From: Ryan Gammon
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Subject: Sorting out bitscope schematics
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Date: Mon, 07 Oct 2002 23:55:08 GMT
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I've been trying to sort out the schematics of the bitscope
(http://www.bitscope.com/design) and I have a few questions that I was
hoping someone would be able to help me with. I've tried to answer my
own questions as best I can, but some of these answers are probably
A. Power supply
My question here relates to the inductor separating the digital ground
from the analog ground (see the analog power circut schematic).
I understand the role of the inductor in keeping current spikes from
digital switching out of the analog side of things -- for a large di/dt,
the inductor will effectively separate the two grounds with a large
I've also seen this done a different way at:
http://www.pjrc.com/tech/mp3/sta013.html figure 6 cs4334 chip
... where the inductor is placed on the 5V rail instead of on the
ground, and a 10uF cap is used instead of the additional set of regulators .
Q1. Why don't ceramic bypass capacitors take care of this? Don't they
"fix" noise? Don't inductors suck in terms of parasitics, as compared
A1. The ceramic bypass cap does a good job of keeping voltages constant:
i = C dv/dt => It's good at instantaneously supplying current (if the
voltage sags because of, say, inductive wires and an ic switching). It
cannot instanteously change its voltage (without an infinite current,
So what happens when digital inputs switch from, say, a 1 to a 0? A
bunch of current gets dumped into the digital ground line. Some of this
comes from the brief period when both the nmos and pmos transistors of
the driver are simultaneously on, and some comes from the discharging
the capacitance of whatever is being driven. This current, combined with
the natural inductance of the ground wire, creates a voltage drop along
the ground line, called ground bounce. This ground bounce is the noise
that we're trying to block with the inductor/rf choke.
In a perfect world, we want capacitors to remove the resulting bounce.
To a capacitor, ground bounce looks a lot like a voltage sag (it now
wants to have a smaller drop across it), which makes the capacitor want
to discharge. It can do this in one of two ways:
-Remove charge from the top plate
-Add charge to the bottom plate.
The latter implies that the capacitor will soak up some of the current
spike that's causing the ground bounce. It cannot totally get rid of the
spike, however -- the current is divided according to the impedances of
the capacitor and the impedance of the natural inductance of the ground
Now consider an inductor: Faced with the same current spike, an inductor
will create a voltage to oppose its flow, and this attenuates the
v = L di/dt. The bigger the L, the greater the generated voltage will
be, and the more attenuated the current spike will be.
A big L will, however tend to make analog ground float farther away from
In conculsion, bypass capacitors should help get rid of noise just like
inductors. Bypass capacitors need ground bounce to work, however,
making their effects dependant on the inductance properties of the wire
in the circuit.
An inductor, on the other hand, works directly on the current spike, and
is therefore independant of the properties of the wire. That said, the
voltage change from the ground bounce will affect the inductor's ability
to block noise, and so the whole capacitor vs inductor issue is still
rather muddy for me.
Q2. Doesn't a drifting analog ground create voltage noise if the VCC
rail is shared between the digital and analog parts of the circuit?
Say a current spike causes the inductor to drop analog ground to -0.3V
relative to digital ground. The difference between AVCC and AGND is now
5.3V -- in other words, noise. Bypass capacitors filter out this noise.
In the case of bitscope, a second set of regulators are used, so this is
not a problem.
Q3. Does it matter whether the inductor goes on VCC or GND?
A3. No. It "opens" the ground circuit either way.
Q4. What happens if something on the analog side of the design does
something "digital" (ie - does something that will cause a spike in
A4. The inductor will oppose it, and the GND's will drift apart -- the
digital circuit is also isolated from the noise created by the analog
Q5. Getting back to the bitscope schematic, what is the purpose of D12
The digital regulators could cause the voltage out of the rectifier to
sag. D12 and D13 prevent current from flowing out of the analog
Q6. Wouldn't the same be true on the digital side? Why aren't there
diodes in front of the digital regulators?
I don't know.
How'd I do?