The Sound of Music

 One of the "funnest" projects I did was to build a Direct digital synthesizer (DDS). This part takes in a digital control word and provides a sine wave output. The frequency of the sine wave is proportional to the digital control word. Apparently one of the customers was a  toy manufacturer manufacturing musical toys spewing out classic songs. It is interesting but us engineers tend to skimp over applications of our ICs - you'd be surprised how many different ideas people can come up with (especially if your product is marked for the consumer space)

This part had digital logic, memory (ROM to store the sine codes) as well as DACs, buffers and amplifiers. In essence it was an SOC even before that term had been coined :-) At that time there was no AMS flow. In fact the company did not even have a verilog license (given the analog roots).  Thankfully we did have waveform viewers on screen and not reams of paper to look at (that was my undergrad school!).

Most of the time was spent on the current steering DAC and opamps (used in the low pass filter and the drivers). The customer wanted at least 50db of output fidelity so that translated back into quantization noise and DAC resolution/accuracy. The rest of the time was spent making sure that spice converged :-) No, it wasn't that bad but we did have clunky tools - making sure we actually knew how spice worked. I could design and simulate the individual blocks well but spice used to choke when doing a full chip simulation (remember everything including the memory block was in schematic form). Then you needed multiple cycles to actually do an FFT and make sure your fidelity is not broken. Ugh!

It took me about 5 months of design and layout supervision to get it to fab. I was proud of being the sole designer on it. In hindsight I can now appreciate how much EDA tools have helped increase productivity and made possible much more complicated ICs. AMS, verilogA, database sharing - some of my favourites. More of these in a late blog.

All the functionality and specification was met. However, one of the clock outputs (used only for debug/test) was showing a voltage swing between 5 and .5V (instead of 5 and 0). After beating down on the board designer and finding no leakage paths, I went back to my schematics and layout. To make a long story short, turns out one of the nmos transistors was not connected to ground but to the substrate which ultimately got connected to ground (but via a high Impedence path).

I miss the simplicity of being sole designer on an IC but I don't miss the EDA tools of that time.

The Power of Simplicity

One of the first projects I did straight out of school did not make sense to me.

I and another RCG (recent college graduate) were told to build a 6MHz video filter in .6u BiCMOS process. Our manager came up with a simple scheme of sallen key biaquads and used a simple bipolar source follower as the opamp. (well there was some local feedback but lets not get there) We protested - we wanted to build a fancy schmancy folded fully-differential cascode opamp. Our manager stood firm. The product was supposed to cost only 60c and replace the present solution of using discrete Ls and Cs by our customers. Apparently these customers all used custom solutions with discrete bulky components and we were going to "IC" it.

So we built this simple video filter family (there were multiple bandwidths for the different standards - NTSC and also different gain values etc) which consisted of 6th order filter made up of biquads. It had 2 trim steps for Bandgap reference and temco and some cool equalizer with a nice BiCMOS video driver. We did such a great job that the part worked first time met all specs and sampled to customers ahead of time. Turns out that simple IC sold more than 15mil units in the first year of its introduction.

Interestingly, another engineer was given the task of building a filter family with tighter specs - ladder filters, fully differential configuration and 5 trim steps. The firm thought that they good enter the professional video market as well. Turns out neither the value proposition nor the costing worked out so this complicated product never made it big.

Two lessons

a) The power of integrated circuit comes from ability to replace discrete bulky components with a clear value proposition.

b) KISS. Doesn't have to be cutting edge but has to have a clear value proposition for the customer. Lets see how a DSP person responds to this comment

Footnote: The product won best product award at EEDN in 1996.