DAC and 4-20ma output

What is the resolution of the DAC generally used in temperature
controllers for a 4-20ma output ?
I have made a temperature controller using PIC16f877a taht gives gives
either relay outputs or pulses.Now i want to give 4-20ma output as
well.So i need a DAC to convert the o/p from the PIC.
I was looking at AD420 which has a resolution of 16bits.However, in
such apps where the proportional band is around 20degrees, i think i
dont need so much resolution.A 8bit DAC iwth even a 5V reference will
give me 19.5mV, which is more than i need.Is that correct ?
I am confused about this whole DAC thing.Please help.

Thanks.
.

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