Grounding ADCs & DACs

Data Converters (ADCs and DACs)are accurate and sensitive analog devices whose

analog ports are vulnerable to unwanted noise(most advice in this lecture applies to both ADCs & DACs)

Mixed Signal Systems(systems with both analog and digital processing)

often have separate analog and digital ground planesin order to isolate their sensitive analog signals fromthe noise which is often present on the digital ground

(READ THE NOTES SECTION OF THIS PPT FILE)

Grounding ADCs & DACs

ADCs & DACsfrequently have separate analog and digital ground pins

(labelled, respectively, AGND and DGND)

These should be connected together andto the analog ground plane of the system

Even if the data sheet suggests otherwise!

Grounding ADCs & DACs

A PHILOSOPHICAL PROBLEM!

AGND and DGND should bothbe connected to the analogground plane of the system

The pin description DGND doesNOT imply that this pin shouldgo to the system digital ground

Grounding ADCs & DACs

WHY NOT USE ONE PIN?At high current or high frequency the

impedance of the converter leadsprevents the use of a single ground pin

Low current/low frequency convertersoften do have just one

Grounding ADCs & DACs

SO WHY MUST THEY BEJOINED AT THE PACKAGE?Ground noise at X can affect theanalog circuitry of the converter

via stray capacitances

This noise can be minimizedby minimizing the impedancebetween DGND, AGND and the

system analog ground

Grounding ADCs & DACs

SUPPLY DECOUPLINGThe supply to the digital part of theconverter must be decoupled to the

DGND pin with a low inductancecapacitor having minimum possible

lead and PC track impedance

Digital VDD may be fed from thesystem analog or digital supplies,but should be isolated by a small

impedance in either case

Grounding ADCs & DACs

GROUND RETURN CURRENT

The only current which flowsbetween Analog and digital

system grounds is the returncurrent of the digital interfaces

Grounding ADCs & DACs

BEWARE OF THE BUS!NEVER connect a major data bus

directly to an ADC or DAC

It is a source of noise and mostADCs cannot drive the load

Grounding ADCs & DACs

BUFFER ITPut a buffer between a

data bus and a converter

Even if the converter hasan internal buffer

It minimizesnoise feedthrough

And may improve ADCaccuracy by lowering

power dissipation

Grounding ADCs & DACs

SLOW DOWN!(If you can)

Fast logic edges at aconverter’s digital portsare a source of noise

Slowing them down withRC networks canreduce this noise

But system timing maynot allow it – take care

Grounding ADCs & DACs

SAMPLINGCLOCKS

In order to minimize phasenoise (jitter), which can

devastate the performanceof a sampled data system,

the sampling clock oscillatorshould be built on the system

analog ground

Grounding ADCs & DACs

THIS GROUNDING SCHEMEIS ALMOST UNIVERSAL

If a converter contains nocomputation, or draws lessthan 30mA supply currentit should use this scheme

If the data sheet suggestsotherwise the data sheet is

probably incorrect

Even the MicroConverter® should be grounded this way System

analogground

Grounding ADCs & DACs

BIG DSP DEVICESWITH CODECS ARE

AN EXCEPTIONThese devices have high

(>100 mA) transient currenton DGND and are usuallydesigned to have goodnoise isolation between DGND and the analog

circuitry – they should have DGND and AGND separately

grounded unless thedata sheet says otherwise

Grounding ADCs & DACs

IN CONCLUSIONIf in doubt – join AGND & DGND

and connect them tosystem analog ground