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!