Taming fully differential circuits

Manufacturers make fully differential amplifiers for designs requiring differential drive voltages. Example applications are high-speed ADC inputs, high-speed analog-signal transmission, high-frequency noise rejection, and low-distortion applications. Most fully-differential-amplifier applications are high-frequency; fully-differential-amplifier gain bandwidths are in the multiple-gigahertz region. Thus, fully-differential-amplifier designs require an understanding of high-frequency pc-board layout and construction.

High-frequency pc-board design is a unique art form, and some people imply that it's black magic, requiring a wizard's touch. Not really. Following a few simple concepts and carefully considering appropriate physics laws are adequate for achieving excellent performance.

Each digital and analog IC requires decoupling capacitors connected from the power supply to ground (one for each supply). You must place the decoupling capacitors as close to the power pins as conditions permit. And, because fully differential amplifiers come in SOPs, use nonleaded capacitors. Because it is a low-impedance path to ground, the digital decoupling capacitor neutralizes the current spike that a switching gate causes. This neutralization prevents the current spike from propagating throughout the ground system and happily generating voltage noise across the distributed plane impedance. The ac analog theory you use to design circuits needs an ac short across the power supplies to make it valid. And analog decoupling capacitors provide low-impedance current paths between the IC pins and the power pins and across the supplies.

The minimum value for a decoupling capacitor is 0.01 µF; 0.1-µF capacitors and larger are better, but a trade-off exists among size, dielectric, and value. Use the better grades of ceramic capacitors to obtain low impedance at high frequencies. Mica has the highest frequency response, but mica capacitors have the least volumetric efficiency. Aluminum and tantalum-electrolytic capacitors are useless at high frequencies (greater than 1 and 10 MHz, respectively).

Use copper planes for ground and power distribution wherever possible. Planes ensure that return currents can take the shortest return path, thus minimizing the distributed voltage drop (noise) that these currents cause. The copper planes are low-impedance return paths for currents and heat sinks for the ICs. Heavier copper lowers the electrical impedance, but not by much because of skin effect. However, it enhances the heat-sink function by lowering the thermal impedance. Separating the analog and digital planes—except for a connection usually at the power supply—lowers analog noise by isolating the gate-switching spikes and digital noise. You may have to etch away the planes near or under critical nodes (amplifier-input leads), because plane-induced stray capacitance can cause instability.

Never run analog and digital traces parallel; the trace-to-trace capacitance couples digital noise into analog signals. If physical conditions dictate parallel digital- and analog-signal traces, separate the traces with a trace connected to analog ground at one end. The center trace is a Faraday shield that virtually eliminates coupled noise. These hints can prevent most pc-board problems, but pc-board design-and-layout books provide much more material to help with subtle problems. One way to get an excellent layout on the first try is to copy the IC vendor's evaluation-board artwork. This step isn't theft or cheating—most IC vendors will give you layout tapes.

Author Information

Ron Mancini is staff scientist at Texas Instruments. You can reach him at 1-352-569-9401, rmancini@ti.com.