One unfortunate thing about the DAC

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One unfortunate thing about the DAC

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NXP Employee
NXP Employee
Content originally posted in LPCWare by IanB on Sun Mar 27 09:04:51 MST 2016
In those few moments whilst the microprocessor is configuring its outputs, it the program has crashed, whilst programming and debugging, the DAC output isn't a DAC output, it's just a standard I/O; and that means it has a pull-up resistor.

There's a bit of a snag with this arrangement, that the analogue circuitry attached to the DAC output doesn't know this, and it thinks the DAC is set to full output.

Anyone know a good workaround? All I can think of doing is using 0V to mean "full output" and 3.3V to mean "off", but that's not so convenient for the analogue circuitry.
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NXP Employee
NXP Employee
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NXP Employee
NXP Employee
Content originally posted in LPCWare by IanB on Tue Mar 29 04:14:47 MST 2016
Thanks.

The DAC output specs are a bit confusing (perhaps the NXP folks could clarify?)

The output impedance is specified as 300Ω, which means that any load <1.2MΩ would cause an error greater than 1 LSB; but the load resistance is specified at 10kΩ, which, in theory, would introduce a 3% error.

The worst-case pull-up current is 85µA, so even into the specified 10kΩ load, would still leave an output voltage of 0.85V; or if more I wanted less error when the DAC is running.

Analogue circuitry with an enable input (especially just transistors and op-amps) isn't that easy to arrange, but looks like the only option. Of course, it would have to be a negative enable, otherwise the same problem with the pull-up appears!
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NXP Employee
NXP Employee
Content originally posted in LPCWare by wmues on Tue Mar 29 01:07:42 MST 2016
- add an external pulldown resistor to the DAC output.
- use another pin as enable for the analog circuity.
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