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K60 ADC damage despite input protection

Question asked by Mike Luetzner on Nov 30, 2015
Latest reply on Dec 2, 2015 by Mike Luetzner

We see ADC failures on meanwhile the third unit with a K60 (MK60DN512VLQ10 - 4N22D) from different pcb lots. They return gross stepping values from a linear voltage ramp on the input. There are no failures (yet) reported by the customer, the problems all appeared during software development.

The board already underwent overload tests on the inputs of -24 to +48V and full EMI testing (4kV burst and 30kV ESD) without failure.

We are using the ADC0 & 1 inputs ADCx_DPx for measuring 0..10V (AIN3..4 in our schematic) and 10..100kOhms resistance (AIN 1..2)  and the K60 temperature sensor.
The ADCs are set to 6,25MHz, 16 bits, DIFF, Vref=3.3V, AV=32.
DAC0 drives a 4..20mA output circuit and continues to work correctly on the faulty boards.

Faulty stepping response from a ascending input signal (ADC Mode for this Test: 16Bit, DIFF=Off, REF=ALT)

Differences in register states (Temp Sensor, 16Bit, Diff, REF=3.3V) ("K60ADC-good-registers.jpg" and "K60ADC-faulty-registers.jpg")

ADC part of the schematic ("K60ADC-schematic.jpg")

In our overload tests we have seen the following currents and voltages which all should not lead to a circuit damage.


U_AIN3_4: -24V (20x 1s):
U_SP721:     -0,69V
UinADC0_DP1: -0,561V
IinADC0_DP1: -130uA

U_AIN3_4: +48V (20x 1s):
U_SP721:     +4,08V
UinADC0_DP1: +4,00V
IinADC0_DP1: +80uA

U_AIN1_2: ESD Contact +/-2KV (20x) : OK
U_AIN3_4: ESD Contact +/-2KV (20x) : OK

Hence we have no idea where the faults have there origin. Any ideas?