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EEG or PCEEG Gizmo Hardware
Analog to Digital Converter
As the first Gizmo for this project we have created the Analog to Digital converter Gizmo. This Gizmo can be used separately as a stand alone device for high accuracy Analog readouts or it can be used in the application it was initially designed for which is EEG (Electroencephalography)
Main features of this Gizmo are:
ADS1256 which is extremely low-noise,24-bit analog-to-digital (A/D) converter. It provides complete high-resolution measurement solution for the most demanding applications.
8 Analog Inputs
SPI Interface which has Opto-Couplers on each line, thus opto-isolating completely the output and input signals on the entire circuit. The opto-couplers used in this circuit are ACPL-772L-000E
Operates on 12v, outputs an extra 5v line which is also filtered for Low Noise operations as the rest of the circuit's power.
Reduced Noise Power management allows the device to concentrate mostly on the Analog readouts and eliminating most of the noise from the power source.
On board DRL which puts 2V to the body for reference voltage and the DC voltage from the body comes back to it and if it shifts it inverts the signal as a feedback loop to level it out.
Pin-outs & Connectors
There are several headers on the board and each one has it's purpose. Below we shall describe each one and the signals they represent.
The analog inputs are sorted as shown on the picture below, compared to the board in the orientation on the picture.
D0 - D2
There are 4 general-purpose Digital I/O pins on the ADS1256 IC. These pins are revealed on the P5 header on the schematics and are as shown on the image below.
Note that these pins are not passed through Opto-isolation and are bare outputs from the IC with only 100R resistors used for filtering and are all pulled LOW with 100K resistors.
The SPI and other pins used for controlling the ADS1256 IC are all passed through Optocouplers and are converted from 3v to 5v signals. So in order to control the IC the Signal voltage level should be 5V. The pin outs are as shown on the image below.
The DRL signals are as shown below.
Voltage Reference Jumpers
There are 2 headers for selecting the Voltage Reference of ADS1256 for Positive and Negative. This headers are there for experimenting, debugging and operational reasons. For the Positive reference you can select ether to use the filtered 5v supply on the board or to use the DRL's VGND2V signal which you can examine more closely in the schematics. For the Negative reference you can select Filtered GND of the Gizmo or again to use the VGND2V from the DRL circuit. Below is the visual representation of this setup as it is in the schematics.