Five Tips to Improve Dynamic Range

Dynamic range is the ratio of the maximum signal level to the smallest signal level achievable in a measurement. Tools with good dynamic range are especially helpful for analyzing wide dynamic range signals in which a full-scale signal must be acquired, while at the same time, very small amplitude signal details must be visible. Here are five tips for improving the dynamic range of your measurement instrument.

1. Start with the Highest Vertical Resolution Possible

Figure 1: Measuring saturation voltage
of a power FET using 8-bit and 12-bit
resolution oscilloscopes.

It would be easy to discount this first tip as shameless marketing, but the fact remains that not all instruments are equal, and sometimes, you do just need better tools to get the job done.

An oscilloscope’s dynamic range is based on the resolution of the analog-to-digital converter used in the acquisition system, and the architecture that supports it. Each bit of vertical resolution corresponds to 6 dB of dynamic range.

Figure 1 shows two measurements of the saturation voltage of a power FET. The saturation voltage is about 1 Volt, but the full dynamic range of the signal is 46 dB. The resolution of the 8-bit oscilloscope is 48 dB, barely enough to accommodate the full dynamic range of the signal, while that of the 12-bit oscilloscope is 72 dB. There is a visible difference in how each is able to resolve that very small signal.

Teledyne LeCroy High Definition Oscilloscopes (HDO and HD models) will deliver 12 bits of vertical resolution regardless of sample rate, number of active channels, concurrent digital inputs, etc. When we say “12 bits all the time,” we mean it, so with our instruments, you will have excellent dynamic range out of the gate.

2. Use a Separate Grid for Each Waveform

Figure 2: Don't reduce vertical sensitivity and add
offset to display all traces on a single grid.
Instead, use a multi-grid display.

Use all the display grids you need to avoid losing your oscilloscope’s dynamic range. Before the advent of multi-grid displays, it was commonplace to group multiple traces on a single grid by reducing the vertical sensitivity (gain) of each by whatever factor was needed to fit them all, then adding offset to separate them on the display. For some, this may still be common practice.

But if you display, say, four traces on a single grid by reducing the vertical sensitivity of each by a factor of 4, you will consequently also diminish your dynamic range by a factor of 4 (Figure 2). The display may be reasonable, but the dynamic range of each channel has been reduced by 2 bits, or 12 dB. An 8-bit oscilloscope is now a 6-bit oscilloscope, a 12-bit oscilloscope is now a 10-bit oscilloscope, and so on.

On Teledyne LeCroy oscilloscopes, each display grid maintains the full vertical resolution and dynamic range of the oscilloscope, and on most oscilloscopes, you can open one or more grids for every channel. So, don’t skimp on the grids—use them!

3. Use Averaging

Figure 3: Do use averaging to improve
the dynamic range of a measurement.

You can use averaging to actually improve dynamic range. If the signal is repetitive, then acquiring multiple acquisitions and averaging the result can improve the dynamic range by the square root of the number of signals averaged. So, for four averages, you get a 2x improvement or 6 dB (1 bit). For 64 averages, you get an 8x improvement or 18 dB (3 bits). This will also result in a visibly crisper image on the display. Figure 4 shows the improvement due to averaging applied as a math function.

Averaging does not reduce the bandwidth of the measurement. It eliminates white noise, but does not reduce distortion products, so you will still be able to capture waveform anomalies.

4. Use Filtering

Figure 4: Do use ERES to improve resolution
by low-pass filtering the acquired signal.

Another technique to improve dynamic range is to filter the signal. Modern oscilloscopes have several software tools to implement filters. On Teledyne LeCroy oscilloscopes, the easiest of these tools to apply is enhanced resolution, or ERES. ERES uses a finite impulse response (FIR) low-pass filter to reduce the signal bandwidth. This reduces white noise, but at the expense of reduced measurement bandwidth.

There are six settings of resolution enhancement, from 0.5 to 3 bits. Each half-bit step reduces the bandwidth by a factor of 2 and improves vertical resolution by 3 dB. In Figure 4, 2.5 bits of ERES added to the trace reduces the bandwidth to 36.5 MHz but improves resolution by 15 dB.

Note that both the acquired signal and the processed signal can be observed simultaneously when averaging or filtering are applied as math functions, instead of as a form of channel pre-processing. This helps you verify the effects of the operation on the signal and measurements and decide if the “cost” is affordable.

5. Adjust Coupling

Trying to measure small signals riding on large signals is difficult—for example, measuring power supply ripple on top of the nominal rail voltage. Old timers used to overdrive the oscilloscope to obtain the high vertical sensitivity necessary to see the small amplitudes. In older oscilloscope designs this may have been possible, but with modern digital signal processing in the front end, it is a definite "no-no". Do not overdrive the channel input!

If you need to use the full bandwidth of the oscilloscope, it is advisable to use DC coupling. Then, you can add channel vertical offset to counteract the nominal voltage. The problem that arises with this is that the offset voltage range on the high sensitivity settings may be limited. For instance, an HDO oscilloscope operating in the 1 mV to 4.95 mV of V/div range has an offset range of ±1.6 V. That is fine for a 1 or 1.1 V signal, but it will not work for a 3.3 V signal. Keep in mind also that the vertical offset introduces an error of ± (0.5% of offset + 0.5% of FS + 1 mV).

You could use a 10:1 probe, which increases the offset range by a factor of 10, but that also attenuates the small signal by the same factor, so it isn’t a good option.

An alternative is to use AC coupling, which blocks the DC component of the voltage. You have to keep in mind that AC coupling has a high-pass characteristic, and that frequencies below about 10 Hz will be attenuated. This can be problematic if you need to see frequencies below the cutoff frequency. The DC component of the signal is also lost. But, if you don’t care about the low frequencies or the DC component, this can be a good option.

Watch Steve Murphy's on-demand webinar about Optimizing Your Vertical Gain.

Test Happens.

01 June 2021