Design Ideas: February 15, 1996

The circuit in Figure 1 is a simple, adjustable square-wave generator. It provides a low-cost means to generate precise square waves that rival those of expensive function generators. The circuit can swing ±2V into a 50 Ohm load, and it operates from ±5V supplies with 55- and 35-mA quiescent currents from the positive and negative supplies, respectively. The key component in this circuit is the OPA678 switchable-input op amp (swop amp), IC₄. This device is a fast-settling op amp with high output-drive capability.

The swop amp has two independent differential inputs, A and B. The TTL input CHA [overscored] selects the active input: A for CHA [overscored] low and B for CHA [overscored] high. The OPA678 can switch rapidly (typically, in 4 nsec) between inputs. The VFC121 voltage-to-frequency converter (IC₁) provides a convenient adjustable clock. The duty cycle of IC₁ is a function of its output frequency. The D flip-flop IC₂ provides a 50%-duty-cycle clock for IC₄ by toggling on every rising edge of IC₁'s output. RTERM "squares up" IC₂'s output and is optional.

C_OS, C_INT, and R_IN determine the frequency range of IC₁'s output. The temperature drift of these components affects the temperature stability of IC₁'s output frequency. You'll obtain best results with NP0 capacitors and low-TCR resistors. For the values shown, the clock signal at IC₄'s CHA [overscored] input is adjustable between 10 and 750 kHz. A convenient feature of IC₁ is its 2.6V reference output. You use this in setting the frequency with potentiometer P₁. The reference also serves in setting the levels of the output square wave.

IC_3A inverts the reference, allowing you to use P₂ to adjust the offset in the range ±2V. P₃ adjusts the amplitude between 0 and +2.3V. IC_3B and IC_3C buffer the offset and amplitude signals. The buffered offset voltage goes directly to IC₄'s A input. IC_3D sums the offset and amplitude voltages and supplies IC₄'s B input. IC₄ is configured as a follower, so when CHA [overscored] is low, IC₄'s output is at the "lower" level, or the offset voltage, and when CHA [overscored] is high, IC₄'s output equals the "upper" level, which equals offset plus amplitude.

Even with RP₂ and RP₃ attenuating the reference, it is still possible to exceed the output range of IC_3D. In that case, increasing the offset or the amplitude further produces no increase in IC_3D's output. To overcome this limitation, S₁ allows you to set the lower and upper levels independently by feeding P₃'s buffered output directly to input B.

Figure 2 shows a scope photo of the adjustable-frequency clock from IC₁ (Trace A) and the output from IC₄ driving a back-terminated 50 Ohm coax cable (Trace B). The offset and amplitude are -1V and +1V, respectively. The output signal swings ±500 mV because of the 50% attenuation from the back-termination. The output frequency is 750 kHz. This rate requires a 1.5-MHz output from IC₁, its maximum operating frequency. IC₁ could operate at higher frequencies, but at the expense of increased nonlinearity, jitter, and temperature drift. For higher speeds, replace IC₁ and IC₂ with a faster clock generator. The transition and settling times of the OPA678 allow clock speeds exceeding 20 MHz. For best results, follow sound high-speed layout techniques, use a heavy copper ground plane, and place bypass capacitors close to the swop amp's power-supply pins. (DI #1829)