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.

If that light came from fluorescent lamps, the result would be injection of 120 Hz noise into the signal paths which would have been really bad news for that particular project. 

 

Fortunately, the circuit board on which these op-amps were used was inside of a light-tight box where darkness ruled by night and day, so everything stayed nice and quiet.

 

Comments

[George Storm]

Did burr-Brown use uncoloured glass?

[John Dunn]

I couldn't tell.

[Jaya Bijor]

If its exposed to light it will be affecting the performance upto some extent..But, its good that the Circuit-Board is inside of light-tight box..

[Rudy Schneider]

This reminds me of a time, back in the 60's and 70's, when a firm I joined (in 1975) was using a lot of discrete analog circuitry for +/-125V op-amps in analog computer applications for flight motion simulation platforms. They'd experienced leakage problems with glass diodes, and from that time forward always added some black heat-shrink tubing around the diode before assembly.