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There is a differential-output CMOS logic gate, after all

A project log for The Rise and Fall of Pulses

A project in which I try to go faster and faster

Ted YapoTed Yapo 03/22/2019 at 15:391 Comment

Until now, I believed that such a gate was the stuff of myth, like rainbow-farting unicorns. Now that I've finally found the 74LVC2G157, I'm going to take a little closer look at every horse I see, just in case.

So, here's the deal: the Y and Ybar outputs of the 74LVC2G157 2:1 multiplexer are as close to a differential output as you can find. I had previously measured the Q and Qbar outputs of a 74AC74 flip-flop and found them to have pretty low skew, but the fact that it's a flip-flop makes it a little difficult to use as an output buffer. The 74LVC2G157 seems even better, plus being 74LVC logic, has rise and fall times around 500 ps or so.

I designed a lavish breakout PCB to test this part mostly because I didn't feel like soldering the tiny SSOP pins by hand. The board has solder bridge footprints to allow any of the inputs to be connect to ground, +V, or the input signal. It turns out I only needed to test one combination. The logic of the mux is straightforward: Abar/B selects one or the other input to be routed to the output, when the Gbar input is low. The output is provided as Y and Ybar, which can be abused as a differential pair for all sorts of nefarious purposes. This is my kind of part!

By tying the A input low and B input high, the output follows the Abar/B input, producing a differential-output buffer. The output looks really good on the scope: the outputs transition in around 500 ps, and cross at about 1/2 the supply voltage, indicating a relatively low skew. I didn't measure the skew between the outputs directly, but it looks small enough to be useful. I'm not sure where the ringing is coming from; the outputs are probed with 10:1 resistive Z0 probes which should be fine at these speeds. It may be cheap RG174 cables from ebay - I'll have to re-test with some decent RG316 ones I have around somewhere. At least the ringing is relatively symmetrical and balanced.

Zooming in a bit, we get a better look at the transitions:

The traces don't cross exactly at 1/2 the supply voltage, indicating some possible skew, but it's relatively small. This could also be due to some other factors: I didn't check the two oscilloscope channels for perfect alignment, for instance. In any case, this is good enough to do some further work with. It's certainly better than tuning a couple of XOR gates.

Just to put this into perspective, this part is generating a 10 V differential step in half a nanosecond (it could be pushed to 11 V and still be within the recommended range according to the datasheet). This is plenty good enough to create a sampling pulse for an sampling oscilloscope of several GHz, since the diodes will switch on a much shorter segment of the transition. There was an old design for a sampling scope in Electronic Design (nearly 20 years ago now!) that achieved 1 GHz bandwidth with 2.2 ns transitions from the output of a comparator. This transition is nearly 5x faster. That's pretty exciting.

I also have some breakout PCBs for laser diode drivers with 25 ps transitions being fabbed at the moment, but this CMOS multiplexer might enable some incredibly-dirt cheap applications, like maybe a standalone (no oscilloscope required) TDR with spatial resolution in the cm range.

EDIT:

You know, in the rush I designed this PCB, I didn't check that the traces on the PCB are exactly the same length. There probably isn't much difference, but it could be a contributing factor.

Discussions

Dave's Dev Lab wrote 03/24/2019 at 03:06 point

great find!

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