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Another Source of Front End Noise.

A

Anonymous

Guest
About four years ago, I was looking at the noise performance of various front end arrangements, and it seemed that the transmitter also contributed to the noise level. When the TX Mosfet is disconnected from the circuit, rather than being just off, the noise was lower. This could do with being looked at again as there may be various reasons for this. e.g. the Mosfet mounting tab (drain) is connected to a heat sink which can act as an antenna for noise external to the circuit, or even on board sources such as unsynchronised 7660's or other psu inverters.
As to opamp data sheets, for PI front ends, the only sure method is to try one and see if it works. I have bought countless opamps on the basis of a good paper spec, only to find that it is useless in this application. Clean recovery after the flyback pulse is the main problem area.
Eric.
 
Even when devices are off they can still have thermal noise (but not shot or flicker), perhaps this noise couples through the fairly large parasitics.
You're right about trying opamps, I grabbed some different hot-rod ADI amps and found that several looked like el crappo in the flyback region.
- Carl
 
Hi Eric,
What about the clamp diode that just had a good shot of current put through it? Well the mosfet just had more current put through it, is Johnson noise (themal noise) at work here? If you split the input resistor to the front end amp, and from the diode you have a 100 ohm resistor with a 1 Megaohm feedback (gain 10,000), sure taking a hard look at that diode.
JC
 
Hi Eric,
Noticed a post by you on the subject of getting killed by a 9 volt battery. I had heard of automotive mechanics getting killed off 12 volt car batteries. But never knew exactly how.
Was reading some European Safety Agency stuff a year or so ago, and it talked about a German computer technician that killed himself on a 5 volt computer supply.
Of course this is blood to blood (punch through skin on both ends), current through the heart, and they stated the resistance can be as low as 20 ohms. I'm not going to measure this (and kids don't try this at home), and it probably depends on how many salty pretzels you've eaten.
The numbers I have heard (and I've heard different ones) is 35 milliamps through the heart, or 3-5 milliamps through the temples (this one should be harder to do). Could make a good english murder mystery.
Anyway something to keep in mind when playing with gel cells and other batteries. Seriously.
JC
 
Well... sorry... it looks like a horrible misunderstanding. The main resistance barrier is a co called double layer (electrode/electrolyte contact) - a few examples from Annals of Biomedical Engineering, Vol. 29, pp. 181
 
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