Minimizing Switching Regulator Residue in Linear Regulator Outputs (Linear Technology) - 10

Application Note 101
APPENDIX C
switching regulator spike measured within a continuous
Probing Technique for Sub-Millivolt, Wideband Signal
coaxial signal path. The spike’s main body is reasonably
Integrity
well defi ned and disturbances after it are contained. Fig- ure C2 depicts the same event with a 3 inch ground lead Obtaining reliable, wideband, sub-millivolt measurements connecting the coaxial shield to the circuit board ground requires attention to critical issues before measuring plane. Pronounced signal distortion and ringing occur. anything. A circuit board layout designed for low noise The photographs were taken at 0.01V/division sensitiv- is essential. Consider current fl ow and interactions in ity. More sensitive measurement requires proportionately power distribution, ground lines and planes. Examine the more care. effects of component choice and placement. Plan radiation management and disposition of load return currents. If the Figure C3 details use of a wideband 40dB gain pre-amplifi er circuit is sound, the board layout proper and appropriate permitting text Figure 12’s 200µV/division measurement. components used, then, and only then, may meaningful Note the purely coaxial path, including the AC coupling measurement proceed. capacitor, from the regulator, through the pre-amplifi er and to the oscilloscope. The coaxial coupling capacitor’s The most carefully prepared breadboard cannot fulfi ll its shield is directly connected to the regulator board’s ground mission if signal connections introduce distortion. Con- plane with the capacitor center conductor going to the nections to the circuit are crucial for accurate information regulator output. There are no non-coaxial measurement extraction. Low level, wideband measurements demand connections. Figure C4, repeating text Figure 12, shows care in routing signals to test instrumentation. Issues to a cleanly detailed rendition of the 900µV output spikes. In consider include ground loops between pieces of test Figure C5 two inches of ground lead has been deliberately equipment (including the power supply) connected to the introduced at the measurement site, violating the coaxial breadboard and noise pickup due to excessive test lead regime. The result is complete corruption of the waveform or trace length. Minimize the number of connections to presentation. As a fi nal test to verify measurement integrity, the circuit board and keep leads short. Wideband signals it is useful to repeat Figure C4’s measurement with the to or from the breadboard must be routed in a coaxial signal path input (e.g., the coaxial coupling capacitor’s environment with attention to where the coaxial shields center conductor) grounded near the measurement point tie into the ground system. A strictly maintained coaxial as in text Figure 13. Ideally, no signal should appear. environment is particularly critical for reliable measure- Practically, some small residue, primarily due to common ments and is treated here1. mode effects, is permissible. Figure C1 shows a believable presentation of a typical 0.01V/DIV 0.01V/DIV AC COUPLED ON 3VDC AC COUPLED ON 3VDC 200ns/DIV 200ns/DIV
Figure C1. Spike Measured Within Continuous Coaxial Signal Figure C2. Introducing 3" Non-Coaxial Ground Connection Path Displays Moderate Disturbance and Ringing After Main Causes Pronounced Signal Distortion and Post-Event Ringing Event Note 1:
More extensive treatment of these and related issues appears in the appended sections of References 1 and 2. Board layout considerations for low level, wideband signal integrity appear in Appendix G of Reference 3. an101f AN101-10