Friday, December 3, 2010

Creating a Differential Signal with a Function / Arbitrary Waveform Generator

Single-ended signals are referenced to a common level, such as ground, that they can share with other signals, so a single-ended signal requires only a single path or wire. Differential signals are made up of a pair of paths that are both dedicated to a single signal at any given time. One path is used at a higher potential than the other. Differential signals do add complexity, since they require two wires instead of just one, but they provide a number of performance advantages over single-ended signals.
Differential signal advantages include better signal-to-noise ratio, fewer timing errors, and less crosstalk. These advantages make differential signals common in applications such as ADC inputs, instrumentation amplifiers, measurement sensors (like accelerometers), and communication signals. When engineers design and test devices that use differential signals, simulating the differential signals for testing can be challenging. These challenges are caused by the fact that most function/arbitrary waveform generators (FAWGs) have single-ended outputs; instruments that can generate differential signals tend to be fairly costly. In this post I will explain two ways to create a low cost differential signal: using a FAWG with some custom hardware and using a 2-channel FAWG
One way to use custom hardware at the output of a single-ended source to create a differential signal is to use a differential amplifier circuit design as shown it the figure.

The resistors in the differential circuit were chosen to achieve a gain value of 1. I set the DC offset to 0 V. When building the circuit, be careful to keep signal paths and wiring as short as possible to keep parasitic reactive affects low for better signal integrity.
FAWG’s with two single-ended channels (isolated from ground) can have their channels combined into a single differential signal channel. To do this, you need to tie together the two "low" or "common" connections of each channel. The "high" of one channel must be used as the high signal path of the differential channel and the "high" of the other channel must be used as the inverse return wire or low signal path, as shown in the figure.

In addition to the two channels, it is also a lot easier to do this on a two-channel FAWG that has channel tracking capability, like Agilent’s 33522A two-channel function/arb waveform generator. This feature gives you the ability to create an inverted mirror image of the output signal from channel one onto channel two, which is exactly what is needed to create a differential signal. Also, this capability means you only have to set up the arb or built-in waveform on one channel and the inverted version of the waveform automatically tracks to the other channel. Without this feature you would have to setup an arb or built-in waveform on both channels and try to output them in sync using triggering.
As an example I measured and captured three signals with a differential input high-resolution digitizer. The example signal we used was a squarewave at 500 KHz. The figure below shows a digitizer screen shot of the signals. The three signals:
  1. Signal in yellow is a differential signal from the output of the differential amplifier connected to the single-ended FAWG
  2. Signal in green is the differential signal output created by the two channels from the 33522A
  3. Signal in purple is the output of the single-ended FAWG before the differential amplifier input


As you can see from the figure there is quite a bit of ringing on the differential signal created with the custom hardware. When I built my diff amp circuit I was careful to keep wiring as short as possible and I provided a large ground plane. Now with further time and engineer effort I could probably further improve the signal integrity of the circuit. But the point of this example is to show you can save test time and achieve better signal quality by using a 2-chan FAWG with tracking to create a differential signal. Also the cost of a 2-chan FAWG is still typically much cheaper than a differential output waveform generator.



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