ADC supply/vref

[Jamie Morken]

Hi,

I am using an ADC that has a quiescent current of 2mA and the voltage
reference chip I am using can supply 20mA so I would like to use it as the
supply to the ADC as well as the vref input. (only have 3cm by 3cm board
area) Would this be a bad idea? The ADC datasheet says that the vref input
can vary from 500mV to VCC, but I don't know if that is true at the uVolt
level?! The vref chip is +-1% at 5volts (+-500uV).

The ADC chip is the ADS8344:
http://focus.ti.com/docs/prod/productfolder.jhtml?genericPartNumber=ADS8344

datasheet:
http://focus.ti.com/lit/ds/symlink/ads8344.pdf


cheers,
Jamie Morken

 

[Bill Sloman]

Hi,

I am using an ADC that has a quiescent current of 2mA and the voltage
reference chip I am using can supply 20mA so I would like to use it as the
supply to the ADC as well as the vref input. (only have 3cm by 3cm board
area) Would this be a bad idea? The ADC datasheet says that the vref input
can vary from 500mV to VCC, but I don't know if that is true at the uVolt
level?! The vref chip is +-1% at 5volts (+-500uV).

The ADC chip is the ADS8344:
http://focus.ti.com/docs/prod/productfolder.jhtml?genericPartNumber=ADS8344

datasheet:
http://focus.ti.com/lit/ds/symlink/ads8344.pdf

It is a bad idea, unless you put unusually effective passive filters
between the A/D converter power pin, the A/D converter reference input
pin, and the Vref ref source output pin, as discussed in the "Layout"
section at the end of the data sheet. I've used ferrite bead or chip
inductors in addition to the the low value resistors suggested, taking
care that the resonance of the LC circuit so formed is well damped.

A successive approximation register (SAR) converter like the ADS8344
incorporates a fast, high gain comparator, which can be sensitive to
small spikes on the power supply or on its inputs, and you can expect
problems if you aren't careful with your layout and decoupling.