LTC2662 - Troubleshooting

Question

We are trying to use the LTC2662 but we are experiencing some problems. We are able to command some current through SPI (between 0mA to 20 mA) to the ADC 0 but when we command larger current that 20mA it drops close to zero and any command that we send doesn't respond IC doesn't respond except Power Down (the current drops to zero). 
 
 At the moment only the VDD0 is connected to 12 V and a resistor of 120 ohms is connected in the low side (pin 24). All the other VDD are floating. Is that ok? All the bits from the fault register that is sent by the ECU is equal to zero for every message that we send through SPI. We were considering deactivating all the protection modes through the config command but we are a bit afraid to damage the IC because we don't know what it is the root cause of the problem. 
 
 Has anyone faced this problem?

Thanks in advance,

Jaume

ADIApproved · Accepted Answer

Are you sending data in 8 bit chunks or in 24 bit chunks? If you are using 8 bit chunks you are programming the part incorrectly. Every time chip select comes high the part is programmed. So 8 bits are shifted in and that command is executed, then 8 more bits the part is executed again, then eight more bits and finally the code you want is in the DAC register and is programmed. So at some point I suspect you are inadvertatly shifting in a code that is putting the part into a mode you aren't expecting. I would make sure you are shifting in 24 bits at a time, not 8.)

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LTC2662 - Troubleshooting