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DAC121C081: Responds When Not Addressed

JULIA TRUCHSESS
Intellectual 310 points
Part Number: DAC121C081
Other Parts Discussed in Thread: ADS1115, , DAC121C085

Tool/software:

A DAC121C081 and an ADS1115 share my I2C bus. The DAC is the SOT-8 variant and has A0 connected to ground, so its address should be 0x0D. The ADS1115 has its ADDR pin connected to ground so has an address of 0x48. The DAC responds correctly to a write and outputs the expected voltage, but when a subsequent write is made to the ADC the DAC responds to that as well, even though the command is not addressed to it. In the first screenshot the DAC is written with the code for ≈1V. About 3ms later (second screenshot) a write to the ADC's configuration register where the first two bytes are 0x01C9 causes the DAC to output ≈0.56V, which is what I'd expect it to output if 0x01C9 were written to it. Each transaction is properly framed with valid Start/Stop sequences as shown by the green and red brackets in the scope display.

  • +1
    TI__Guru* 97465 points

    Julia,


    Unfortunately, this is part of the design of the device. All DAC121C081 (and DAC121C085) respond to a "broadcast address" of 0x48. This means that all devices of this type will respond and update synchronously to the broadcast address. This is discussed in the data sheet on page 18.

    It's also listed on the bottom of Table 1, with the other non-broadcast addresses on page 19. To use this device, I'd set the ADS1115 to a different address not listed in Table 1.


    Joseph Wu