JFET in-amp housed in MSOP

Norwood, Mass. — Analog Devices Inc. said it's breaking ground with its first junction field effect transistor-input instrumentation amplifier, which is the smallest in the industry and offers the highest common mode rejection ratio.

The JEFT input instrumentation amplifier is also said to enhance patient vital sign detection in both fixed-line and transportable electrocardiograms (ECGs), electroencephalograms (EEGs) and other types of patient monitoring devices.

Housed in an 8-lead plastic MSOP, the AD8220 in-amp allows designers of medical equipment to break new ground by freeing up additional board space to significantly increase ECG and EEG channel density, allowing for smaller medical monitoring systems, said Lew Counts, vice president, Advanced Linear Products at Analog Devices.

Monitoring equipment with high channel density also gives medical professionals more measurement points for very small signals like heart pulses and brain activity while protecting patient vital signs from electrical interference. In addition, the AD8220 operates on a single low-voltage power supply that draws only 700 μA, making it suited for use in portable patient monitoring systems.

The AD8220 features an extremely low input bias current of just 4 pA, which is less than half the level of competing components and addresses a significant source of signal error in precision instruments, Counts said. The AD8220 also achieves an 80 dB common mode rejection ratio (CMRR) up to 10 kHz (G=1), while competing in-amps guarantee only 72 dB to 200 Hz.

With significantly lower input bias current and higher CMRR, health care professionals can now measure heart pulses, electrical brain waves, and other patient vital signs at previously undetectable levels, Counts said. The high CMRR, meanwhile, shields these signals from outside interference by rejecting electrical noise from other parts of the human body.

With JFET inputs, the AD8220 in-amp achieves a guaranteed input bias current of 20 pA, maximum, and 4 pA, typical. It specifies a 1-nA input bias current over temperature, and 2 μV/°C input offset voltage temperature drift. The rail-to-rail output of the AD8220 allows designers to improve dynamic range by increasing the gain further before reaching the supply rails. Furthermore, the AD8220’s 80 dB CMRR (G=1) over frequency ensures that only the desired signals are monitored and unwanted common-mode signals are rejected.

The AD8220 is suited for patient monitoring systems, where its small size, low current noise and low input bias current increase the sensitivity and noise rejection of fixed-line and portable medical equipment, such as ECG and EEG systems. The low input bias and high CMRR characteristics also make the AD8220 a fit for industrial automotation applications, such as electrometers and other precision instrumentation systems where extremely small currents must be measured in electrically noisy environments.

The AD8220 is currently sampling, with production planned for November. The AD8220 is priced at $2.29 per unit, in 1,000-piece quantities. Click here for the AD8220 data sheet.

Analog Devices , 1-800-262-5643,

See related block diagram

Keeping in line with current industrial and medical instrumentation trends, ADI's AD8220 JFET input instrumentation amplifier touts a 50 percent reduction in package size and improved performance, while reducing cost and power consumption.

While ADI currently offers many different types of JFET input operation amplifiers, the AD8220 is ADIs first JFET input instrumentation amplifier.

The AD8220 is a gain programmable, high performance instrumentation amplifier that draws a max input bias current of 20 pA and rejects high frequency common mode signals. The CMRR of instrumentation amplifiers on the market today falls off at 200 Hz. In contrast, the AD8220 maintains a CMRR of 80 dB over an extended frequency at G = 1. The combination of extremely high input impedance and high CMRR over frequency makes the AD8220 useful in applications such as patient monitoring where input impedance is high and high frequency disturbances must be rejected.

ADI is able to make the precision instrumentation amp half the size of competing devices via its propritary complimentary bipolar process and engineering techniques. “The competition does not offer a JFET in-amp in a MSOP package. Getting the die size small enough to fit into this package size will be a challenge for them,” said Scott Pavlik, ADI's product marketing manager. Texas Instruments Inc. makes these devices as well.

The smaller footprint allows for higher channel densities in precision instruments such as patient monitoring equipment, either portable or line connected. Portability is a growing part of ECG and EEG systems; particularly monitors that patients can wear, Pavlik said. ADI's tiny JFET-input instrumentation amplifier helps make portability possible, he said.

Medical monitoring system sizes vary depending on the feature sets and number of channels; from the standard lab top down to a PDA size. Increasing ECG and EEG channel density is crucial for portable and multifunction systems. This is the most important feature about this product since it enables the stacking of more channels in a given area.

Many patient montiors offer more than just ECG and EEG. “They will measure blood pressure and other vital signs offering more functionalilty in less space, which is why channel density is so key,” Pavlik said.

Patient monitoring devices must pass stringent electromagnetic interference (EMI) requirements. ADI's CMRR, which Pavlik says is 50 percent higher than competing devices up to 10 kHz, makes it easier for designers to achieve a high level of CMRR with minimal or no additional circuitry, he said.

Previously, interference was blocked at a potentially greater cost and used up more printed-circuit board space to achieve acceptable CMRR levels, Pavlik said. “Similar products were much bigger, taking up more space and requiring additional filtering to get this type of CMRR performance,” he said.

Low power is also important in multichannel systems, particularly portable and battery operated devices. The fact that this instrumentation amp operates down to less than 1 mA makes it especially suited for portable patient monitoring systems, Pavlik said.

In a nutshell, ADI's AD8220 provides the smallest package FET input in-AMP out there, high CMRR over frequency, a low input bias current, rail-to-rail output, and single and dual power supply operation.

 See related functional block diagram

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