Phototransistor fed to a transimpedance amplifier

SilverLeo · 2016-07-17T07:56:45+00:00

Hi all. I'm running through some test of a phototransistor signal conditioner, and I'm facing with wild noise issues. The conditioning is a basic TIA stage: Feedback resistor is 5K, feedback capacitor is 470pF, negative bias is -12V, and photocurrent is in the range 8uA-800uA. The bandwidth should be about 50kHz. Opamp is OPA2277UA. Problem is that phototransistor if far away from TIA, with four meters of cable running along with power cables (valves, actuators, motors in the 5kW range). This causes tenth of uA of noise well inside the signal bandwith. The same phototransistor, with same cabling, works great with the simple resistor between collector and supply rail: TIA is better for me because it will keep phototransistor far away from saturation, but maybe the finite opamp bandwidth is actualy amplifying noise. Hints?

Jul 17, 2016 45 Replies

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ChesterW 10 years ago

First stop on the trip was Homer. Agree it's a great little town. There was an ultrasonic level sensor at the head of a new wastewater treatment plant, controlling a couple of pumps with VFDs. Of course a very glamorous assignment :). The PE had run the instrument power with the motor power and this caused the sensor electronics to reset every 20 seconds or so. They had to control it by hand 24/7. It was part of the contractors check-off list, and they wanted to make sure it was really fixed, so I got to spend a few day wandering around Homer. The fish restaurants on the spit really spoil you.

Commercial fishing is big in Alaska, and the Fish and Game guys cut off the season after the boats reach a certain catch limit. They weigh the fish when the boats off-load. This leaves a lot of capacity still at sea after cut-off, so they don't get much precision in their control. So after Homer, I got to go out to Dutch Harbor to judge the feasibility of weighing the catch right on board the boats.

I didn't get a chance to go to Kodiak. The area is certainly lovely, but what is the draw of Spruce Island?

Best Regards, ChesterW +++ Dr Chester Wildey Founder MRRA Inc. Electronic and Optoelectronic Instruments MRI Motion, fNIRS Brain Scanners, Counterfeit and Covert Marker Detection Fort Worth, Texas, USA www.mrrainc.com wildey at mrrainc dot com

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ChesterW 10 years ago

The inductors on the outside look really cool. I'm doing that on my next design! :)

Best Regards, ChesterW

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Phil Hobbs 10 years ago

It's where St. Herman of Alaska lived. He was a monk from Valaam in north-w estern Russia who with his brothers walked all the way across Siberia and c rossed the sea to bring the Gospel to the indigenous people of what was the n Russian Alaska. They all died there. Herman spent his life in service to them, often opposing the Russian authorities on their behalf.

Really a remarkably holy place--there are pilgrimages there all the time. M aybe next year.

Cheers

Phil Hobbs

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Robert Baer 10 years ago

Did you try adding a 4th toroid as a tri-filar common mode filter? BTW, did the red and blue toroids get close to saturation on power peaks? That increases their loss and tends to (at minimum) shift the spike phase for an even lower interference.

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