Electrochemical sensors are widely used as a sense mechanism for gas and chemical sensing. Common applications include carbon monoxide detectors, chemical species identification, Amperometric sensors etc. Electrochemical sensors can be considered simply as transducers that convert the physical characteristic of gas/chemical concentration to an electrical signal which can be processed by instrumentation.
The programmable Analog Front End (AFE) is perfect for use in micro-power electrochemical sensing applications. It provides a complete signal path solution between a sensor and a microcontroller that generates an output voltage proportional to the cell current. The programmability enables it to support multiple electrochemical sensors such as 3-lead toxic gas sensors and 2-lead galvanic cell sensors with a single design as opposed to the multiple discrete solutions. The AFE supports gas sensitivities over a range of 0.5 nA/ppm to 9500 nA/ppm. It also allows for an easy conversion of current ranges from 5µA to 750µA full scale. The adjustable cell bias and Transimpedance amplifier (TIA) gain are programmable through the I2C interface. The I2C interface can also be used for sensor diagnostics. An integrated temperature sensor can be read by the user through the VOUT pin and used to provide additional signal correction in the µC or monitored to verify temperature conditions at the sensor. The AFE is optimized for micro-power applications and operates over a voltage range of 2.7V to 5.25V. The total current consumption can be less than 10μA. Further power savings are possible by switching off the TIA amplifier and shorting the reference electrode to the working electrode with an internal switch.
Signal Acquisition and Processing:
TI's high resolution differential ADCs have low power consumption, wide dynamic range and low noise. This can be used to digitize the conditioned analog bridge output for high resolution, precision measurements. Alternately, one could use TI's MSP430 microcontrollers with integrated ADCs and DACs. Further post processing algorithms can be run on this MCU.
Interface and Communication:
Traditional analog (4 - 20mA) interface remains the popular choice for industrial control and sensor applications. The other popular protocols include HART, Profibus and IO-Link. TI’s IO-Link interface products have integrated regulators and diagnostic outputs. In addition, wireless options based on IEEE 802.15.4 protocols are becoming more prevalent. TI is committed to provide solutions for both traditional and emerging industrial interfaces.
The Field Transmitter can be powered in one of three ways. Line powered transmitters are commonly powered by voltage rails of 12V, or 24V. Loop powered transmitters are powered by the 4-20 mA loop. Such transmitters require extremely low power architectures as the entire solutions has to be powered off the loop. TI provides high efficiency Step Down converters with low quiescent current and low output ripple appropriate for Line and Loop powered transmitters. Battery powered transmitters powered can be designed using TI's low power Buck and Buck-Boost converters. The DC/DC buck converters offer over 95% efficiency over a wide battery voltage range, even with input voltage down to 1.8 volts extending battery life. Special Buck-Boost converters generate a stable required output voltage and supply constant current for over- and under-input voltage conditions and support various battery configurations.
Electrocardiogram (ECG or EKG) Solution from Texas Instruments
TI's new ADS1298 provides eight channels of PGA plus separate 24-bit delta-sigma ADCs, a Wilson center terminal, the augmented Goldberger terminals and their amplifiers, provide for a full, standard 12-lead ECG integrated analog front end. The ADS1298 reduces component count and power consumption by up to 95 percent as compared to discrete implementations, with a power efficiency of 1 mW/channel, while allowing customers to achieve the highest levels of diagnostic accuracy [view video].
ECG System Functionality and Evolution
Basic functions of an ECG machine include ECG waveform display, either through LCD screen or printed paper media, and heart rhythm indication as well as simple user interface through buttons. More features, such as patient record storage through convenient media, wireless/wired transfer and 2D/3D display on large LCD screen with touch screen capabilities, are required in more and more ECG products. Multiple levels of diagnostic capabilities are also assisting doctors and people without specific ECG trainings to understand ECG patterns and their indication of a certain heart condition. After the ECG signal is captured and digitized, it will be sent for display and analysis, which involves further signal processing.
Signal Acquisition challenges:
Measurement of the ECG signal gets challenging due to the presence of the large DC offset and various interference signals. This potential can be up to 300mV for a typical electrode. The interference signals include the 50-/60-Hz interference from the power supplies, motion artifacts due to patient movement, radio frequency interference from electro-surgery equipments, defibrillation pulses, pace maker pulses, other monitoring equipment, etc.
Depending on the end equipment, different accuracies will be needed in an ECG:
Standard monitoring needs frequencies between 0.05-30 Hz
Diagnostic monitoring needs frequencies from 0.05-1000 Hz
Some of the 50Hz/60Hz common mode interference can be cancelled with a high-input-impedance instrumentation amplifier (INA), which removes the AC line noise common to both inputs. To further reject line power noise, the signal is inverted and driven back into the patient through the right leg by an amplifier. Only a few micro amps or less are required to achieve significant CMR improvement and stay within the UL544 limit. In addition, 50/60Hz digital notch filters are used to reduce this interference further.
Analog front end options:
Optimizing the power consumption and the PCB area of the analog front end is critical for portable ECG's. Due to technological advancements, there are now several front end options:
Using a low resolution ADC (needs all filters)
Using a high resolution ADC (needs fewer filters)
Using a sigma-delta ADC (needs no filters, no amplifier aside from INA, no DC offset)
Using a sequential Vs simultaneous sampling approach.
When a low resolution (16 bit) ADC is used, the signal needs to be gained up significantly (typically 100x - 200x) to achieve the necessary resolution. When a high resolution (24bit) sigma delta ADC is used, the signal needs a modest gain of 4 - 5x. Hence the second gain stage and the circuitry needed to eliminate the DC offset can be removed. This leads to an overall reduction in area and cost. Also the delta sigma approach preserves the entire frequency content of the signal and gives abundant flexibility for digital post processing.
With a sequential approach the individual channels creating the leads of an ECG are multiplexed to one ADC. This way there is a definite skew between adjacent channels. With the simultaneous sampling approach, a dedicated ADC is used for each channel and hence there is no skew introduced between channels.