SLOS776A September 2012 – December 2015 THS789
PRODUCTION DATA.
All the high-speed time-measurement circuitry in the TMU is implemented in differential emitter-coupled logic (ECL). Besides high speed, a characteristic of differential ECL is good rejection of power-supply noise and variation. However, there is a great deal of CMOS logic, FIFO and output-serial interface circuitry that is an excellent source of power-supply current noise. Therefore, to maintain the best accuracy, the TMU power supply must be low-impedance. This is accomplished in the usual ways by careful layout, good ground and power planes, short traces to the power and ground pins, and capacitive bypassing. TI recommends placing a quality, low inductance, high-frequency bypass capacitor of approximately 0.01 μF close to each power pin. The 0402 size works well. Additional bypass capacitors of larger value should be placed near the TMU, making low-inductance connection with the power and ground planes. With a typical power-supply sensitivity of 30 ps/V, a 1% power supply shift yields a 1-picosecond additional error, making power-supply regulation important for the best accuracy.