Squarewave oscillator needs no external components

Systems often require a squarewave (clock) generator for driving digital or AC-powered subsystems, but are not fussy about the frequency of the signal. Such frequency-tolerant digital systems include multiplexed panel indicators and display sequencers.

Certain sensors also require AC drive. Electro-chemical sensors (used for monitoring tilt, conductivity, and moisture) cannot tolerate DC current because it degrades the materials of which they are made. Capacitive sensors operate on a principle that calls for AC drive, and for piezoresistive sensors an AC drive cancels DC errors and 1/f noise. Fluxgate sensors also need an AC drive.

The circuit of Figure 1 , requiring no external components except the bypass capacitor, generates a squarewave that is useful as a clock signal or as AC drive for the excitation of sensors.

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Figure 1: Formed from a single IC (an inverting charge pump) this circuit produces a squarewave useful in many applications.

Producing a powerful 3V to 5V squarewave output with very low source impedance (less than 10 ohm), it also provides rail-to-rail excursions in a small SOT23 package. The output frequency is reasonably stable with variations in supply voltage and temperature, Figure 2 .

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Figure 2: Output frequency for the circuit varies only approximately 1% over the allowable extremes of temperature and supply voltage.

The IC shown is an inverting charge pump (MAX1697), originally designed to supply a negative voltage in positive single-supply systems. Its rail-to-rail capability means the output-amplitude precision depends directly on the level of VIN . Duty-cycle error is a fraction of 1%.

The circuit has a dependable start response, even with supply voltages below 1V (depending on the MAX1697 version). Note that the price paid for simplicity in this circuit is a lack of flexibility in output frequency. As shown in Figure 1, four nominal frequencies are available according to the IC version chosen (12 kHz, 35 kHz, 125 kHz, and 250 kHz). Figure 3 is a scope shot of the output waveform.

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Figure 3: Output waveform for the circuit.of Figure 1

About the Authors
Alfredo H. Saab and Steve Logan are with maxim Integrated Products

1 comment on “Squarewave oscillator needs no external components

  1. lj oineorin
    April 17, 2014

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