[hsdd] High-Speed Digital Design Newsletter - - DC Loading
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Thu, 17 Jul 2008 13:54:29 -0700
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<http://www.sigcon.com/images/blueshirtHJsm.gif> 2008 Signal
taught by Dr.
Integrity Seminars
Howard Johnson
JUST ADDED:
<http://www.sigcon.com/seminars/Rochester.htm> Rochester, NY
<http://www.sigcon.com/seminars/SanJose.htm> San Jose, CA
<http://www.sigcon.com/seminars/seminarHSDD.htm> High- Speed
Digital Design: <http://www.sigcon.com/seminars/Rochester.htm>
September 29 - 30
<http://www.sigcon.com/seminars/SanJose.htm> October 27 - 28
<http://www.sigcon.com/seminars/seminarAHSSP.htm> Advanced
High-Speed Signal Propagation:
<http://www.sigcon.com/seminars/SanJose.htm> October 29 - 30
<http://www.sigcon.com/seminars/seminarHSNG.htm> High-Speed
Noise and Grounding:
<http://www.sigcon.com/seminars/SanJose.htm> November 3 - 4
DC Loading
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 11 Issue 04
I crave the unexplained, the unusual, and the downright
counterintuitive sorts of problems that make you really think
about what's going on in a circuit. This issue, I'll tell you
about a driver whose output gets bigger when loaded.
_____
DC Loading
by Dr. Howard Johnson
<http://www.sigcon.com/images/news/11_04pic1.jpg> I just got a
new differential probe.
Whether the probe accurately reports the voltages to which it is
exposed, I do not doubt. I'm willing to assume the probe measures
things the way its manufacturer says, unless it's broken. What I
must always check, though, is the degree to which the probe loads
down or distorts the signals in my system when applying the
probe.
A trivial setup will suffice for this measurement (Figure 1).
Take any driver and run its output directly into your scope with
coaxial cables (no probes). Then, while observing the coaxial
output, touch a probe onto the output pins of the driver. See
what happens.
My setup incorporates an LVDS-style differential driver. The
driver feeds a short (1-in.) pair of microstrip traces. The
traces go out through SMA connectors, then through 24 inches of
RG316 coaxial cables to a scope. At the scope, I connect DC
blocking capacitors ahead of the 50-ohm scope inputs. Then I
display the resulting differential signal.
<http://www.sigcon.com/images/news/11_04Fig1.jpg>
When doing this test, the expected result depends on the relation
between the input impedance of the probe and the impedance of the
circuit under test. Think of the circuit under test as a voltage
generator with output voltage v[source] and output impedance Z1.
When loaded with a probe having impedance, say, Z2, the expected
value of the measured signal should be, according to the
resistor-divider theorem:
V[measured] = V[source] * (Z2 / (Z2+Z1))
[1]
I'm using a LeCroy D600A-AT 7.5 GHz differential probe with a
differential DC input impedance of Z2=4000 ohms,
http://www.lecroy.com/tm/products/Probes/Differential/WaveLink/de
fault.asp.
My 100-ohm differential circuit, terminated at both ends, has an
effective differential driving-point impedance of 50 ohms. That
happens because, from the perspective of the probe, it "sees" a
differential impedance of 100 ohms to the left, at the driver, in
parallel with another differential impedance of 100 ohms to the
right, at the scope. Two 100-ohm loads in parallel together make
a 50-ohm differential driving point impedance.
Given those numbers, I expect this attenuation factor when
connecting the probe:
A[expected] = (4000 / (4000+50)) = 0.988
If my calculations are correct, the expected loading effect of
the probe should shrink the measured signal by only 1.2 percent,
a tiny change. Let's try it.
Figure 2 plots the results when measuring a National
Semiconductor DS25BR100 driver. This high-powered driver is
designed for long-distance transmission applications. It
incorporates transmit pre-emphasis, a feature that is turned off
for this test. The figure shows the results of experiments
conducted with zero, one and two LeCroy probes connected to the
output terminals of the device. All measurements are made using
the coaxial cable setup in Figure 1. The illustration
superimposes both positive and negative signal edges.
Obviously, the probes affect the size of the signal. That's
normal. But what totally, completely surprised me in this example
is the direction of the change. Look closely. As you add probes
the signal grows. Compared to the unloaded amplitude, one probe
enlarges the signal by 6.5%. With two, it gets 13% bigger.
