原创 Fundamentals of signal integrity(2)

measurements are made using the coaxial cable setup in Figure 1. The illustration shows 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.With one probe, the signal gets 6.5% bigger. With
two, it's 13% bigger.

   What?? I can't believe it. I re-tried the experiment several times under different conditions. I doublechecked 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 tostabilize its common-mode output voltage (a good idea). The feedback loop reacts to changes in
common-mode loading.
    What changes might there be? Well, a simple 100-ohm resistor across the output terminals of the driver provides a 100-ohm termination for differential signals, but draws zero common-mode current. 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 DCblocking 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 commonmode 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.
     You can't get 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. Under that condition, the outputs GREW. Then I connected the same resistors from the outputs to VCC instead of ground (now ourcing
common-mode current INTO the driver) and, guess what, the outputs SHRANK.

     The DS25BR100 is the first case I can recall of a transceiver whose output gets bigger when loaded. Not all LVDS outputs do this. Figure 3 shows the outputs of a TI DL100-44T. I captured these output waveforms with zero and one probes present (two
wouldn't fit). 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.