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Application of Capacitance-to-Digital Converters in PLL

Question asked by Shahrokh on May 21, 2013
Latest reply on Feb 13, 2014 by seanbrown



I need to control and program the value of a mechanical variable capacitor (several pF ) in a closed loop. The attached block diagram shows the basic idea.


As depicted, I need an accurate oscillator with an external capacitor (as the control element) which acts similar to a VCO in the PLL. Parts like 74HCT9046A or CD4046B meet the requirement but they do not provide the flexibility of newer PLLs. Basically I prefer to use one of Analog Devices' PLLs to make use of various design tools, ANs, etc. I am not sure if such a device is available from the Analog! Other solutions might be available.


In short, I seek for any of the following solutions:

1- A VCO with external capacitor like the above mentioned components but with more features suitable for integrating with Analog PLLs.

2- A Capacitance-to-Frequency Convertor which can act like a VCO in the loop.

3- A Capacitance-to-Digital Converter (eg. AD7745/46/47) which can be used to control a Programmable Oscillator. So the series combination of CDC and digital programmable oscillator substitutes the VCO in the loop.


I appreciate any solutions, comments, and hints.


Thanks and Regards,



Message was edited by Shahrokh:  It should be noted that due to actuator hysteresis, look-up table can not be used to set the capacitor value. Basically, this problem has led to the idea of using PLL.