How to decide output current in dac?

[surno]

Hi. I design 7bit 1GS/s DAC that is used in DDFS. I calculate the output current of dac by using output swing voltage and output impedence. If output voltage is 250mV and output impedece is 50ohm, output current is 250mV/50ohm=5mA.

Is this way alright?

 

[sunking]

it is not right.
what is the load impedence?

 

[surno]

The load Cap. is 5pF.

 

[sankudey]

If your DAC is a voltage output DAC; you have to design a I to V converter too. In this case you have to set the spec. yourself and the output impedence need to be taken care by the I to V converter.

For a current output DAC; the spec should be given by the user. Definitely the the load is not going to be purely resistive. It should be a parallel combination of Resistance and capacitance. In that case you can calculate back from the load. Mind that according to the step (LSB) and speed ; the DAC output current should be able to drive the above load.

hopw would help..
sankudey

 

[Btrend]

the equations u need:
(1) time constant of load (τ) =Rload*Cload=50*5=250 ps
(2) for any N-bit resolution converter, the following equation must be hold.
2^N*LSB*exp(-t/τ) < LSB/2
==> t > τ*(N+1)*ln(2)
==> 1/f > τ*(N+1)*ln(2)
(3) in ur case, f=1GHz, τ=250ps, ln(2)~=0.7, ==> N < 4.8
So, in theorem, u will never achieve 1GHz operation with 7 bit resolution
(4) to increase operation, u should decrease τ first, either Rload or Cload should
be decreased, after that Rload is calculated, then from your full scale (FS) spec. u can calculate the required current = FS/Rload

 

[surno]

thank you for your kindness. ^^

 

[surno]

Dear Btrend. I don't know the meaning of full scale spec. Please explain that the word in detail. ^^;;

 

[sankudey]

Hi,
Full Scale menas when the input is swingiging full scale, i.e., for DAC 000..00 to 111...11 etc. In this case the output will also go full scale...
sankudey