EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Dan Moos on April 29, 2016, 02:32:03 am
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Ok, I think I know the answer here, but I want some affirmation or correction. Apologies to those that helped with a similar question earlier. I still need clarification i guess ::)
I have a design that uses a crosspoint switch chip (AD75019) to distribute audio signals, some of which are relatively high gain (guitar pedal signals to be exact)
I plan to use opamp buffers (t074's) every where the signals enter and exit the switching chip. There will be a main PCB withe the switching chip and the MCU, and a sub board with the signal jacks on it. I currently plan to use screw terminals to join the signal lines to the main PCB, which will be on individual shielded cables to the board with the jacks.
Ok, I feel which end of the signal wires I place the buffers is key to good noise/interference performance. My intuition says that the lines going IN to the buffers should be as short as possible, so the opamps should be close to the input jacks for signals entering the device (ie, on the jack board), and the lines where a signal leaves the switch chip to a buffer should also be short, so for signals exiting the device, the opamp should be close to the switching chip.
In other words, any line going into the high impedance input of a buffer should be short in order to reduce noise/interference.
Is my thinking correct?
My reasoning for wanting the buffers is that the impedance of the switch chip inputs/outputs is apparently pretty dependent on signal level. My current breadboard setup works much better with the buffers in place, but when the gain levels in the pedal signals is high, there is a lot of noise, especially noise from the LCD digital signals. I realize alot of this is grounding and power, not to mention alot of cheap breadboard jumpers, but I still want to be sure my bases are well covered when I make the PCBs. The PCBs will be DIY, so I can afford a few bad designs, but still don't want to waste my copper clad board supplies making unnecessary duds.
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Correct. The high impedance input should be as close to the signal source as possible. Gives then a low impedance output which is less susceptible to interference.
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Once you done layout, you can post it here and lot of people may come and help you out.
Analog devices and TI have nice public appnotes and handbooks regarding opamp layout and placement questions, be sure to check those out. Most of basics and design approaches are covered well there.