February 2, 1998

Layout and probing techniques ensure low-noise performance

Jim Williams, Linear Technology Corp

A cavalier attitude regarding a low-noise design is a direct route to disappointment. Achieving and maintaining the low-noise potential requires judicious layout, probing, and connection techniques.

When you think of low noise, switching regulators do not typically come to mind, but this situation is changing. A switching-regulator design can achieve low-noise performance of 100 µV (Reference 1). However, this low-noise switching-regulator design or any low-noise circuit doesn't achieve low-noise performance in a vacuum. You want the circuit to exhibit low noise in the real world on a real pc board with real connections to the circuit it powers and to the instruments that measure performance. Proper techniques for breadboarding, for pc-board layout, for probing, and for making valid connections are necessary to ensure the lowest possible noise.

The low harmonic content of an inherently low-noise switching regulator allows its noise performance to be less layout-sensitive than other switching regulators (Figure 1a). However, some prudence is in order. As in all things, a cavalier attitude is a direct route to disappointment. Obtaining the absolute lowest noise figure requires care, but you can readily achieve performance lower than 500 µV.

In general, you obtain the lowest noise by preventing the mixing of ground currents in the return path. An indiscriminate disposition of ground currents into a bus or ground plane causes such mixing, raising the observed output noise. The LT1533's restricted edge rates mitigate corrupted ground-path-induced problems, but the best noise performance occurs in a single-point ground scheme. Single-point return schemes may be impractical in production pc boards. In such cases, provide the lowest possible impedance path to the power entry point from the inductor associated with the LT1533's power ground pin (Pin 16). Locate the output-component ground returns as close as possible to the circuit load point. Minimize return-current mixing between input and output sections by restricting such mixing to the smallest possible common conductive area.

Control ground connections

The layout of the low-noise breadboard of the switching-regulator circuit in Figure 1 makes it fast and easy to modify in keeping with a breadboard's purpose. Single-point returns arrive separately from the output area (right side) and Pin 16 of the LT1533 IC (center left). The ground plane carries no current. The dummy load resistors do not terminate at the plane but return to the transformer's center tap. The center tap and plane separately tie into the ground system at the power-input common jack. 

Layout considerations for the floating-output circuit in Reference 1 are similar to those in Figure 1a, although the floating output mandates a few changes (Figure 2). The output load (right side, above the BNC connector) returns directly to the transformer secondary, which floats from input and plane ground potential. The main ground plane ties to the input common at the power entry port (left banana jack). The layout refers the floating-output potentials to a separate, smaller planed area (lower right), which ties to the transformer secondary center tap.

The most carefully prepared breadboard cannot fulfill its mission if signal connections introduce distortion. Connections to the circuit are crucial for accurate information extraction. The low-level, wideband measurements demand care in routing signals to test instrumentation.

Ground loops and 60-Hz pickup are common problems. Figure 3a shows the effects of a ground loop between pieces of line-powered test equipment. Small current flow between the test equipment's nominally grounded chassis creates 60-Hz modulation in the measured circuit output. You can avoid this problem by grounding all line-powered test equipment at the same outlet strip or by otherwise ensuring that all chassis are at the same ground potential. Similarly, you must avoid any test arrangement that permits circuit current flow in chassis interconnects. Figure 3b also shows 60-Hz modulation of the noise measurement. In this case, a 4-in. voltmeter probe at the feedback input is the culprit. Minimize the number of test connections to the circuit, and keep leads short.

Avoid poor probing techniques

A short ground strap affixed to a scope probe connects to a point that provides a trigger signal for the oscilloscope (Figure 1b). The oscilloscope monitors circuit output noise via the coaxial cable. A ground loop on the board between the probe ground strap and the ground-referred cable shield causes apparent excessive ripple in the display (Figure 1c). Minimize the number of test connections to the circuit, and avoid ground loops.

