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Triangle Wave Generator

The triangle wave is generated by linearly charging and discharging a capacitor between two reference voltage levels, provided as the inputs for generation of triangular waveform. The simplified block diagram of triangular waveform generator is shown in figure 2. The potential range is scanned starting at the Initial potential ($V_{lo}$) and ending at the Final potential ($V_{hi}$). The input to the potentiostat $V_{pot}$ is then changed as a linear function of time. The rate of change of the potential with time (termed as scan rate) is set by changing the bias current ($I_b$) used to charge/discharge the capacitor ($C_{tri}$).

Figure 2: Block diagram of triangular wave generator. $V_{hi}$ and $V_{lo}$ set the maximum and minimum value of the waveform generated
\resizebox{!}{2.3in}{\includegraphics{triwave_gen.eps}}

The two comparators used in this design were designed to work at fast speed so that the output state changes immediately as the triangular wave touches $V_{hi}$ or $V_{lo}$ assuring that triangular waveform is confined within their specified range. We have used high-speed strobed comparator [4] for our design. The delay involved during the comparison of $V_{hi}$ with the voltage on the capacitor was determined to be around 4.5ns from simulation results. The corresponding delay on the other direction is 4.3ns. Depending upon bias current, it will results in little error but it is well within the tolerance of the circuit. With the change in the bias current, frequency of the generation of triangular waveform can be changes. As the frequency of the generation of triangular waveform is reduced, overshoot of the peak-to-peak voltage of the triangular waveform from the limits specified by $V_{hi}$ or $V_{lo}$ reduces and is near negligible below 50kHz. The outputs of the two comparators is fed into SR-latch whose output controls the switching of the transistors PCH and NDS. Either one of the two transistor is always OFF. The bias current ($I_b$) is generated using a single source and mirrored using modified Wilson current mirrors (not shown in the diagram). Table 1 shows the positive and negative overshoot of the triangular waveform at different frequencies.

Table 1: Triangular Wave Generation: Simulation Results
Frequency(Hz)) Overshoot(pos, neg)(mV)  
9.6 1,1  
69.6 1.1,1.1  
188.3 1.1,1.1  
9.6k 1.3,1.4  
57k 0.9,1.1  
305k 4.3,5.1  
1.2M 13.6,16.2  
3.2M 31.2,37.1  

From the above results, we can observe that the amount of overshoot is less than 10mV in the operating range of the signal generator from 10Hz to 1MHz. Average mismatch between the waveforms generated in this frequency range was 4.5
next up previous
Next: Comparator Up: Design Modules Previous: Design Modules
mr harpreet 2003-12-11