Hi,

I am trying to build a 10-bit ADC circuit... and the ADC I have chosen has both analogue and digital power and ground pins. The circuit I have, will be have a DC source and no analogue input. In the ADC datasheet, it says, I can share the analogue supply with the digital.

I am confused as to how to introduce an analogue supply to a system with a DC source. The final application will be powered by battery - so using a seperate analogue supply is not really an option. Is it possible to use the DC power supply rails for the ADC analogue supply as well - or is there something I need to do to convert it to analogue.

The ADC chip I am using is a AD7470 from analog devices.

Any help would be much appreciated.

 

The only design considerations I know of(which may not be all the ones you should consider)...are,

keep your "power" circuits(relays, motors etc) ground traces completely isolated from the digital circuits ground.

Tie them as close as possible, if not directly at the ADC chip.

Use plenty of bypassing on the Vcc pins of any logic devices in the circuit.

You want any surges of current to the power devices to want to flow ONLY to those devices, so you want large, low resistance pathways for them. You do not want any of the surges to be present on the Digital ground so these ground pathways must NOT be common with the power pathways except at the one point.(some chips will have two named pins-one for Analog and one for digital ground).

If you get the grounds messed up in the design you will notice it, but it shouldn't stop the circuit from operating and some amount is going to have to be tolerated by the circuit anyway. Keep them separate except at one point.

 

OK what I am confused about is, the two supplies rails for the ADC one the analog supply rail and the other digital supply rail - what is different about them?

Are they both DC voltages, just taken out from two different Linear voltage regulators? or is one DC and one AC?

also is there a difference between LDO's and linear regulators when using for the DC supply?

 

The point is to keep switching noises associated by the analog and digital separated to avoid erratic responses from the micro.

 

so they are both actually DC voltages?

 

Yes, with different levels of decoupling. Respective to the design.

 

OK what I am confused about is, the two supplies rails for the ADC one the analog supply rail and the other digital supply rail - what is different about them?

Are they both DC voltages, just taken out from two different Linear voltage regulators? or is one DC and one AC?

also is there a difference between LDO's and linear regulators when using for the DC supply?

They are both DC voltages. They can be implemented by using two voltage regulators. It it quite common to separate the digital and analog supply. Then you use a voltage regulator, you need to have an input voltage that is higher than the output voltage. The dropout voltage of a voltage regulator is the smallest possible difference between the input voltage and output voltage to remain inside the regulator's intended operating range. for the LM7805 this is at least 2 volt. But for some low dropout (LDO) the dropout voltage may be as low 0.2-0.3 volt.

 

Thanks for that... all this time I was thinking analog supply meant an AC voltage!

so, I jst need to make sure the analog supply is seperate from the digital supply, which is the same as what will power the rest of the circuit ?

 

Something like tht.
The ground needs to be separate completely but should be connected at one single point only

 

You can power both the analog and digital rails from the same supply, but put an inductor filter between the two, and use bypass caps on each. The digital pin will draw digital currents from the supply, relating to the digital control of the ADC - which you don't want to pollute the analog pin with. If you use two seperate regulators to supply the rails then you have to worry about sequencing of the voltages on startup, and keeping them within a certain range. The datasheet should outline these common design concepts - but possibly 10 bit is not so critical - but it becomes an issue with 12bit and above. Different grounds are also employed - look for an app note describing the normal techniques to use.

Ciao, Tim