High speed comparator with output voltage reference? - Page 1
Forum section: electronics / projects

author: kizzap
author_rank: Unknown
timestamp: March 26, 2014, 01:10:15 pm
content: Hey guys, got an interesting problem that I just can't figure out.I am working on making a Signal Generator, trying to get a device capable of outputting a sine, triangular and square wave output, from "DC" up to a minimum 20MHz, with an output amplitude of between 1 to 10 volt. Simple, right? Well yesandno:So far the only DDS chip that I have found which is suitable for the purpose is theAD9834.Depending on the version of the chip, it is either capable up to either 25MHz or 37.5MHz, so check that tick-box. Where it falls short though is that it is only specified to output a sine or triangular wave, which leaves me to solve the square wave output, which I think I have, to an extent.I figured that if I compared the output of the sine, or even better, the triangular wave against a known voltage, I will get my square wave, and if I can control the known voltage, I can then also control the duty cycle of the squarewave. (read: BONUS!)Here is where I problem lies: I can easily work out the comparator to get an output, where I struggle is to latch the voltage down to a known voltage, in this case 1.000V, when high. So far the only solution I can think of is to use an op-amp to attenuate the signal down, controlling the gain on an op-amp with a trimmer, setting the output voltage to the level you need. However this solution poses issue in that trimmers are shit.So the facts are as follows:I have a square-wave output.The square-wave output has an adjustable duty cycle.As it stands, the output will have a fixed high and low value.So basically what I am after is a comparator, with a way to limit the output to a known voltage, Ideally through a voltage reference pin. Does such a device exist?-kizzap

author: nickm
author_rank: Regular Contributor
timestamp: March 26, 2014, 02:25:50 pm
content: The LT1715 is an example of something that might fit your requirements. There are many comparators that have separate Vcc and/or Vee rails for the output. You still might have to attenuate in one way or another because of minimum voltage requirements. You also might try to roll your own high speed buffer stage which can tolerate 1V-10V and can be hung off a regular comparator.

author: jpb
author_rank: Super Contributor
timestamp: March 26, 2014, 02:48:03 pm
content: This is the technique that Agilent used in the 33250 - see page 300 of the manual:http://cp.literature.agilent.com/litweb/pdf/33250-90002.pdfThey use a DAC to set the threshold on the comparator but they have to use a different wave shape below 2MHz to reduce jitter (different from sine that is - you're idea of using triangle might be better though I think you'll need to set a large amplitude to get a decent slope.

author: kizzap
author_rank: Unknown
timestamp: March 26, 2014, 03:04:45 pm
content: Quote from: nickm on March 26, 2014, 02:25:50 pmhmmm, itisan interesting chip, functionally what I am after too. The only catch is it is limited to the output working off a 3V supply.Ironically, I have just realised that I already have a solution on board. I am using an analog switch, to switch between the unfiltered output of the DDS, with the output I am forming the square-wave signal on. I just realised that if I connect the output of the comparator to the switch control of the analog switch, I can set whatever reference I want to the inputs of the switch, namely 1.000V and GND, and simply switch the output between the two values, generating exactly what I want. I swear I am a genius some times....Off to find a nice and fast analog switch-kizzap

author: dannyf
author_rank: Super Contributor
timestamp: March 26, 2014, 03:16:25 pm
content: AD9850 has a comparator built-in. Your own chip has a comparator as well.A comparator to 20Mhz is not difficult. The adjustable reference can by done with a fixed reference + a digital pot.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2014, 03:56:03 pm
content: Ew, don't use a DDS for that. At least, not such a slow one. The lumpiness of the waveforms will plague you for anything over, oh, 400kHz or so. The triangle will look steppy or jagged and the sine will be steppy and bouncy and stuff. How much will vary by code. You'll only get true 10 bit cleanliness with enough steps to make sense of it -- roughly 1000, or down around 37kHz. (There's actually some pi involved in that statement, but it's the right ballpark without.)As for the comparator, you're looking for a 1 bit DAC. Simple solutions include: 74HC1G04 (or 74LVC) powered by VREF (VREF > 3V or so), lightly loaded; diode gated current source/sink into a resistive load or TIA; various switching or clamping methods using diodes, BJTs and etc.Note that the output buffer isn't anything to scoff at, either; my 20MHz Wavetek 193 uses a strange complementary emitter follower circuit with a pair of 2N3866s (and a pair of whatever the complement is, I forget) for 10ns risetime, +/-15V into 50 ohms. You might be able to buy an off-the-shelf buffer for this (well, I know you can), but it's probably not cheap, either.Not that I would recommend you build a Wavetek 193 from scratch. Sucker's chock full of 10k series ECL, fast transistors (of the PN3563 sort), and obviously, a fancy output stage (though sadly not a beefy one). Many things can be done differently today. I wouldn't recommend a DDS approach, but doing it the analog way isn't as hard as it may seem, I think. And, what do you want out of it? A long term project, or something that's stable, accurate and easy to use? If the latter, just buy one; $100 should be enough on eBay. If the former, it would be an excellent introduction to analog, precision and high speed circuit design.Tim