IEEE Consultants' Network of Long Island

A Sneak Noise Source - John Dunn, Consultant, Ambertec, P.E., P.C.

I once chose a TO-99 packaged, low-noise op-amp, the OPA2111 when that device was a product of Burr-Brown. Today, that device is being sold by Texas Instruments.

I got some really nice results with it, but I was just plain lucky to have avoided trouble with that part from a noise source I hadn't reckoned with, ambient light.   

The OPA2111 uses a pair of junction FETs for its differential input stage. The trademark for this feature  is now Difet ®.

According to a Burr-Brown tech-rep with whom I spoke way back then, those two FETs were quite large, occupying approximately 50% of the chip's area all by themselves. Being large, those FETs were highly sensitive to ambient light that could sneak into the package via the glass beads that held the eight leads of the TO-99 can.

The Sculptor - John Dunn, Consultant, Ambertec, P.E., P.C.

The company had hired an engineer for a microwave project and the fellow dug into the work with energy and gusto. He seemed genuinely happy, until......

The planned design was going to need some stripline structures, but an examination of several circuit boards convinced this engineer that the available geometric precision of circuit board artwork at that company was inadequate for the purpose at hand. Therefore, he brought in from home an utterly exquisite set of precision carving knives and working by hand, he actually carved the required stripline structures out of raw copperclad circuit boards without ever etching them.

His stripline carvings were wonders to behold. Engineer after engineer, upon seeing them, walked away awestruck.

Ground - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this 12-bit analog-to-digital converter (ADC) which was set up for testing in a fixture that this fellow had assembled in a 19-inch rack that was maybe seven feet tall.

First, let's look at some numbers:

The ideal mV steps of this ADC were one part in the twelfth power of two, take away one (for zero to +5V exactly). That comes to 1/(2¹² - 1) or 1/4095. At the ADC's full scale voltage of five volts, that comes to a nominal resolution of 5/4095 volts which is very nearly 1.221 mV. The ADC tests were failing their test limits and I was asked to find the reason why which did not please the fellow who had set up that rack. I began the process anyway.

Opto-Coupler In Linear Service - John Dunn, Consultant, Ambertec, P.E., P.C.

We may examine the linear properties of an ET1104 opto-isolator as follows.

Note that there was no reason in particular for choosing an ET1104 to examine. There just happened to be one on hand. Also, the 1.33K to the LED was used only because it happened to come to hand at the moment. It could just as well have been a 1K.