<http://www.sigcon.com/images/news/11_04Fig2.jpg>
What?? I can't believe it. I re-tried the experiment several
times under different conditions. I double-checked the stored
waveform file names to see if I had them reversed. My assistant
looked over the setup. Everything appears right. As far as I can
tell, this signal actually GROWS when loaded.
How can that be? The explanation involves a subtle interaction
between the common-mode loading of the probe and the
differential-mode gain of the driver. There is not a problem with
either circuit; it's just how they happen, in this circumstance,
to work together.
The DS25BR100 employs a feedback control loop to stabilize its
common-mode output voltage (a good idea). The feedback loop
reacts to changes in common-mode loading.
What changes might you expect? Well, let's first consider a
simple 100-ohm differential load. If you tie a 100-ohm resistor
across the output terminals of the driver it provides a 100-ohm
termination for differential signals, but draws zero common-mode
current. That is, if you exercise such a load with a common-mode
signal (same on both sides) no current flows through the resistor
-- it's as if the resistor weren't there. The simple 100-ohm load
draws no common-mode current.
Similarly, the scope in Figure 1, configured with DC-blocking
capacitors, draws zero common-mode current at DC.
The LeCroy differential probe is different. It presents a load of
2K ohms to ground on each side. From a common-mode perspective,
that's a 1K load to ground. This load draws a small amount (1.2
mA) of common-mode current from the driver. As differential
probes go, that is pretty good. Some high-speed probes draw much
more. I think the common-mode current drawn by this differential
probe is causing the waveform amplitude artifacts in figure 2.
To validate my thinking, I measured the common-mode output
voltage from the driver when the probe was removed, using a
high-impedance digital voltmeter, and again with the probe
present. The DC droop under that condition amounted to only about
1 mV, indicating an effective common-mode output impedance from
the driver of approximately 1 ohm. A practical IC driver would
never obtain such a low output impedance without feedback
regulation. So, I conclude that the DS25BR100 incorporates an
internal feedback loop designed to regulate the common-mode
output voltage. (Lee Sledjeski at National Semiconductor confirms
my suspicions).
In practice, when you apply a differential probe to the driver,
the feedback loop inside the driver raises the common-mode gain
to make up for the DC droop caused by the common-mode loading of
the probe. As a side effect, the act of raising the common-mode
gain also raises the differential-mode gain, causing the signal
growth reported here.
To check my assumptions, I loaded the DS25BR100 with a 2.2K-ohm
passive metal-film resistor on each side to ground, drawing 1.2
mA of DC common-mode current. Under that condition, the outputs
GREW. Then I re-connected the same resistors, this time going
from the digital outputs to VCC instead of ground. That
arrangement sources common-mode current INTO the driver, instead
of taking it out. Guess what-under that condition the outputs
SHRANK.
The DS25BR100 is the first case I can recall of a transceiver
whose outputs get bigger when loaded. Not all LVDS outputs do
this. Figure 2 shows the outputs of a TI DL100-44T. I captured
these output waveforms with zero and one probes present (two
wouldn't fit on the circuit). This output, when loaded, shrinks
as expected by 1.2 percent.
I do not mean to imply that one transceiver is better than the
other; only that they are different, and that the difference
affects your voltage margin budget. If you are trying to measure
output levels with any accuracy it's worth knowing that the
DS25BR100 outputs can GROW when probed. That affects your voltage
margin budget calculations, and that's worth knowing about.
<http://www.sigcon.com/images/news/11_04Fig3.jpg>
Best Regards,
Dr. Howard Johnson
_____
New for 2008! We are pleased to announce the addition of a
High-Speed <http://www.sigcon.com/seminars/seminarHSDD.htm>
Digital Design class in Rochester,
<http://www.sigcon.com/seminars/Rochester.htm> NY on September
29-30. Registration is now open with early-bird discounts
available through July 29th. Or come to all three of Dr.
Johnson's courses in San Jose, CA
<http://www.sigcon.com/seminars/SanJose.htm> at the end of
October! A full schedule of cities and dates appears at:
www.sigcon.com <http://www.sigcon.com/seminars.htm> . Use Promo
Code NL78.
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