in an industrial machine and located in distance (<1m).
I will use the following topology:
My questions are:
1) Do you recommend to add protection devices on the A/D input (to suppress spikes)?
2) If yes what type of devices do you suggest so it will have minimal impact on the measurement?
Thank you,
Ohad
Hi Ohad,
Welcome to the E2E forum! This is not necessarily an easy thing to answer as I have no knowledge of the operating environment. I can make a guess that there may be some EMI issues. But if you place devices on the input path you can also create error if the devices are prone to leakage.
Most thermistors are relatively high impedance which means the current used may be small. So noise in general may be an issue. You may want to add a Schottky at the IDAC source pin to prevent large voltage spikes from driving back into the ADS114S08.
As far as the input pins are concerned, you only want to add the protection that is required to protect the ADS114S08 from damage. Primarily you need to make sure that no more than 10mA of current pass into the input pins. This requires the maximum input voltage to be limited to a specific voltage and use resistance in the input path to limit the current in over-voltage conditions.
The input protection can consist in any or all of the following shown in the diagram below. I emphasize the devices should be low leakage.
Going from left to right, there are TVS diodes (such as TI TVS bidirectional series) followed by ferrites, resistance and a diode array (such as BAV199 which is low leakage compared to BAT54 which is high leakage). The right-hand side is normal RC filtering for noise and antialiasing. The components and values chosen depend on the conditions that can possibly take place. Board space and cost often limit the devices that can be placed on the PCB.
Best regards,
Bob B
Hi Bob,
Thank you for your detailed response, you are right I concern mainly with EMI injected noise.
Regarding the Diode array (e.g: BAV199), Could I count on the ADS114S08 internal integrated diodes for this protection, see below:
Or you advise to add the extremal diodes as well ?
Thank you.
Best regards,
Ohad
