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High CMRR Instrumentation Amplifier (Schematic and Layout) design for biomedical applications

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Instrumentation amplifiers are intended to be used whenever acquisition of a useful signal is difficult. IA’s must have extremely high input impedances because source impedances may be high and/or unbalanced. bias and offset currents are low and relatively stable so that the source impedance need not be constant. Balanced differential inputs are provided so that the signal source may be referenced to any reasonable level independent of the IA output load reference. Common mode rejection, a measure of input balance, is very high so that noise pickup and ground drops, characteristic of remote sensor applications, are minimized.Care is taken to provide high, well characterized stability of critical parameters under varying conditions, such as changing temperatures and supply voltages. Finally, all components that are critical to the performance of the IA are internal to the device. The precision of an IA is provided at the expense of flexibility. By committing to the one specific task of
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communication using visible light

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Wireless optical communication using visible light is one of the emerging green technologies that has not been fully utilized. Many versions of white-LED transceivers have been built, but mainly due to the high cost of good photodiodes and the existing popularity of fluorescent lamps, visible-light communication (VLC) only receives luke-warm attention to date. I shall introduce two low-cost and efficient transceiver circuit designs that can be constructed using on-the-market components.
They can be applied in moderate-speed data communications such as smart phones and tablets for ad-hoc transmission of photos and files. The potentials for these designs are wide if we are able to scale down the circuits to tiny PCBs or manufacture them at IC level.


Introduction

LED is a very green technology. Since very little heat is produced, it can reduce interior temperatures by 1 to 2 degrees, thus lowering air-conditioning costs and carbon dioxide emissions. LED lighting is also much safer for the living and working environment because it is mercury free and does not produce IR or UV rays which can be harmful to human eyes and skin [1].
White LED communication uses light in the visible spectrum as the carrier medium. The functional duality of LEDs - both as a light source and a communication medium - creates many new and interesting applications [2][3][4][5][6][7][8][9] based on the fast-switching characteristic of LEDs and the ability to modulate lightwave for free-space communications. In this kind of technology, it is possible to achieve high-speed data transmission for high data loads with low implementation complexity. Also, lightwave cannot penetrate walls, thus making it easy to secure transmissions against casual eavesdropping. Furthermore, unlike radio frequencies, the visible-light spectrum does not need licensing.
Given the strengths of LEDs [10] - long lifetime, high tolerance to humidity, small size, and low power consumption - a white-LED communication system is therefore potentially feasible for indoor wireless networks. VLC is somewhere between Bluetooth and WLAN, but in the near future, it will replace these two technologies if the transceiver circuit can be fabricated with the LED and photodiode together on a single chip. We should expect a sudden burst of popularity in white-LED communication in the next generation of personal computers after IBM and Intel have successfully fabricated and tested their new optical core processors. Optical I/O ports would most probably replace our LAN and USB interfaces. These ports would be able to connect to ceiling lights, wall lights, or desk lamps via a repeating adaptor.
In the following sections, two types of transceiver circuit prototypes are introduced. The design principles in this work are based on cost cutting, simplicity, and the most common electronic components on the market. The intended application is serial peer-to-peer, ad-hoc communication.

Hardware Description: Prototype 1


The components of Prototype 1 (Fig. 1) consist mainly of a microcontroller, a repeater, and a USB-RS232 converter. The transmitter consists of a microcontroller PIC12F508 which is used for the modulation of the TX signal from the µUSB-MB5 (USB-RS232 converter). When the TX pin transits from logic low to logic high, the 12F508 generates a 40 kHz carrier. During low periods, the carrier is suppressed. After the modulation of the signal, the output of the microcontroller is passed on to an NPN darlington transistor to drive the white LEDs.
The repeater receives white-light signals using the BS520 eye-response photodiode and retransmits them at a higher power. Signals received by the BS520 is usually weak and easily affected by ambient light. The repeater circuit is used to shape and boost the received signal from the photodiode before passing it to the IR transmitter. The other use of the repeater is to fine-tune the signal to the correct frequency, so that it can be readily accepted by the IR receiver. The path between the IR transmitter and the IR receiver must be enclosed to ensure that the receiver does not receive reflected signals from the transmitter. Alternatively, an optoisolator IC (e.g. 4N25) can also be used in place of the IR transmitter and receiver.
After the IR receiver receives signals from the repeater and demodulates them, the demodulated signals are sent to the RX pin of the µUSB-MB5, which then passes the converted signals back to the computer. The µUSB-MB5 RS232-to-USB converter is one of the more expensive items in the transceiver (USD $58), but there should be less expensive serial converters out there in the market.
The design of Prototype 1 is based on the established IR technology by creating a hybrid combination (Fig. 2) of IR and visible-light devices. The components are cheap and widely available.


VLC Transceiver: Prototype 1





VLC Transceiver: Prototype 2


Prototype 2

In Prototype 2 (Fig. 3), a PIC12F508 microcontroller is used to generate a 40 kHz clock which is passed to the one-shot IC DM74121N. The TX signal from the USB-MB5 is then pulse-position modulated (PPM) by the one-shot device 74121 before being transmitted out through the LEDs.
The receiver consists of a Centronic OSD50-E eye-response photodiode, a preamplifier KA2181, a phase-locked loop MM74HC4046N, and an inverter 74LS04. Light signals received by the photodiode is preamplified and shaped by the preamplifier before being demodulated by the phase-locked loop. For the KA2181 circuit, the user must find its optimal inductor value which determines the receiver's sensitivity.

Both Prototype 1 and 2 can only operate at 9600 bps.


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