Producing analog sawtooth waveforms (linear ramp adopted by a fast reset to zero) from digital timing alerts is a typical perform. It often requires a detrimental provide as sketched in Determine 1, the place Vsaw = -(-V) Tsq/(R1C1).
Determine 1 The standard topology for changing digital sq. waves to analog sawtooth requires a detrimental rail.
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Unfavorable present by means of R1 ramps up inverting integrator A1’s output till change U1 resets it to zero. This topology is easy and works nicely, however it has the drawback of needing a separate detrimental rail so the integrator might be referenced to floor. Additionally, the RC community in entrance of U1, which must differentiate the optimistic fringe of the sq. wave, is type of messy and fewer than exact.
Determine 2 reveals an alternate sawtooth topology that employs flying capacitor C2 to reset A1. This permits it to be referenced to the optimistic rail as a substitute of floor, eliminating the necessity for Determine 1’s detrimental rail and pulse differentiation. Right here’s the way it works.
Determine 2 Flying capacitor C2 resets the RRIO ramp to zero volts, permitting it to be referenced to the optimistic rail as a substitute of floor.
RRIO op-amp A1’s optimistic enter being tied to the VL optimistic rail permits R1 to behave as a present sink between floor and A1’s summing level. This ramps A1’s output positively to:
Vsaw = VL Tsq / (R1C1)
Concurrently, flying capacitor C2 expenses to Vsaw by way of its connection to A1’s output by means of U1a. Then, on the positive-going fringe of “Sq. In,” the underside finish of C2 is related to A1 summing level whereas its high finish switches from floor to VL. This dumps a quantum of cost into C1:
(VL + (Vsaw – VL))C2 = VsawC2
On condition that C1 = C2, this resets “Noticed Out” to floor as proven in Determine 3.
Determine 3 “Sq. In” and “Noticed Out” voltage waveforms the place Vsaw = VL T/(R1C1).
For finest accuracy, R1, C1 and C2 ought to be precision sorts.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a good distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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