Datasheet LTC6900 (Analog Devices) - 10

LTC6900
APPLICATIONS INFORMATION
Once RIN/RSET is known, calculate RSET from: Example 2: 20k Vary the oscillator frequency by one octave per volt. As- RSET = 10MHz • • N f sume f OSC(MAX) OSC(MIN) = 1MHz and fOSC(MAX) = 2MHz, when the input voltage varies by 1V. The minimum input voltage is ⎡ ⎛ ⎞ ⎤ half supply, that is V V ⎢ IN(MIN) = 1.5V, VIN(MAX) = 2.5V and IN(MAX) − V+ ( )+VRES 1+ RIN ⎥ ⎝⎜ R V+ = 3V. SET ⎠⎟ ⎢ ⎥ ⎢ ⎛ R ⎞ ⎥ Equation (2) yields R ⎢ V IN IN/RSET = 1.273 and Equation (3) yields RES ⎥ ⎢ ⎝⎜ R ⎠⎟ ⎥ RSET = 142.8k. RIN = 1.273RSET = 181.8k. For standard SET ⎣ ⎦ (3) resistor values, use RSET = 143k (1%) and RIN = 182k Example 1: (1%). Figure 11 shows the measured fOSC vs VIN. For VIN higher than 1.5V, the VCO is quite linear; nonlinearities In this example, the oscillator output frequency has small occur when VIN becomes smaller than 1V, although the excursions. This is useful where the frequency of a system VCO remains monotonic. should be tuned around some nominal value. Let V+ = 3V, f
Maximum VCO Modulation Bandwidth
OSC(MAX) = 2MHz for VIN(MAX) = 3V and fOSC(MIN) = 1.5MHz for VIN = 0V. Solve for RIN/RSET by The maximum VCO modulation bandwidth is 25kHz; that Equation (2), yielding RIN/RSET = 9.9/1. RSET = 110.1k by is, the LTC6900 will respond to changes in VIN at a rate Equation (4). RIN = 9.9RSET = 1.089M. For standard resis- up to 25kHz. In lower frequency applications however, the tor values, use RSET = 110k (1%) and RIN = 1.1M (1%). modulation frequency may need to be limited to a lower Figure 10 shows the measured fOSC vs VIN. The 1.5MHz rate to prevent an increase in output jitter. This lower limit to 2MHz frequency excursion is quite limited, so the curve is the master oscillator frequency divided by 20, (fOSC/20). of fOSC vs VIN is linear. In general, for minimum output jitter the modulation fre- quency should be limited to fOSC/20 or 25kHz, whichever is less. For best performance at all frequencies, the value for fOSC should be the master oscillator frequency (N = 1) when VIN is at the lowest level. 2.00 3000 RIN = 1.1M RIN = 182k 1.95 RSET = 110k RSET = 143k 2500 1.90 V+ = 3V V+ = 3V N = 1 N = 1 1.85 2000 1.80 (MHz) 1.75 (kHz) 1500 f OSC 1.70 f OSC 1000 1.65 1.60 500 1.55 1.50 0 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 VIN (V) VIN (V) 6900 F10 6900 F11
Figure 10. Output Frequency vs Input Voltage Figure 11. Output Frequency vs Input Voltage
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