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Re: [Discuss-gnuradio] frontend4702 schematic and layout

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] frontend4702 schematic and layout
Date: Mon, 23 Feb 2004 22:48:29 -0800
User-agent: Internet Messaging Program (IMP) 4.0-cvs

> The MAX518 needs a +5V supply.  Matt and are currently discussing the
> right way to do this.  In the mean while, you should probably use the
> +5V from the connector.  Ooops.... using +5 will mean that you won't
> meet the SDA/SCL VIH_lo spec (0.7 VDD == 3.5V).  Hmmm, looks like
> this needs a different solution.

Should we add to the spec that all I2C devices have to be 3.3V?

> Maybe you could run a single supply op-amp configured with a
> non-inverting gain of 2?
> Then use one of the aux
> ADC outputs available on the connector as the control voltage for the
> input of the op-amp.

This is the best way to do it.

> Put a small linear regulator on board to
> generate, say 9V, from your 33V supply.

I wouldn't do this.  First, that 33V supply is very low current.  Second, there
really is no need to do that.  You can just use a single-supply rail-to-rail 5V
opamp.  That will let you send 0 to 5V to the AGC input, and will have the
required drive strength.

> I'd also add some bulk bypassing for the +5V.  We don't want your
> oscillator feeding back to the other boards.

Yes.  All boards should bypass all power supplies which they use.  If you draw
from 5V, use a 10 uF tantalum, a .1uF and some 1000pF caps onit.  If you draw
from both DVDD and AVDD, do it for each of them.  And don't join the digital
and analog grounds.

The example schematic I posted doesn't do this because there are no active
components other than the 24LC025, which is basically only used at power up and
is extremely low power.


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