You can replace the coaxial cable that transmits the circuit's output noise to the amplifier oscilloscope with a probe (Figure 4a); a short ground strap acts as the probe's return. This case eliminates the error-inducing trigger-channel probe of the previous case. Instead, a noninvasive isolated probe triggers the scope. The probe makes no galvanic connection to the circuit, which eliminates any possibility of a ground loop. Unfortunately, the breakup of the coaxial signal environment causes excessive display noise (Figure 4b). The probe's ground strap violates coaxial transmission, and RF noise corrupts the signal. To avoid the felony of violating coaxial signal transmission, maintain coaxial connections in the noise-signal-monitoring path.

The probe connection in Figure 5a also violates coaxial signal flow but to a less offensive extent--a misdemeanor. This probing setup eliminates the probe's ground strap and replaces it with a tip grounding attachment. The result (Figure 5b) is much better than that in Figure 4b, although signal corruption is still evident. Again, maintain coaxial connections in the noise-signal-monitoring path.

Maintain coaxial connections

In theory, using a coaxial cable to transmit the noise signal to the amplifier-oscilloscope combination affords the highest integrity cable-signal transmission (Figure 6a). Figure 6b's trace shows this theory to be true: The aberrations and excessive noise in Figure 6a have disappeared. The switching residuals are now faintly outlined in the amplifier noise floor. Once again, maintain coaxial connections in the noise-signal-monitoring path.

One way to verify that no cable-based errors exist is to eliminate all cables between the breadboard, amplifier, and oscilloscope (Figure 7a). The result (Figure 7b) is indistinguishable from that of Figure 6b, indicating no cable-introduced infidelity. When results seem optimal, design an experiment to test them. Whether results of this experiment are as expected or poor, design another experiment to test them.

In theory, attaching a voltmeter lead to the regulator's output should introduce no noise. However, an increased noise reading under this condition contradicts the theory (Figure 8). The regulator's output impedance, albeit low, is not zero, especially as the frequency scales up. The RF noise that the test lead injects works against the finite-output impedance to produce the 200-µV noise in the figure. If you must connect a voltmeter to the output during testing, provide the connection through a 10-kilo-ohms to 10-µF filter. This network eliminates the problem and introduces minimal error in the monitoring DVM. Minimize the number of test-lead connections to the circuit while checking noise. Prevent test leads from injecting RF into the test circuit.

Use an isolated trigger probe

The isolated trigger probe in Figure 4 is simply an RF choke terminated against ringing (Figure 9). The choke picks up a residual radiated field, generating an isolated trigger signal. This arrangement furnishes a scope-trigger signal essentially without measurement corruption. For good results, adjust the termination for minimum ringing while preserving the highest possible amplitude output. Light compensatory damping produces a signal with high-amplitude ringing, which causes poor scope triggering. Proper adjustment results in a more favorable output, characterized by minimal ringing and well-defined edges.

The field around the switching magnetic components is small and may be inadequate for reliably triggering some oscilloscopes. In such cases, a trigger-probe amplifier is useful (Figure 10a). This amplifier uses an adaptive triggering scheme to compensate for variations in probe-output amplitude. A stable 5V trigger output is maintained over a 50-to-1 probe-output range.

IC₁, operating at a gain of 100, provides wideband ac gain. The output of this ×100 stage (Figure 10b, Trace A) biases a two-way peak detector, comprising Q₁ through Q₄. Q₂'s emitter capacitor stores the maximum peak, and Q₄'s emitter capacitor retains the minimum excursion. The dc value of the midpoint of IC₁'s output signal appears at the junction of the 500-pF capacitor and the 3-Mega-ohm resistors. This point always sits midway between the signal's excursions, regardless of absolute amplitude.

IC₂ buffers this signal-adaptive voltage to set the trigger voltage at the LT1116's positive input. IC₁'s output directly biases the LT1116's negative input. Signal-amplitude variations of greater than 50-to-1 do not affect the LT1116's output, which is the circuit's trigger output (Figure 10b, Trace B).

Reference

1. "Switching-regulator design lowers noise to 100 µV," EDN, Dec 4, 1997, pg 151.

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