How to set the frequency of AD9560 PWM generator

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Question I am working on the design of the analog interface of a SmartCard 
 reader/tester/ encoder.
 
 I am currently evaluating the AD9560 pulse width modulator. This product, (or 
 its equivalent AD9561) would be perfect for our PWM needs, except for the Rset: 
 this resistor implies that operation has to be done at a single frequency. 
 However, in my application, I would like to change the frequency from 500kHz to 
 30MHz...
 
 Is it possible to replace Rset with a DAC or a digital potentiometer, for 
 example...? Which voltage or current range should I use with a DAC? NB: It is 
 important that the precision on the duty cycle remains under 2%. Answer Here are some ideas for setting the Rset resistance.
 
 The purpose of Rset is to set the current, which charges an internal capacitor 
 to generate the internal voltage ramp. This ramp is the basis of the PWM 
 generator timing. Since the on-chip timing circuits vary from lot to lot and 
 can drift with temperature, an auto-calibration function adjusts the current 
 source so that the ramp duration is matched to the CLOCK input.
 
 The attached diagrams show the simplified internal circuitry connected to the 
 Rset pin and a suggested way of connecting an external digital potentiometer. 
 The Rset node is sensitive to external capacitance, and since digital pots can 
 contribute significant parasitic capacitance, it is recommended that a fixed 
 resistor Ra be connected to pin 15, to isolate the device from external 
 capacitance. Choose Ra to be slightly less than Rset_min and choose Rb to be 
 slightly twice more than Rset max
 requirement. Then, Rpot range should be 0 ohms to Rb's value.
 
 The Rset only needs to set the current source to APPROXIMATELY the correct 
 value; the auto-calibration circuit will "fine-tune" the current source, so 
 that the datasheet specifications can be met. The guideline given in the 
 datasheet is that the nearest 1% resistor to the nominal value should be 
 chosen. So our "automatic" Rset should contribute an error no greater than the 
 nearest 1% resistor would. There are 96 standard values of 1% resistors in each 
 decade 
 (10k,10.2k,10.5k ... 97.6k,100k...) So it is normally possible to choose a 
 resistor whose nominal value is within 1% to 1.2% of the exact value you 
 require. This resistor may in turn have a value within +/-1% of it's nominal 
 value. So we end up with a worst case error band of +/-2.2% at room temperature.
 
 For your stated frequency range, the resistor values are as follows
 500kHz => 115.9kOhms
 30MHz => 1.687kOhms
 
 The problem with all "variable resistance" ICs (DACs, digital pots) is that the 
 ABSOLUTE value of the resistors is not so accurate (you must reckon with +/-20% 
 to +/-40%). What allows the very high precision is the RELATIVE MATCHING of the 
 on-chip resistors. In your application you need the absolute value of the 
 resistor to be within 2% to allow the autocalibration to operate. 
 
 1. Digital potentiometer. Easy to use. You can use three in series to give you 
 the variation in gain that you're looking for. The attached application note 
 shows the considerations when combining several digital pots.
 
 2. Using the variable resistance from a Multiplying DAC. This offers higher 
 resolution and a wider choice of parts than the digital potentiometer solution. 
 However, you need to be careful to keep certain nodes at the same potential or 
 else the DAC behaves non-linearly. It may be harder to select a DAC with the 
 absolute rsistance value you require. I have attached a full reference from our 
 CMOS DAC Application guide. Suitable DACs would be any CMOS Iout, multiplying 
 DAC. e.g. AD7524, AD7533, AD7943, AD7945, AD7948.

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