| Electronics > Projects, Designs, and Technical Stuff |
| Simple single-supply, negative ramp/sawtooth generator idea? |
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| ebastler:
Well, lets face it -- my analog design skills are marginal. ::) So I could use some help... From an FPGA, I would like to drive a simple negative sawtooth output: With approx. 3 kHz repetition rate, set the output "high" (rise time up to 1µs), then let it fall reasonably linearly to the "low" level (for approx. 300µs). "Resonably linear" means within a couple %, so no high precision requirements, but exponential capacitor charge/discharge curves won't be good enough. I have a 3.3V supply available. To drive/trigger the ramps, I can provide one or two (complementary?) digital signals with whatever duty cycles makes sense, toggling them between 3.3V and 0V, or high-Z if that helps. The output does not have to operate rail-to-rail (0V to 3.3V), but at least 1Vp-p output amplitude would be desirable. There must be a simple solution to this, using an op-amp or two, or an NE555 -- but I have not found any "cookbook" solutions, and can't figure out anything simple myself. Any ideas or pointers are much appreciated! |
| temperance:
I see several options: -A SPI DA converter if you have four pins available -A resistor network DA converter if you have 6 or more pins available. -A resistor network DA converter connected to the outputs of a 74xx595 like shift register if you three pins available. -A PWM output with a low pass filter. (one op-amp, two or four resistors and two capacitors) -An integrator with the non inverting input biased around 1V. (the integrator input is pulled up to the op-amp VCC. To reset the integrator, the input is pulled low with an open collector output with 100R in series connected to the inverting input. Make the integrating capacitor just a few nF) |
| MT:
Dont really understand what you want but you can rearrange a 7555 (dont use 555) to generate up or down ramps or drive it from negative voltage and do the same but then ramps are -V going. Some 7555 can be driven from +2V. |
| temperance:
Attached you will find the integrator based version. For the op amp you will need to select a rail to rail op amp like the OPA350 or something alike. |
| magic:
ICs are boring :P |
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