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