High Frequency Voltage Clamping Circuit - Page 1
Forum section: electronics / projects

author: Lolucoca
author_rank: Contributor
timestamp: August 15, 2023, 05:38:49 pm
content: Hey y'all!I'm a computer science student designing and optimizing a circuit for his master thesis. The circuit consists of an AD9520 PLL driving an AD9954 DDS, the output of which goes into a THS3215 DAC output amplifier. The whole purpose of this circuit is to generate a 5, 10, 20 or 40MHz sine wave with a set phase to an input clock. Long story short, I'm working on a fail-safe system where the amplitude of the output is limited to the range of [-0.5V, 3.5V].I've looked into usable circuits for this and found a clamping circuit using two schottky diodes. I've tried that, however, the diodes I tried (BAT54 and 1N4148) didn't clamp the output very well, losing effectiveness with increasing frequency.There are also TVS diodes, but those usually clamp at around 7V minimum.There's also the option of using a unity-gain op amp stage with built in voltage limiting, such as the OPA698, which I may fall back to, if there are no other options.Do you fine people know of any ways to clamp such a high frequency signal effectively?Thanks a lot in advance!

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 15, 2023, 08:08:31 pm
content: At around $8 a pop at Digikey why not usethe OPA698? Looks like it's exactly what you need.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2023, 08:35:12 pm
content: Why would the DAC generate any more voltage than it's set to? Is this an output mismatch thing perhaps?Tim

author: Lolucoca
author_rank: Contributor
timestamp: August 15, 2023, 08:49:57 pm
content: There shouldn't be any unusually high voltages at the output during normal operation. The thing is, it's connected to a really expensive electro-optical modulator and I really don't want to take any chances, in case something goes wrongMy reasoning behind looking for something else than the OPA698, the additional component would introduce noise into the system, which I try to avoid.

author: Benta
author_rank: Super Contributor
timestamp: August 15, 2023, 09:26:42 pm
content: Look at USB protection TVS devices.

author: moffy
author_rank: Super Contributor
timestamp: August 15, 2023, 11:09:43 pm
content: If for the final output amplifier you can use a Rail to Rail output opamp and limit the voltage supply to that to say -0.7V and 3.7V your output will be clamped. The only clamping you then need is on the supply rails which is much easier to do since they are DC. The extra little +/- 0.2V gives the output some headroom, but if you don't want that just go with -0.5V to 3.5V supplies.P.S. If worried about inductive ringing you can load the output of the opamp with a suitable low value resistor, that should dampen any ringing.

author: Lolucoca
author_rank: Contributor
timestamp: August 16, 2023, 07:21:31 am
content: Speaking of rail-to-rail op amps, couldn't I just limit the supply voltage of the THS3215 to something like +- 5V since that has a typical 1.4V output headroom to the supply voltage? That would give me *some* clamping, at least for positive voltages.

author: moffy
author_rank: Super Contributor
timestamp: August 16, 2023, 01:41:49 pm
content: Just another possibility, if you look at page 36 of the datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/AD9954.pdf, they use an RF transformer to do the differential to single ended conversion. If you used a transformer with a 3:1 ratio you would get 3Vp-p out for 1Vp-p input, and the transformer doesn't contribute any noise. The output impedance of the RF transformer would be 9 times the input impedance, in this case 50R in and 450R out, I don't know if that is an issue. A possible candidate would be:https://www.minicircuits.com/pdfs/ADT9-1T+.pdf

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 16, 2023, 03:25:56 pm
content: Is beyond the range of [-0.5V, 3.5V] where the "really expensive electro-optical modulator" is actually damaged?Or is the range of [-0.5V, 3.5V] simply it's recommended operating range?Do you have a part number or spec sheet for this device? It might be able to take quite a bit of overdrive before it's damaged.

author: Lolucoca
author_rank: Contributor
timestamp: August 17, 2023, 10:28:45 am
content: Thanks for all of your replies!The electro-optic modulator is an NIR-MX-LN-10 (https://www.ixblue.com/wp-content/uploads/2022/01/NIR-MX-LN%20SERIES.pdf) which needs a 50 ohm terminated input, so the balun solution would probably not work here, as far as I can tell.I like the idea of using USB clamping diodes such ashttps://www.mouser.de/datasheet/2/916/PESD2USB5UX_T-2938465.pdfsince those shouldn't alter my signal too much, with that nominal capacitance of 0.47pF. I'm not sure, I understand entirely how the datasheet works though. Figure 3 shows the VI-Characteristics of said diode. The X axis shows the voltage, the Y axis shows the current flowing at said voltage across the diode. But why is -3V3 all the way to the left, -8.7V in the middle and -Vrwm (Max. 5V) on the right?

author: moffy
author_rank: Super Contributor
timestamp: August 17, 2023, 12:01:10 pm
content: Quote from: Lolucoca on August 17, 2023, 10:28:45 amIf you wanted to try the balun solution you would only need a unity gain buffer to buffer the output of the balun and drive the 50R input to the modulator.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 17, 2023, 03:31:16 pm
content: So I don't really understand where you get the [-0.5V, 3.5V] limit for the 50input when the absolute maximum in the spec-sheet is 28dbm.. (7.9V peak / 15.8V pk-pk / 5.6V rms)

author: David Hess
author_rank: Super Contributor
timestamp: August 17, 2023, 09:53:49 pm
content: A current driven diode bridge can be used to limit signal swing. Tektronix used this to protect 50 ohm inputs.