Posted on Jul 22, 2012

The uA2240 timer/counter, combined with aprecision resistor ladder network and an op amp, form the staircase generator. In the astable mode, once a trigger pulse is applied, the uA2240 operates continuously until it receives a reset pulse. The trigger input at pin 11 is tied to the time base output at pin 14, resulting in automatic starting and continuous operation. The frequency of the time-base oscillator, TBO, is set by the time constants R1 and C1 (f = 1/RIC1). For this example, a 10-KO resistor and a 0.01-I`F capacitor form the timing network.

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The counter outputs are connected to a precision resistor ladder network with binary-weighted resistors. The current sink through the resistors connectedJo the counter outputs correspond to the count number. For example, the current sink at Q7, the most significant bit, is 128 times the current sink at QO, the least significant bit. As the count is generated by the uA2240 eight-bit counter, the current sink through each active binary-weighted resistor decreases thepositive output of the op amp in discrete steps. The feedback potentiometer is set at a nominal10 K{l to supply a maximum output voltage range. An input of 12 V allows a 10-V output swing. With a 0. 5-V input reference on pin 3 of the TCL271, the output will change from 10.46 V maximum, in 256 steps of 38.9 mV per step, to a 0.5 V minimum. Each step has a pulse duration of 100 l"s and an amplitude decrease of 38.9 mV. The waveform output is repeated until a reset is applied to the uA2240.

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