T1042 Analog Domain Filtering

[layneberge]

Hi,

I am currently working with a design which uses a T1042 processor.  In designing the analog domains, I have run across an oddity in the analog domain filtering.  The datasheet for the T1042 processor requires the AVDD_PLAT, AVDD_CGA1, AVDD_CGA2 and the AVDD_D1 analog domains to use an RC filter with a 5 ohm series resistor and two caps in parallel: a 10 uF and a 1.0 uF.  Each domain pulls around 20 - 40 mW from a 1.8 V domain.  With the required amount of power for each domain, this either comes near or exceeds the maximum tolerated DC droop of the domain in the filtering resistor alone.  This leaves me without any IR loss budget for each domain.  Is there a different recommended design which does not use such a large series resistor?  Perhaps there is a method to design the filter using a much lower DC loss component, such as a ferrite bead?

[LPP · Accepted solution]

Don't need to worry about the drop across the filter resistors. We are aware of the voltage drop and it is in fact intentional. This is why we spec the AVDD voltages at the inputs to the filter, not at the AVDD pins themselves. Note that we do not spec what the voltage drop will be; though you could estimate it based on the info in the HW spec, it's not very important from a customer's point of view. Our advice on this is always to just use the filter we recommend and everything will be fine.     

> using a much lower DC loss component, such as a ferrite bead?

Parameters of the filters are proven by Freescale. We do not recommend to alter resistor and capacitor values essentially.

Ferrite bead filter could be used but requires special verification. We havent check this type of filters and we do not provide recommendations in this way. In particular, LC filter with ferrite bead may show resonance in the target frequency range and thus it may amplify the noise at this frequency.

Have a great day,
Pavel

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