Grounding ADCs & DACs - PowerPoint PPT Presentation

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

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!

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

WHY NOT USE ONE PIN?

At high current or high frequency theimpedance of the converter leadsprevents the use of a single ground pin

Low current/low frequency convertersoften do have just one

SO WHY MUST THEY BEJOINED AT THE PACKAGE?

Ground noise at X can affect theanalog circuitry of the convertervia stray capacitances

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

SUPPLY DECOUPLING

The supply to the digital part of theconverter must be decoupled to theDGND pin with a low inductancecapacitor having minimum possiblelead and PC track impedance

Digital VDD may be fed from thesystem analog or digital supplies,but should be isolated by a smallimpedance in either case

GROUND RETURN CURRENT

The only current which flowsbetween Analog and digitalsystem grounds is the returncurrent of the digital interfaces

BEWARE OF THE BUS!

NEVER connect a major data busdirectly to an ADC or DAC

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

BUFFER IT

Put a buffer between adata bus and a converter

Even if the converter hasan internal buffer

It minimizesnoise feedthrough

And may improve ADCaccuracy by loweringpower dissipation

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

SAMPLINGCLOCKS

In order to minimize phasenoise (jitter), which candevastate the performanceof a sampled data system,the sampling clock oscillatorshould be built on the systemanalog ground

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 isprobably incorrect

Even the MicroConverter®should be grounded this way

Systemanalogground

BIG DSP DEVICESWITH CODECS AREAN EXCEPTION

These devices have high(>100 mA) transient currenton DGND and are usuallydesigned to have goodnoise isolation betweenDGND and the analogcircuitry – they should haveDGND and AGND separately grounded unless thedata sheet says otherwise

IN CONCLUSION

If in doubt – join AGND & DGNDand connect them tosystem analog ground