Grounding ADCs & DACs Data Converters (ADCs and DACs) are accurate and sensitive analog devices...
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Transcript of Grounding ADCs & DACs Data Converters (ADCs and DACs) are accurate and sensitive analog devices...
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