31/32 VADIN limit (e3863) still applicable to newer K60 revisons ?

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31/32 VADIN limit (e3863) still applicable to newer K60 revisons ?

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mikeluetzner
Contributor II

After two ADC damages in a well-filtered MK60DN512VLQ10 (4N22D) we are trying to chase the root cause. Possible candidates:

1) VADIN cannot tolerate VrefH
The last K60P144M100SF2V2 (6/2013) data sheet still requires a differential input of no more than 31/32 of VrefH on VADIN.
This was required together with e3863 for the older mask 4N30D:

Could an input voltage of VrefH degrade/damage the ADC in 16-bit differential mode and not only give a conversion error ?
Having seen no conversion errors on the 4N22D we used this mode for the full 3.3V (ADCx-DPx =
VrefH = VSSA , ADCx-DMx = Ground) input.
Please see forum post [https://community.freescale.com/message/593170?et=watches.email.thread#593170] for damage description and circuit schematic.

2) Analog inputs not to be grounded
It has been a requirement for many years to ground open analog (only) inputs. Lately, for instance with forum post (https://community.freescale.com/message/572445#572445) and AN4747, it was suggested to leave them open. Having grounded all ADCx-DMx analog inputs we wonder whether this could be a damage cause?

3) Excessive Vref filtering
In our circuit we tried to provide good filtering for Vref & VSSA
and might have overdone as forum member Chip Weller suspects.
While we definitely want to follow the advice, we are unsure about the best ADC solution.

We tried to follow the TWR-K60N512 schematic here (see Analog Supply TWR-K60N512.JPG). Now we see just a 0.1uF filter on the FRDM64 board (see Analog Supply FRDM-K64.JPG).
Question: Is the K64 ADC so much better that it doesn't need the extensive TWR-K60 filtering or has this TWR filtering caused unwanted side effects ? Would it make sense to switch to a K64 then?

We have stopped assembly of a 200 unit first series lot to hopefully sort out the damage seen in the pre-series units. Quick help would be very much appreciated.

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jeremyzhou
NXP Employee
NXP Employee

Hi Mike,

After had a brief look through the sch, I don't think the filter configuration is the root cause of this issue.

And I'd like to know the MCU under what specific situation can cause the ADC break down definitely after you testing.

I'm looking forward to your reply.
Have a great day,
Ping

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mikeluetzner
Contributor II

Hi Ping,

it would probably be easier if we'd know a root cause - we don't know.

This is how they failed:

1) 3 months ago one of the sample run units failed on the programmers desk. Both ADCs dead. This unit was subject to frequent JTAG adapter change so we suspected something coupled in that way (ground loop etc.). Could also have been a slipped probe, so we replaced the CPU and disregarded the incident.

2) Four weeks later, next sample assembly lot, same environment. This time the programmer said it was only one ADC affected but when we tested it both ADCs had the problem that I described.

3) Another four weeks later the customer calls. They moved a preproduction unit (3rd lot) from one machine to another and on the new one it wouldn't read the analog inputs. No problems otherwise in all field tests on 24 more units.

On inspection same problem as the lab units. This unit had only once seen a (spring probe) JTAG for an initial flash, after that we used our Ethernet boot loader to update the firmware.

The design is ruggedized for EMI, it passed emission tests well below home level and immunity tests for RF, coupling and ESD easily well beyond highest industrial levels. Nobody we asked suspects an external cause to the problem. We think it is a processor sensitivity that comes from within the board. So the idea with the reference voltage being (due to the filter) momentarily higher than processor supply is not that absurd. We'd especially like to know the reasons behind that 31/32*VADSS restriction and whether the K64 is more rugged than the K60 - ADC wise.

Thank you for trying to help

Mike

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jeremyzhou
NXP Employee
NXP Employee

Hi Mike,

I've contacted with other AEs about this issue, and we've also not jumped to conclusion until now.

There are not any documents illustrating the restriction that you want to confirm after I checked.

We all suggest that you'd better to make a appointment with the local FAE, we think it's the efficient way to find out the root cause of this issue.
Have a great day,
Ping

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