ADS1015: part failing with unused inputs in a 24v system

Hello Scott,

Thank you for your question.  I'm working on it and will get back to you later today.

Best Regards,

Henry Sarmiento

Hello Scott,

Thank you for your patience.  Answers are shown after your question below.

1. Is it normal for these kind of failures when there is no input protection circuitry?

Answer: The analog inputs have internal protection diodes to protect against overvoltage.  However, if the analog inputs continue to experience voltages that exceed 300mV, then the ADS1015 can be permanently damaged.  The type of failure seen by the user depends on the extent of device damage, circuit configuration, or application.

2. Will input protection circuitry prevent these failures or is there some other possible cause?

Answer: To prevent analog input damage, the analog input voltages should be within the range show by equation GND – 0.3 V < V(AINX) < VDD + 0.3 V.  To aid in protecting against overvoltage, place current limiting resistors on the input lines that will allow up to 10mA of current.  As a general practice, work inside an ESD safe environment.

3. What is the recommended input protection?

Answer: Please, see answer to question 2 above.

4. Should unused input pins be tied to something?

Answer: The unused analog input pins can either float (not-connected) or be tied to GND.

Best Regards,

Henry Sarmiento

Thanks for the info Henry. Besides the lack of input protection, is there any other cause you can think of that would result in a short developing between the Vcc and Gnd within the device? In the latest instance of failures, twice within a few hours, we had back-to-back failures of this kind without any known reason, and the operator was not aware of ever touching one of the input pins with a voltage source.

Hello Scott,

Following the datasheet's specifications for proper input operating levels and input protection against overvoltage, should provide the conditions for the part to function correctly.  Not being familiar with your circuit configuration, it is difficult to give you a complete answer.  Can you share the schematic showing the circuit configuration surrounding the part?

Best Regards,

Henry Sarmiento 

Sure.

From the schematic, I have one pin floating, one pin connected to a current sense resistor, and the third pin connected to a user input expected to be less than 10 volts. The third pin is currently unused, but it is possible (but not likely) it could have been briefly touched by 24 volts.

In posting these schematics, I see what may be the problem. The 2nd input (AIN1) connected to the current sense resistor has nothing limiting the current to it. When I did this design, I was not aware that the input current had to be limited. I expected the PGA to be a high-impedance input.

1. Is the zero resistance input the cause of the failures?

2. Could it cause the device to create a short between VCC and Gnd?

3. I am curious, what is the internal circuitry that has a low impedance load that will draw excessive current?

Hello Scott,

Thank you for providing the schematic.  Please, give a couple of days to respond.

Best Regards,

Henry Sarmiento

Hello Scott,

Thank you very much for your patience.  Please, see answers to your questions below.

Observations:

• AIN1 in the schematic is connected to a 1 Ohm resistor that will drop up to +0.5V based on F3 rating and that is within specification.
• AIN3 being 3.197V with Ain being 10V is also within specification.
• If Ain gets to about 12V, then AIN3 will be exceeding the 300mV limit.
• With Q1 ON, will IOGND go below GND.
• AIN0, AIN1, AIN2, AIN3 are specified to be between GND and VDD.
• If there is high degrees of EMI, then a floating analog input line may be susceptible to this EMI.

1. Is the zero resistance input the cause of the failures?

Answer: You are correct in that the analog inputs are high impedance and in the mega Ohm range.  The concern is damage to the analog input protection diodes if exceeding approximately 300 mV beyond the supply voltage.

2. Could it cause the device to create a short between VCC and GND?

Answer: Due to the complexity and density of the integrated circuit, a heavily damaged analog input due to an overvoltage event will cause a short or open.  We don’t test for types of failure modes.

3. I am curious, what is the internal circuitry that has a low impedance load that will draw excessive current?

Answer: The protection diodes will provide the low impedance path for current due to an overvoltage event up to 10mA.

Best Regards,

Henry Sarmiento