Datasheet LTC1799 (Analog Devices) - 9

LTC1799
applicaTions inForMaTion Jitter
When VIN = V+, the output frequency of the LTC1799 as- The typical jitter is listed in the Electrical Characteristics sumes the highest value and it is set by the parallel com- and shown in the Typical Performance Characteristics. bination of RIN and RSET. Also note, the output frequency, These specifications assume that the capacitance on SET fOSC, is independent of the value of VRES = (V+ – VSET) so (Pin 3) is limited to less than 10pF, as suggested in the Pin the accuracy of fOSC is within the data sheet limits. Functions description. If this requirement is not met, the When VIN is less than V+, and especially when VIN ap- jitter will increase. For more information, contact Linear proaches the ground potential, the oscillator frequency, Technology Applications group. fOSC, assumes its lowest value and its accuracy is affected by the change of VRES = (V+ – VSET). At 25°C VRES varies
A Ground Referenced Voltage Controlled Oscillator
by ±8%, assuming the variation of V+ is ±5%. The tem- The LTC1799 output frequency can also be programmed perature coefficient of VRES is 0.02%/°C. by steering current in or out of the SET pin, as conceptually By manipulating the algebraic relation for fOSC above, a shown in Figure 8. This technique can degrade accuracy simple algorithm can be derived to set the values of external as the ratio of (V+ – VSET) / IRES is no longer uniquely resistors RSET and RIN, as shown in Figure 9. dependent of the value of RSET, as shown in the LTC1799 Block Diagram. This loss of accuracy will become notice- 1. Choose the desired value of the maximum oscillator able when the magnitude of I frequency, fOSC(MAX), occurring at maximum input PROG is comparable to IRES. The frequency variation of the LTC1799 is still monotonic. voltage VIN(MAX) ≤ V+. Figure 9 shows how to implement the concept shown in 2. Set the desired value of the minimum oscillator fre- Figure 8 by connecting a second resistor, R quency, fOSC(MIN), occurring at minimum input voltage IN, between the SET pin and a ground referenced voltage source, V VIN(MIN) ≥ 0. IN. For a given power supply voltage in Figure 9, the output 3. Choose VRES = 1.1 and calculate the ratio of RIN/RSET frequency of the LTC1799 is a function of V from the following: IN, RIN, RSET and (V+ – VSET) = VRES: RIN = R f 10MHz SET OSC = • 10k • N RIN RSET ⎛ f ⎞ V OSC(MAX) ⎜ ⎟ ⎡ ⎛ ⎞⎤ IN(MAX) − V+ ( )−⎜ f ⎟ VIN(MIN) − V+ ( ) ⎝ OSC(MIN) ⎠ ⎢ V ⎜ ⎟⎥ −1 (2) 1 IN − V+ ⎢ ( ) ⎡ ⎤ + • 1 ⎜ ⎟⎥ f ( OSC(MAX)) ⎢ V R (1) V ⎢ −1⎥ RES ⎜1+ IN ⎟⎥ RES ⎜ ⎟ ⎢ f ⎣ OSC(MIN) ⎥ ⎣⎢ ⎝ R ⎥ ⎦ SET ⎠⎦ 1 5 1 5 V+ V+ OUT V+ V+ OUT fOSC 0.1µF LTC1799 + 0.1µF LTC1799 2 R 2 SET 5V GND VRES RSET 5V GND ÷100 – ÷100 3 4 RIN 3 4 SET DIV ÷10 OPEN SET DIV ÷10 OPEN + IRES ÷1 ÷1 IPR VIN 1799 F08 1799 F09 –
Figure 8. Concept for Programming via Current Steering Figure 9. Implementation of Concept Shown in Figure 8
1799fd For more information www.linear.com/LTC1799 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Theory of Operation Applications Information Typical Application Package Description Revision History Typical Applications