Dear all,
i am using an ADS1256 in my project.
Last days one ADS was destroyed with following malfunction:
The reading of the 2 used channels was only approx. 75 % - 85 % of what it should be.
Then I have measured a voltage drop from 1.25 V down to 1.1 V over the input protection series resistors of 100 Ohm on both channels.
So the input current into one channel is approx.. I= 0,15V / 100 Ohm = 1.5 mA !
The analogue inputs seems to have had a low resistance.
I switched the whole unit off and on again, but still have the same behavior. So it was not a latch-up ?
After unsolder and putting a new ADS1256 everything works fine again without any software change.
Now I am thinking about the reason of what has destroyed the ADS chip.
If I look at the maximum ratings of the ADS1256, the maximum voltage to the analogue inputs is AVdd + 0,3 V.
I am using input protection diodes type BAV199 to +5V and GND, which have a low leakage current to get the best performance of the system.
As these type of diodes are not Schottky, they have a forward voltage drop of 0.6…0.8 V according the data sheet.
So in case of a short circuit of the external sensor, the amplifier will supply +12 V to my ADS1256 input protection circuit. As a result, the voltage on the analogue input pin can be 5.7 V (not measured). Current into the ADS pin is limited to less than 10 mA . I = 0,7V / 100 Ohm= 7 mA.
Schottkey diodes have a lower forward voltage drop to give a better protection, but the leakage current is much more higher.
So my question is, can my poor protection circuit be the reason for damaging the ADS ?
What is the perfect way for good and accurate input protection of the ADS1256 ?
