LMH6702 Pulse Shaping PCB Issues - Drawing too much current, no physical short. - Page 1
Forum section: electronics / projects

author: lskop
author_rank: Newbie
timestamp: August 27, 2015, 11:04:00 am
content: Hello all,I am currently working to make a nice PCB for my pulse shaping circuit that's part of my final year project.I've got a prototype working, with lower bandwidth (rail-to-rail op amps OPA350), now, I know this is no real test for the LMH6702, but the gist is the circuit can work.I've built the + supply regulator, and the negative rail (through the LM2662 voltage inverter). When the circuit was turned on, open circuit, I was reading the +5.5 volts, and -5.3v which is within the range I expected. Of course, as I soldered in one half of the circuit, things changed.There was 4 LMH6702 op amps soldered in, and when I begun to test them, they were oscillating like crazy, without any input, and no difference was had with an input.I then measured the voltage output from the negative rail, and noticed the voltage was down to around -3.8v, and quickly realised this was due to a large current it was drawing (around 350mA). So I turned the supply off and begun inspecting the soldering for shorts or any stupid things (like incorrectly placed components etc). Could not spot anything noticeable.I then desoldered all op amps, the voltage inverted and added a single LMH6702 (the first stage). Note, all cap's and resistors for the circuit are in place. I tried to power this, from a reliable power-supply (+ and - 5.5V), directly inserting the negative rail and it was drawing 70mA from both the positive and negative sources, and so I quickly disconnected it. I had the grounds of the two supplies tied together, and measured the correct voltage with the multimeter.So basically I have the LMH6702 looking like a buffer op amp, the output is in series with a 1nF capacitor, which is in parallel with a 1kOhm resistor. The, output was oscillating like crazy (explaining the high current draw), but I have no idea why! I feel like this basic circuit set-up shouldn't be too much trouble.The only thing I can think of is, although the LMH6702 is unity-gain stable, the fact that I don't have a resistor in the feedback, nor in the input (even if equal for unity gain), may be loosing stability?So has anybody got anything to share, ever used the LMH6702? Managed to use it without a feedback resistor.I guess, if it's worth anything (I doubt it), but the circuit works as expected with TI's provided spice model ... but not too certain if it'd capture this kind of behaviour (highly doubt it).Please, if you have anything I can try, or any experiences you've had with something similar that'd be great. I really don't have that much time left to debug nor redesign/remanufacture the board. I may just go with the working veroboard prototype.Thank you in advanced for all your help and suggestions.Cheers.

author: toli
author_rank: Frequent Contributor
timestamp: August 27, 2015, 11:24:41 am
content: 1. In general, with all types of amplifiers, connecting large capacitive load can be problematic for stability. This is because it moves the non-dominant pole created by the output resistance of the op-amp with the load capacitance to lower frequency, and might place it within the area where the loop gain is >0dB. A typical solution is to add a resistor between the output of the amplifier (where the feedback is taken) and the capacitive load.2. For a CFA (current feedback amplifier), like the 6702, the feedback resistor is a very significant part of the feedback loop, and cannot be simply omitted. I suggest that you not only place the resistor as explained in the datasheet, but also try and read more about it to understand why it is critical. There's no point in writing it here since this is something that has already been explained many times in the past. Here's just one of these:http://www.intersil.com/content/dam/Intersil/documents/an94/an9420.pdfyou can find many more such app notes by googling the current feedback amplifiers.

author: lskop
author_rank: Newbie
timestamp: August 27, 2015, 11:41:56 am
content: Thanks Toli,I've just been researching in the time since I've posted this, and have come to that conclusion. I guess it was silly of me to use a current feedback op amp without fully understanding it's architecture, hell, even a quick look at the laplace equation and I could have realised the impact. We live and learn! (Never going to make this mistake again).