Grounding ADCs & DACs

1. Grounding ADCs & DACs 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)

2. 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!

3. 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

4. Grounding ADCs & DACs 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

5. Grounding ADCs & DACs 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

6. Grounding ADCs & DACs 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

7. Grounding ADCs & DACs GROUND RETURN CURRENT The only current which flowsbetween Analog and digitalsystem grounds is the returncurrent of the digital interfaces

8. Grounding ADCs & DACs 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

9. Grounding ADCs & DACs 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

10. 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

11. Grounding ADCs & DACs 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

12. 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 isprobably incorrect Even the MicroConverter®should be grounded this way Systemanalogground

13. Grounding ADCs & DACs 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

14. Grounding ADCs & DACs IN CONCLUSION If in doubt – join AGND & DGNDand connect them tosystem analog ground