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Topic: Ferrite bead on 5V/GND pin? (Read 2376 times) previous topic - next topic

liuzengqiang

This adafruit design (http://www.adafruit.com/product/1085) includes a ferrite bead to the 5V pin and ground pin:

The design is an ADC breakout board. I wonder if this is standard practice for ADC supply pins. Thanks.
The specsheet has a ref design. It has no ferrite beads.

https://octopart.com/ads1115idgst-texas+instruments-11705599


Here is the bead used:
https://octopart.com/mmz2012y152b-epcos-33188135
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

westfw

extra power supply filtering is typical for analog circuitry.  Especially filtering from digital supplies.
Ferrite beads are one step in that direction.


68tjs

Quote
includes a ferrite bead to the 5V pin and ground pin:
I don't found the schematic but are you sure for a ferrite bead on the Ground pin ?

liuzengqiang

I don't found the schematic but are you sure for a ferrite bead on the Ground pin ?
Yea there are two ferrite beads, one to 5V and the other on GND.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

liuzengqiang

Thanks westfw. I will use ferrite beads then. I suppose I should do the same with atmega chip avcc and agnd, right?
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

ron_sutherland

Some food for thought: The ferrite bead will have a DC resistance (MMZ2012Y152BT000 is 0.4 Ohm), and the TI ADS1115 uses 200uA so that will cause a voltage drop (.2mA*0.4Ohm=80uV) on the ferrite bead that will offset any ground based measured voltage.
my projects: https://github.com/epccs

liuzengqiang

Thank you Ron! I guess I can use differential mode (signal against whatever its ground) if I want the true values. In an application I'm working on, I read voltages from a force gauge (it outputs analog voltages at about 1 gram accuracy weighing mass). So I only need differences from a tare point. This ground issue might be minimized by present voltage - tare voltage, assuming current consumption is stable. 80uV is also below what the force gauge can accurately output, which is about 1mV. For thermocouples, this is surely significant.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

68tjs

The ferrite bead will have a DC resistance (MMZ2012Y152BT000 is 0.4 Ohm)
This was the hidden reason for my question.
 Analog electronics standpoint  I  do not think that a ferrite in the ground connection is a good idea.
Ground must be as  perfect as possible.

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