by John Ardizzoni, Analog Devices
Friday 11 May 2007
Making accurate, high-speed time domain measurements does not have to be a challenge. Employing a few tips can greatly simplify the task.
When choosing the oscilloscope and probe for high-speed measurement, first consider signal amplitude, source impedance, rise time, and bandwidth.
There are hundreds of oscilloscopes available and the variety of probes that accompany them is also quite impressive, but let us consider scopes for high-speed voltage measurements using passive probes.
The scope needs to have enough bandwidth to faithfully reproduce the signal. For analogue measurements, the highest frequency being measured will determine the scope bandwidth. For digital measurements, it is usually the rise time - not the repetition rate - that determines the required bandwidth.
It is important to ensure that the oscilloscope has sufficient bandwidth. Measurements should never be made at frequencies near the oscilloscope’s 3dB bandwidth, as this would introduce an automatic 30 per cent amplitude error in a sine wave measurement.
For digital circuits, rise time is of particular interest. The expected or anticipated rise time can be used to determine the bandwidth requirements of the scope. The relationship assumes the circuit responds like a single-pole, low-pass RC network.
2) Probe anatomy
A probe consists of a probe tip (which contains a parallel RC network), a length of shielded wire, a compensation network and a ground clip. The probe’s requirement is to provide a non-invasive interface between the scope and the circuit, disturbing the circuit as little as possible, while allowing the scope to render a near-perfect representation of the signal being measured.
The most commonly used probes are 10x and 1x passive probes. 10x active FET probes are a close second. The 10x passive probe has 10Mohm input impedance and 10pF typical tip capacitance and attenuates the signal by a factor of 10. The 1x probe has no attenuation, 1Mohm input impedance, and tip capacitance as high as 100pF.
You must calibrate the probe to ensure its internal RC time constants are matched. Some scopes have built-in calibration which should be run before making measurements.
4) Ground clips and high-speed measurements
Their inherent parasitic inductance makes ground clips and practical high-speed measurements mutually exclusive.
The probe LC combination forms a series resonant circuit. The series-LC combination can add significant overshoot and ringing to an otherwise clean waveform. This ringing and overshoot often go unnoticed due to limited bandwidth of a scope.
5) Readying a probe for high-speed measurements
In order to obtain meaningful scope plots we need to rid the circuit of the ground clip and dismantle the probe. Then, the probe needs to be calibrated before it is ready to use. Simply go to a test point and pick up a local ground on the outer metal shield of the probe. The trick is to pick up the ground connection right at the scope probe shield. This eliminates any of the series inductance introduced by using the supplied probe ground clip.
Even better is to design in dedicated high-frequency test points on the board. Such probe-tip adaptors provide all of the above-mentioned advantages for using naked probe tips.
6) Probe-capacitance effects
Probe capacitance affects rise time and amplitude measurements. It can also affect the stability of certain devices. The probe capacitance adds directly to the node capacitance being probed. The added capacitance increases the node time constant, which slows down the rising and falling edges of a pulse.
Active probes are another good choice for probing high speed circuits. These contain an active transistor that amplifies the signal, compared to passive probes that attenuate the signal. Another alternative is to use a probe with a high attenuation factor. Typically, higher-attenuation-factor probes have less capacitance.
Probe tip capacitance can also cause some circuits to ring, overshoot, or become unstable. For example, many high-speed op amps are sensitive to the effects of capacitive loading at output and inverting input. When the probe tip capacitance is introduced at the output of a high-speed amplifier, the amplifier’s output resistance and the capacitance form an additional pole in the feedback response. The pole introduces phase shift and lowers the amplifier’s phase margin, which can lead to instability.
Fortunately, there are a few answers to this problem, such as using a lower-capacitance probe or including a small amount of series resistance, (typically 25 to 50ohm) with the scope probe. This will help isolate the capacitance from the amplifier output and will lessen the ringing and overshoot.
7) Propagation delay
An easy way to measure propagation delay is to probe the device under test (DUT) at its input and output simultaneously. The propagation delay can be easily read from the scope display as the time difference between the two waveforms.
When measuring short propagation delays (<10ns) care must be taken to ensure that both scope probes are the same length. Since the propagation delay in wire is approximately 1.5ns/ft, sizeable errors can result from pairing probes of different lengths.
Therefore make sure the probes are properly calibrated and the difference between the two probes noted.
Numerous factors must be considered when venturing into the lab to make high-speed time-domain measurements. Bandwidth, calibration, rise-time, scope, probe and ground-lead lengths all play important roles in the quality and integrity of measurements. Employing even some of the techniques mentioned here will help speed up the measurement process and improve the overall quality of results.
John Ardizzoni is an applications engineer at Analog Devices