Tag Archives: SPICE Models

Filter Fiasco: Chapter 1

One of the circuits I have to design for my thesis is a bandpass filter.  Based off the specs I was presented with back in the early fall a filter with f_center = 100 MHz and Q = 250 was required; plus f_center needed to be tunable without changing Q. Not exactly the easiest design in the world but I studied up on a few topologies and settled on the Dual Amplifier Bandpass filter (pages 5.74 and 5.93).  According to Matlab and some hand calculations a 4th order filter was all I needed.

Figure 1: Dual Amplifier Bandpass Filter Schematic

Cut to Rev. 1 of the board and not a single aspect of the filter met spec or even remotely functioned as a bandpass filter.  Simply scoping the output showed my design self-oscillated around 50 MHz, fantastic.  Adjusting the potentiometer I put in place for R2 merely shifted the frequency of oscillation. No amount of debugging or rework could make the filter behave and according to one of my professors, my use pots in the first place was a recipe for disaster because of high parasitics along with poor overall performance at high frequencies. Another important thing to note was my use of a current feedback amplifier as opposed to a conventional voltage feedback amp  because of the higher bandwidth and slew rate they offer at high frequencies (foreshadowing, this will haunt me later on…).

After discussing things with my advisor we decided our first attempt was too ambitious and to spin a second revision of the board only this time with a few changes in the specs.  Mercifully, having a tunable center frequency was no longer required. It was determined that this feature wasn’t necessary in the prototype stage and that designing a new tuning method would take too much effort, thus preventing me from completing more important aspects of the project.  The center frequency was also dropped to 10 MHz which lowered Q down to 25 giving a much more achievable design.  Refining my Matlab simulations and hand calculations showed that I was actually incorrect on my first attempt (whoops) in regards to the number of stages.  With these new specs I would need a sixth order filter.  I decided to keep using a current feedback amplifier though I changed parts from Rev 1 and picked the THS3202 from TI.

With my first design having crashed and burned I turned to PSpice to see if I could get my design working in simulation before spending time in hardware chasing something that may prove to be a dead end.  Using  Intersil’s AN1613 (mentioned in my last post here) I downloaded the Spice model for the THS3202 from TI’s website, incorporated it into my schematic and began simulating.  I eventually got my filter working and meeting spec with the help of some compensation techniques from other app notes I discovered and got the results below in Figures 2 and 3.

BPF Mag Plot

Figure 2: Magnitude Plot of BPF

BPF Current Pulse Response

Figure 3: Vout of BPF to Current Pulse Input

From these figures everything appears to be in order, there’s a nice bandpass shape to the filter that met spec, a decent response to being hit with a 100 uA current pulse for 30 us, yadda yadda. All that should have been left was to slap it on a PCB and make sure it functioned right?  Stay tuned for Chapter 2 as our story continues…

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Useful App Notes: Part 1

Over the years I’ve accumulated a rather astounding number of app notes on my hard drive from various companies.  What few ones I’ve read so far are, in my opinion, very useful and should be shared.  The rest I’ve downloaded thinking “This looks like something I should keep around in case it ever comes in handy,” only to never open them and let them collect virtual dust.  Sharing the interesting ones on my blog seemed like a win-win-win situation, I learn things reading app notes, you learn things from reading app notes, and I de-clutter my hard drive/Dropbox account,everybody wins. So without further ado here’s a few of the most useful app notes I’ve come across.

AN1613: From SPICE Netlist to Allegro Design Sub-circuit, Intersil

If you’re like me whenever you come across a part that isn’t in any of the main component libraries when using P-Spice you think to yourself “Is this part really necessary? How well does my design work with this op amp instead of the one I want?”  I did this because up until I found this app note, even if I could find a SPICE model online for a part I wanted, I didn’t know what to do with it.  Enter AN1613.  It goes through an easy step by step process of how to take a SPICE model you’ve found online and actually make it work in a Cadence Allegro Design  simulator.   I’ve used this app note to help simulate parts from a few IC companies with great results.

Op Amps for Everyone, Texas Instruments

While not exactly a single app note, Op Amps for Everyone is still a great design guide to have handy when you need to quickly refresh yourself on a topic or get a basic overview of a concept before researching it in more detail.  It covers everything from basic circuit analysis and feedback theory to filter design, converter interface, and everything in between.  There’s plenty of examples to go along with the theory as well as a whole chapter on layout considerations.

AN95-1: S-Parameter Techniques for Faster, More Accurate Network Design, Hewlett Packard

Here’s a blast from the past, AN95-1 was first released in 1967 by HP. While slightly on the old side, I still found this app note pretty useful when trying to wrap my head around S-Parameters last summer.

Got any favorite app notes you can’t live without? Share them in the comments, I’m always looking to collect more of them.