| The uA2240 timer/counter,
combined with a precision 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/R1C1). For this example, a 10-K ohm resistor and a 0.01-uF capacitor form the timing
network. The counter outputs are connected to a precision resistor ladder network with
binary-weighted resistors. The current sink through the resistors connected to 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 Q0, 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 the positive output of the op amp in discrete steps.
The feedback potentiometer is set at a nominal 10 k ohm 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 ms and an amplitude
decrease of 38.9 mV. The waveform output is repeated until a reset is applied to the
uA2240. |