Datasheet LT1711, LT1712 (Analog Devices) - 12

LT1711/LT1712
U TYPICAL APPLICATIO 1MHz Series Resonant Crystal Oscillator
frequency glitch (middle trace of Figure 8). This glitch is
with Square and Sinusoid Outputs
caused by the fast edge of the comparator output feeding Figure 7 shows a classic 1MHz series resonant crystal back through crystal capacitance. Amplitude stability of oscillator. At series resonance, the crystal is a low imped- the sine wave is maintained by the fact that the sine wave ance and the positive feedback connection is what brings is basically a filtered version of the square wave. Hence, about oscillation at the series resonant frequency. The RC the usual amplitude control loops associated with sinusoi- feedback around the other path ensures that the circuit dal oscillators are not necessary.2 The sine wave is filtered does not find a stable DC operating point and refuse to and buffered by the fast, low noise LT1806 op amp. To oscillate. The comparator output is a 1MHz square wave remove the glitch, the LT1806 is configured as a bandpass (top trace of Figure 8) with jitter measured at better than filter with a Q of 5 and unity-gain center frequency of 28ps 1MHz, with its output shown as the bottom trace of RMS on a 5V supply and 40psRMS on a 3V supply. At Pin 2 of the comparator, on the other side of the crystal, is Figure␣ 8. Distortion was measured at – 70dBc and – 60dBc a clean sine wave except for the presence of the small high on the second and third harmonics, respectively. 2 Amplitude will be a linear function of comparator output swing, which is supply dependent C4 R10 R5 and therefore adjustable. The important difference here is that any added amplitude 100pF 1k 6.49k stabilization or control loop will not be faced with the classical task of avoiding regions of nonoscillation versus clipping. C5 R6 100pF 162Ω 1MHz C3 R4 AT-CUT 100pF R7 V 210Ω S 15.8k 3V/DIV R1 VS V 1k S 2 1 + 7 2 7 SQUARE – R2 1V/DIV LT1711 6 1k R9 3 LT1806S8 SINE – LE 8 4 2k 3 1 5 VS + 6 4 R3 C2 R8 1V/DIV 1k 0.1µF 2k 171112 F07 C1 200ns/DIV 171112 F08 0.1µF
Figure 8. Oscillator Waveforms with VS = 3V. Top is Figure 7. LT1711 Comparator is Configured as a Series Comparator Output. Middle is Xtal Feedback to Pin 2 at Resonant Xtal Oscillator. LT1806 Op Amp is Configured LT1711 (Note the Glitches). Bottom is Buffered, Inverted in a Q = 5 Bandpass with fC = 1MHz and Bandpass Filtered with a Q = 5 by LT1806 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS
LT1016 UltraFast Precision Comparator Industry Standard 10ns Comparator LT1116 12ns Single Supply Ground Sensing Comparator Single Supply Version of the LT1016 LT1394 7ns, UltraFast Single Supply Comparator 6mA Single Supply Comparator LT1671 60ns, Low Power, Single Supply Comparator 450µA Single Supply Comparator LT1713/LT1714 Single/Dual 7ns, Low Power, 3V/5V/±5V, R-R Comparator 7ns/5mA versions of the LT1711/LT1712 LT1719 4.5ns, Single Supply 3V/5V/±5V Comparator 4mA Comparator with Rail-to-Rail Outputs and Level Shifting LT1720/LT1721 Dual/Quad, 4.5ns, Single Supply Comparator Dual/Quad Version of the LT1719 171112f LT/TP 0401 4K • PRINTED IN USA 12 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 2001