| ??? 11/05/04 15:24 Read: times |
#80468 - RE: Excellent Reading for ADC Grounding Responding to: ???'s previous message |
Ashish said:
To interface it with a noisey micro-controller environment, we can put an i-coupler such as an ADuM1402 and have a complete isolation between the Analog and Digital grounds. I agree, but not with the phrase 'complete isolation'. It's often overlooked that these 'isolators' do only isolate DC currents. But for higher frequencies there's always this annyoing stray capacitance, which stops this isolation and allows digital noise to traverse the barrier. It's worth also to have a look at pin-to-pin cpacitance of such isolators. And to choose one which shows very low values! ADUM1402 shows pin-to-pin capacitance of 2.2pF, if I remember correctly. This gives an impedance of about 720 Ohm at 100MHz. And if a parallel accessed converter is used, this impedance decreases by a factor of about ten. So you end up with an impedance of about 70 Ohm at 100MHz between the two sides which should be isolated by AFUM1402... In such cases, it can be useful to switch to converteres which are serially accessed and to use a self built optcoupler isolator using 6N137, for instance. 6N137 shows only 0.6pF and with careful layout and/or shielding measures precision of converter can be maintained. Yes, in such a case, analog and digital ground pins of converter are connected to analog ground plane. But don't make a mistake, very careful filtering and layout is still needed, when the precision shall not be degraded... Kai |
