Oscilloscope and Data Logger Application Notes

Analog offset

Analog offset, also called DC offset, is a valuable feature available on many PicoScope oscilloscopes. When used correctly, it can give you back the vertical resolution that would otherwise be lost when measuring small signals.

Analog offset adds a DC voltage to the input signal. If the signal is out of range of the scope’s analog–to–digital converter (ADC), the offset can be used to bring the signal back in range:

Figure 1: Out–of–range signal

Figure 2: Signal brought into range by analog offset

A typical application: LVDS

LVDS (low–voltage differential signalling) uses a balanced line driven by two signals of opposite phase. The nominal voltages of each signal are as follows:

• Amplitude: 350 mV peak–peak
• Common–mode offset: 1.2 V
• High voltage: 1.2 V + 0.5 × 350 mV = 1.375 V
• Low voltage: 1.2 V − 0.5 × 350 mV = 1.025 V

Figure 3: LVDS V+ and V- voltages

The oscilloscope used is this test is the PicoScope 6404B, a 4 channel 500 MHz instrument with 8 bit resolution. We used a simulated LVDS signal.

Viewing the signal without analog offset

Figure 4: Simulated LVDS signal

The waveform above shows the simulated LVDS signal (one half of the differential pair). We have selected the ±2 V range, which is the most sensitive range that allows the signal to fit on the screen. Although the oscilloscope has 8–bit resolution, equivalent to 256 distinct voltage levels, the signal occupies only a small fraction of this range: 350 mV out of a total of 4 V, or a mere 22 voltage levels. This number of levels means that we are using only log 22 / log 2 ≈ 4.5 bits of the ADC’s 8 bit resolution.

Zooming in on this signal shows the effect of this low resolution:

Figure 5: Zoomed view of low–resolution signal

Using the rulers, we measured the quantization noise to be 16 mV. As expected, this is close to one ADC level: 4 V / 256 ≈ 15.6 mV.

Using analog offset

In the PicoScope software, a drop–down menu for each channel shows all the settings at a glance. We have set the DC Offset to -1.2 V to cancel the common–mode voltage of the input:

Figure 6: Channel Settings dialog

Here is the result of applying -1.2 V of analog offset:

Figure 7: Signal with analog offset

Now that the signal is within 175 mV of ground, we can set the scope to a much more sensitive range, ±200 mV, without saturating the input circuitry:

Figure 8: Offset signal on ±200 mV range

The signal now occupies a 350 mV range out of a total of 400 mV, which corresponds to 224 levels out of 256. We are therefore using about log 224 / log 2 ≈ 7.8 bits of the ADC s 8 bit resolution: over 3 bits more than before. This enables us to make measurements of the waveform with about 10 times greater precision.

Zooming in on this waveform shows a huge improvement in resolution compared with Figure 5 above:

Figure 9: Zoomed view of offset signal

The rulers show that most of the quantization noise now occupies a range of 1.58 mV. Again, as expected, this is about one ADC level: 400 mV / 256 ≈ 1.56 mV, but this time the error is reduced to about a tenth of that obtained with the ±2 V range.

Using AC coupling

On oscilloscopes without analog offset capability, or when the analog offset range is insufficient, it is sometimes possible to use AC coupling to remove the DC offset from the input. This technique works when the signal has a steady DC component, as is the case with ripple on a DC power supply. However, it does not work well for LVDS, because the signals are not DC balanced and therefore do not have a constant average voltage. The average drifts up and down depending on the data pattern, making accurate measurements impossible.

Here, first, is a successful example of the use of AC coupling: a 10 volt rail with some sinusoidal ripple:

Figure 10: 10 V rail with ripple

Zooming in on this reveals the effect of using only a small fraction of the ADC’s input range:

Figure 11: Ripple magnified, showing poor resolution

We can remove the DC offset by selecting AC coupling, which allows us to select a more sensitive input range. Now we can use almost the full resolution of the scope:

Figure 12: Ripple with AC coupling

If we now try the same trick with our LVDS waveform, the result is acceptable if we have a steady data stream. If, however, a burst of data occurs after a long period of inactivity, then the AC coupling capacitor will begin to charge, creating an unpredictable offset voltage that decays with time:

Figure 12: LVDS burst with AC coupling

We could zoom in on this waveform to show individual pulses, but we would be unable to make any DC measurements as there is no fixed ground reference.

Conclusion

In this example of a typical low–level signal, an LVDS line, the analog offset feature of our PicoScope oscilloscope allowed us to increase the sensitivity of the instrument by a factor of ten. This resulted in a ten–fold improvement in vertical measurement resolution. AC coupling, while useful for steady waveforms such as ripple on power rails, is of limited use with serial data streams.