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DAC80508: Using an external reference with the DAC80508

Paul_Frost
Guru 53583 points
Part Number: DAC80508
Other Parts Discussed in Thread: REF6225

Originally asked in this thread:https://e2e.ti.com/support/data-converters/f/73/t/836424

Moving this question to a new thread

Andrzej asked: "

How to connect external reference voltage to this DAC since its output is as default also configured as output?

In DAC8568 chip the internal reference was disabled by default."

Paul: "we anticipated the case where the user wants to use an external reference.  The reference current capability is fairly low (±5mA), so during startup, the internal reference and your external reference will conflict with each other but only with this low current.  The DAC will either sink or source current depending on the voltage of the external reference. As long as the abs max table is not being violated, there will not be any damage.  This conflicted state will exist until the reference is disabled."

Andrzej: "In regards to the reference. It is very strange and unusual solution to connect output to output.

especially given the fact that external source of reference voltage is unknown

e.g. it may be very strong output of OPA or super precision/delicate/expensive external reference voltage source."

  • 0
    TI__Guru 53583 points

    Hi Andrzej,

    This concern is valid, but we feel it is low risk for the following reasons:

    1. While this is an output vs output scenario, the internal reference does not have much current drive capability, so most external references will be able to drive the output to approximately the final value if it can source more than ~5mA.  This state will not damage the DAC80508 and should not cause any other issues other than slightly higher current consumption until the reference can be disabled.  Below is a simplified schematic of the two systems:

    2. The reference input is buffered before driving the DAC resistor ladder.  This has the benefit of creating a high impedance input for an external reference.  This means that that reference/reference-buffer only needs to drive about 25µA of current (when the switch is open).

    This high-impedance input means that a series resistor (RLIM) can be added to reduce/limit the current that will flow between the two references when the switch is closed.  Once the switch is open there will be little voltage drop across the resistor as the input current of the reference buffer is very low.

    Finally, we do not really expect external references to be used on this device (though it is possible).  We invested a lot of design time to make the reference of the DAC80508 very good (with low drift, noise).

    In addition, the REF-DIV and GAIN features gives the output range of 1.25V, 2.5V, and 5V, so that covers most ranges for low voltage applications.  

    I suspect that if you are looking at an external reference, then you either want a very specific output range (eg. 4.096V, 3.3V) or you want even better drift performance (REF6225).

    Thanks,

    Paul