Tiny Programmable Oscillator Spans Three Decades from 5Khz to 20Mhz
By merging a digital potentiometer with an oscillator chip, we can make a very small programmable oscillator (1 uMAX and 1 SOT23 package). Not only does this circuit take up very little board space but it can be controlled with only three signals from a m
By merging a digital potentiometer with an oscillator chip, we
can make a very small programmable oscillator (1 uMAX and 1 SOT23
package). Not only does this circuit take up very little board
space but it can be controlled with only three signals from a
microprocessor.
The MAX5160 is a 200K digital pot with 32 taps. To move up to
the next tap, set UP/DOWN high and pulse INCREMENT low. To move
down, set UP/DOWN low before pulsing INCREMENT.
The LTC1799 is a programmable oscillator from Linear Technology.
The frequency of oscillation is proportional to the value of the
set resistor between V+ and the SET pin. Internal to the chip is a
divider which is controlled by the DIV pin. The following table
illustrates the relationship between the divide ratio and the input
on DIV:
DIV Input
Internal Divider Value
(N)
Ground
1
High Impedance
10
V+
100
The equation for the oscillator frequency is:
Where N is the
divider value. A plot of this equation for each value of N and each
of the tap locations on the digital potentiometer can be seen in
the figure below. Note that there is some overlap between the
curves for each value of N.
In the schematic shown below, R1 was added to satisfy the
minimum Rset requirement for the LTC1799. The circuit can be
further simplified by fixing the DIV input and operating over a
narrower output range.
