Grounding ADCs

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)
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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!
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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
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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
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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
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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
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Grounding ADCs DACs
GROUND RETURN CURRENT The only current which
flowsbetween Analog and digitalsystem grounds
is the returncurrent of the digital interfaces
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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
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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
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Grounding ADCs DACs
SLOW DOWN!(If you can) Fast logic edges at
aconverters digital portsare a source of
noise Slowing them down withRC networks
canreduce this noise But system timing maynot
allow it take care
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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
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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
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Grounding ADCs DACs
BIG DSP DEVICESWITH CODECS AREAN
EXCEPTION These devices have high(gt100 mA)
transient currenton DGND and are
usuallydesigned to have goodnoise isolation
between DGND and the analogcircuitry they
should have DGND and AGND separately grounded
unless thedata sheet says otherwise
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Grounding ADCs DACs
IN CONCLUSION If in doubt join AGND DGNDand
connect them tosystem analog ground