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I have a simple triangle generator circuit soldered and powered by 5V rails shown below. Verified by simulation and equations, it should output a 500kHz triangle wave a \$\pm4.2V\$. Instead, the only thing visible at the output is the power supply noise with additional high frequency noise at the peaks.

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It seems like the issue is with the supply rails. The power supply is showing 5V but at the terminals on the board i'm seeing +1.8V for the positive rail and -3.2V for the negative rail. What is the cause of this and how is it possible for this circuit to bring down the supply rails?

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  • \$\begingroup\$ You win the arbitrary sig. gen award of the year. (lol) asymmetric 40Hz (?) with a chirp burst carrier, but at least you a have a 5.0Vpp avg swing. Is that your integrator output on U1A? U need 0V on U2-5. U1-3 and then +5, -5 with caps. When in doubt, check every pin against spec.. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 5 '19 at 12:32
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I have a simple triangle generator circuit soldered and powered by 5V rails

Given that you said "rails" and not "rail" AND that you said the output should be +/- 4.2 volts I have to assume you are attaching +/- 5 volts to the circuit. If you are then you have probably destroyed the LT1719 because it's maximum supply voltage (Vs to GND) is 7 volts and not 10 volts: -

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  • \$\begingroup\$ This is incorrect: VCC to VEE (LT1719 S8 ) ... 12V \$\endgroup\$ – Huisman Jun 5 '19 at 11:24
  • \$\begingroup\$ @Huisman don't be so hasty with this - read the data sheet and note that pin 8 in the circuit connects to +5 volts and pin 5 connects to - 5 volts - those pins (Vs and GND) should not exceed (or be close to) 7 volts. \$\endgroup\$ – Andy aka Jun 5 '19 at 11:25
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    \$\begingroup\$ You're right. +1 for detecting it. I should better read OP's circuit. It's a nice example that Vee =/= GND and Vs =/= Vcc \$\endgroup\$ – Huisman Jun 5 '19 at 11:29
  • \$\begingroup\$ @Andyaka ah, thanks for that catch. Does there exist such a comparator that can output Vcc to Vee? Or is it not possible to create a triangle generator circuit where the comparator outputs +/-5V. \$\endgroup\$ – dos584 Jun 5 '19 at 22:41
  • \$\begingroup\$ Try and find a decent of amp that can work on a single 5 volt rail and bias the mid rail to be 2.5 volts is my advice. \$\endgroup\$ – Andy aka Jun 5 '19 at 22:49
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I stumbled on this thread last week and was surprised the following fix didn't get mentioned: with three more resistors and one power supply connection change, the circuit can work as originally intended. As previously noted, the LT1719CS8 output stage cannot operate between +/-5V. But it can operate 5V single-supply, including from 0V to -5V. So with the changes below, the left-hand portion of the circuit is limited to that negative span. The OPA2810 non-inverting input is lowered to -2.5V by the additional two 4.99K resistors, and this is a better choice than +2.5V due to an aberration in the input stage at that level, when the "main" input stage is switched over to the the "auxiliary" one; see Figure 18 in the datasheet as well as the specification tables on pp6-7.

Fix with Left-side entirely negative

With the LT1719 output voltage swing half what the circuit was originally designed for, the R1 & R3 values are cut in half to preserve the same current swings. The right half of the circuit works essentially the same as before, but because the current of R3 is biased at 0 or 1mA, whereas before it was biased at +/-0.5mA, the added 10K resistor to +5V is needed to source 0.5mA into the comparison node to compensate.

Note: the actual current through R3 and the new 10K resistor will not be static, fluctuating with the Output waveform. These current levels mentioned are the currents where it counts: near 0V when the LT1719 comparator will trip.

While this circuit will happily preform as a Triangle Generator, the LT1719 output signal should not be connected to any logic gate inputs (to, say, monitor the "clock" of the output) because it is operating from -5V to Ground.

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