about current steering DAC

[yxk]

current steering dac

Hi,guys:

can some one tell the advantage of adding a buffer or a filter after to current steering DAC's output.
why not only resistor load?

thanks

 

[krivan]

current steering dac output

Hi,guys:

can some one tell the advantage of adding a buffer or a filter after to current steering DAC's output.
why not only resistor load?

thanks

I would say that is because after the different current sources [mirrors] you have a high-impedance point -> buffering/opamp needed, and you produce a current but you may convert it to a voltage at the output, so the configuration of the opamp with the resistors plays a transimpedance amplifier role there.

Check this out:

**broken link removed**

Hope it helps.

 

[yxk]

advantages of current steering dac

thank you krivan!!

another question to you: can you tell me how can i choose the specification of the low pass filter of the DAC output. how to choose the bandwidth or gain of the amp according to the specification of the DAC.

can you recommond to me some other papers!!

best regards!!

yxk

 

[winsonpku]

This is really good question.
Who can help us?

Hi,guys:

can some one tell the advantage of adding a buffer or a filter after to current steering DAC's output.
why not only resistor load?

thanks

 

[user0088]

Re: advantages of current steering dac

thank you krivan!!

another question to you: can you tell me how can i choose the specification of the low pass filter of the DAC output. how to choose the bandwidth or gain of the amp according to the specification of the DAC.

can you recommond to me some other papers!!

best regards!!

yxk

Hello all,
Please share your opinion in this regards, i.e. how to define specifications (dc gain, bandwidth, settling time, etc) for OA used in DAC's Filter.

Thanks in advance.

 

[varunkant2k]

how to choose the bandwidth or gain of the amp according

Do you know the purpose of low pass filter in DAC? Do you know the Low pass filter uses? I guess, once it is clear, you may not listen these questions. Anyway , I will start something.
Low pass filter can be used as averager/integrator, if its time constant is 5-10 times of inversion of input signal frequency. So your BW should be at least 10 times higher than max desired input signal frequency.

If you do'nt give gain (to filter, here passive filter) ideally it will pass the signal without attenuation, but in real it does not, and if higher bandwidth/linearity required, active filters can be used. Now, gain it is something, related with your specification.
Gain is chosen with taking account of resolution, offset and stability.
So Gain= (thumb rule) >60dB, BW, 10 times of input frequency.

 

[lamoun]

Using an opamp you can set the output common mode.
Also, fluctuation of the converter's output voltage give rise to second order effects, using an opamp will fix the DACs output voltage, thus increasing the dynamic characteristics of the converter.

 

[user0088]

Using an opamp you can set the output common mode.
Also, fluctuation of the converter's output voltage give rise to second order effects, using an opamp will fix the DACs output voltage, thus increasing the dynamic characteristics of the converter.

What kind of effects do you mean?

 

[lamoun]

Well, a current steering dac is essentially a current source.
Because the output resistance of this current source isn't infinite, the output current changes with output voltage (a little bit)
This changes will increase the the amplitude of the harmonics in the output spectrum, leading to minimized SNDR and SFRD.
Of course the opamp you add, must have better characteristics than the DAC itself!

Using an opamp you can set the output common mode.
Also, fluctuation of the converter's output voltage give rise to second order effects, using an opamp will fix the DACs output voltage, thus increasing the dynamic characteristics of the converter.

Also you can drive larger loads and you can add some gain in the output voltage.