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Nolan Johnson

Nolan Johnson knows his way around CAD tools, and circuit board manufacturing. Having started his career as a CAD tool programmer, with over seven years at Sunstone Circuits, and more than twenty years in IC & PCB manufacturing, Nolan can help with design workflow issues and Manufacturing concerns for your PCB design work.


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  • I'm a postgraduate student at the Indian Institute of Astrophysics, Bangalore, India. I'm working on a 6- chaannel photodiode amplifier, which will be used for site surveys for a new telescope coming up in India.

     

    A dual JFET opamp (LT1464) powered by ±15V supply is used to amplify the signals from a very sensitive photodiode (Thorlabs FDS1010) In the first stage, the current in the range of nanoamperes from the photodiode is converted to desirable volt range by a resistance of 9.1MΩ.


    Thereafter, DC and AC components of the signal are separated and given different amplification coefficients with the second stage, along with another low pass filter that eliminates all frequencies above 400 Hz. AC component of the signal is amplified by 110 times, while the DC is amplified by 2 times so as to bring out the scintillations from what would have been a DC dominated signal. A high pass filter is also integrated into the circuit to only admit all frequencies over 0.1 Hz. Necessity of clean power requires appropriate smoothening capacitors, to be connected across the power supply of the amplifier

     

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    With the current circuit configuration, the circuit tends to oscillate giving an output sinusoidal signal of less than 15V amplitude, without the application of any signal. According to Fig, it’s giving a sinusoidal signal of about 10-12V with NO DC offset, and a frequency of 564Hz in the absence of light from the LED (or for that matter, any light at all). Even when the photodiode was disconnected, the oscillations were still present, as shown in the figure

     

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    Following is the PCB layout

     

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    After reading more about PCB design, I found that the PCB was full of parasitic capacitances and there was plenty of crosstalk between the output of the six channels. So I decided to separate all the channels and adopt a better PCB design

     

    I'm making another board after reading through some PCB guidelines. I'm populating it only with SMD Resistors and caps (hopefully reducing the noise) and giving a proper ground and power plane. So, can you tell me whether there's any problem with this design before I give it for fabrication?

     

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    Following is the PCB design without the ground plane (for better visibility of the layout)

     

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  • in reply to Former Member

    Hello,

     

    I have a startup and we have NO CLUE about any of this and we need something that is small, can play video or a photo slideshow without an OS and can attach a screen and need it as thin/compact as possible... How do we go about doing this and cheap or is their something that already exists?

     

    Thanks,

    Lance

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