Abstract The Electroencephalography EEG is a common diagnostic tool for neurological diseases and dysfunctions, such as epilepsy and insomnia. However, the current EEG technology cannot be utilized quickly and conveniently at the point of care due to the complex skin preparation procedures required and the inconvenient EEG data acquisition systems. This work presents a portable amplifier design that integrates a set of skin screw electrodes and a wireless data link. The battery-operated amplifier contains an instrumentation amplifier, two noninverting amplifiers, two high-pass filters, and a low-pass filter. It is able to magnify the EEG signals over 10, times and has a high impedance, low noise, small size and low weight.

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Its versatile 3-op amp design and small size make it ideal for a wide range of applications. A single external resistor sets any gain from 1 to 10, The INA is laser trimmed for very low offset voltage 50?

V , drift 0. A—ideal for battery operated systems. A3 60k? Street Address: S. Tucson Blvd. Tel: ? Telex: ? FAX: ? Immediate Product Info: ?

INA Printed in U. See text for discussion of low power supply and single power supply operation. Continuous Operating Temperature Burr-Brown recommends that all integrated circuits be handled with appropriate precautions.

Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure.

Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. Offset Voltage Change? Applications with noisy or high impedance power supplies may require decoupling capacitors close to the device pins as shown.

The output is referred to the output reference Ref terminal which is normally grounded. This must be a low-impedance connection to assure good common-mode rejection.

A resistance of 12? The 50k? These on-chip metal film resistors are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifications of the INA The stability and temperature drift of the external gain setting resistor, RG, also affects gain. Low resistor values required for high gain can make wiring resistance important.

Sockets add to the wiring resistance which will contribute additional gain error possibly an unstable gain error in gains of approximately or greater. This is due to the current-feedback topology of the INA Settling time also remains excellent at high gain.

This is a result of its current-feedback topology and is not an indication of instability. A simple pole at kHz or lower will produce a flat passband unity gain response. NC Basic Connections. F 7 A1 25k? INA 60k? A3 A2 60k? For differential source impedances less than 1k? For source impedances greater than 50k? Low frequency noise of the INA is approximately 0.

Vp-p measured from 0. This provides dramatically improved noise when compared to stateof-the-art chopper-stabilized amplifiers. Most applications require no external offset adjustment. Figure 2 shows an optional circuit for trimming the output offset voltage. The voltage applied to Ref terminal is summed at the output. The op amp buffer provides low impedance at the Ref terminal to preserve good commonmode rejection.

Microphone, Hydrophone etc. INA 47k? Thermocouple INA 10k? Optional Trimming of Output Offset Voltage. However, a path must be provided for the input bias current of both inputs. High input impedance means that this input bias current changes very little with varying input voltage. Input circuitry must provide a path for this input bias current for proper operation. Figure 3 shows various provisions for an input bias current path. Without a bias current path, the inputs will float to a potential which exceeds the commonmode range of the INA and the input amplifiers will saturate.

If the differential source resistance is low, the bias current return path can be connected to one input see the thermocouple example in Figure 3. With higher source impedance, using two equal resistors provides a balanced input with possible advantages of lower input offset voltage due to bias current and better high-frequency common-mode rejection. Center-tap provides bias current return.

As a differential input voltage causes the output voltage to increase, however, the linear input range will be limited by the output voltage swing of amplifiers A1 and A2. Thus, the linear common-mode input range is related to the output voltage of the complete amplifier. Input-overload can produce an output voltage that appears normal.

For example, if an input overload condition drives both input amplifiers to their positive output swing limit, the difference voltage measured by the output amplifier will be near zero. The output of the INA will be near 0V even though both inputs are overloaded. Most parameters vary only slightly throughout this supply voltage range— see typical performance curves. Operation at very low supply voltage requires careful attention to assure that the input voltages remain within their linear range.

Voltage swing requirements of internal nodes limit the input commonmode range with low power supply voltage. Figure 5 shows a basic single supply circuit.

The output Ref terminal is connected to ground. Zero differential input voltage will demand an output voltage of 0V ground. Actual output voltage swing is limited to approximately 35mV above ground, when the load is referred to ground as shown. You cannot, for instance, connect the inverting input to ground and measure a voltage connected to the non-inverting input. To illustrate the issues affecting low voltage operation, consider the circuit in Figure 5.

It shows the INA, operating from a single 3V supply. Internal circuitry on each input provides low series impedance under normal signal conditions. To provide equivalent protection, series input resistors would contribute excessive noise.

If the input is overloaded, the protection circuitry limits the input current to a safe value of approximately 1. The inputs are protected even if the power supplies are disconnected or turned off.

The more detailed diagram shown here provides additional insight into its operation. Each input is protected by two FET transistors that provide a low series resistance under normal signal conditions, preserving excellent noise performance. When excessive voltage is applied, these transistors limit input current to approximately 1. The output difference amp, A3, removes the common-mode component of the input sig- nal and refers the output signal to the Ref terminal.

Equations in the figure describe the output voltages of A1 and A2. Input Bias Current Compensation 10? A3 VO 60k?

Single-Supply Bridge Amplifier. F VO R1 1M? AC-Coupled Instrumentation Amplifier. Differential Voltage to Current Converter.

All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, patent infringement, and limitation of liability.

Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except those mandated by government requirements.

Customers are responsible for their applications using TI components. TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such semiconductor products or services might be or are used.


Portable Amplifier Design for a Novel EEG Monitor in Point-of-Care Applications



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