Microcontrollers DAC output is RC filtered.....it's OK?

[treez]

Hello,

We are using the SAM3N00B microcontroller's DAC output to provide a voltage.

The DAC's output voltage goes to an RC filter which is 1K and 100p.
Page 720 of the SAM3N00B datasheet says that the max load capacitance for the DAC can be no more than 50pF.
Since our 100p is not directly connected to the DAC pin, but is after the 1K resistor, will we thus be allright using the 100p capacitor?

Also, page 720 of the SAM3N00B datasheet says that the minimum resistor load for the DAC output is 5K....does this mean that any resistor that shunts to ground from the DAC pin must be greater than 5K?

SAM3N00B DATASHEET:
http://www.atmel.com/Images/doc11011.pdf

 

[upand_at_them]

The 100p capacitor is still a load capacitance, whether it is before or after the resistor. And, yes, the DAC shunt resistor must be greater than 5K.

Why are you using an RC filter on the DAC ouput? That is something you would do on a PWM output, to smooth it. Your values produce a cutoff frequency of 1.59 MHz. What is the purpose of that?

 

[treez]

noise filtering

 

[paulfjujo]

hello,

You must add a buffer to adapt high impedance to low impedance ouput
here is an example..
what is your maximum frequency ?

 

[treez]

there is no frequency, the DAC just puts out a constant DC voltage.
The buffer sounds a bit extreme?

 

[paulfjujo]

Only one buffer is enough..
but it is the only way to get low impedance output ..
What is the purpose of this analog output ?
what device you will connect to it ?

if your receiver has allready a high impedance , just change your filter by
R >= 5K ohms. and 100pF

If output is a constant DC voltage, why a so low capacitor value of 100pF ?
you can use more than 100pf (because after 5K, not directly to DAC output)

 

[treez]

What is the purpose of this analog output ?

it feeds the non inverting input of an opamp, which acts as an error amp......

 

[paulfjujo]

No inverting input, so, input must be high impedance. (Ze)..
you can measure the DAC output voltage without be connected to the amplifier...
and measure connected..
and see difference of voltage due to the impedance value of following amplifier
and decide after what to do ...

Any schematic to propose ?