[SOLVED] The voltage and current reference in ADC/DAC design

[jiangwp]

the reference

When ADC/DAC is designed, a precision reference which has more accuarcy than the resolution of DAC/ADC is need.

In the voltage mode , the reference is voltage which can be implimented by BandGap.

But in the current mode, a reference voltage is transferred to current reference.

Therefore, the reference voltage must be supplied a large current.

So i think whether a LDO is needed for reference voltage or not.

If it is not like this , what way is to impliment a high accurate voltage which can

supply high current capability?

 

[Vamsi Mocherla]

Re: the reference

Well, for current references there have been instances of using other external references. The advantage with a current reference is that we can control the value by changing the resistor across the reference. So, normally it should not matter whether we have an LDO or not . You can also use a simple bandgap and the can control the current externally by comparing the BGR value and the voltage across the resistor. So, it also acts as an LDO.

 

[jiangwp]

Re: the reference

If the reference voltage is a fraction of the external voltage source, how are the internal reference get?

And, the reference noise will be folded into the useful signal that results in the degradation of the SNR+THD. So how to generate a low noise and high driver capability reference for the ADC/DAC?

If the DAC/ADC is use as a part block in a system which has a signal supply, how to get a external reference for ADC/DAC with high accuracy?

 

[Vamsi Mocherla]

Re: the reference

Well,if you want a lower noise and very accurate reference, then you have to go for an internal reference. External references are not as accurate. In case, there is a reference voltage which is higher than the required one, you can use a resistor divider to get the required value. Also at the output of the reference, you can put a huge capacitor so that the high frequency noise is filtered out.

 

[jiangwp]

Re: the reference

I think the resistance divider is the easiest way ot get the reference. but there

are four disadvantages for it.

1. the resistance is noise source which effects the reference accuracy.

2. If the reference is used to supply current to DAC, even a stability large

capacitance is used, i think the reference will be ripple which degrades the

accuracy of reference .

3. maybe the resistance is needed to have good temperature cooeffient and

voltage cooeffient.

4. the large capacitance may implimented with MOS, but the value of capacitor of

MOS will changing as the refernce changes. So the KT/C of resistance and

capacitance will change.

 

[johnyaya]

Re: the reference

Check out Linear Tech's LT1019 series of voltage reference. There is an easy current boost schematic shown in their docs that I have used many times.

 

[Vamsi Mocherla]

Re: the reference

If you are not using the reference for a very high frequency application, you can do the division using caps. The best way is to use MiM capacitors. You will also not have t he thermal noise problem as in the case of resistors. Well, the power is also brought down.