AD8244BRMZ

The AD8244BRMZ is a precision four-channel JFET-input unity-gain buffer from Analog Devices, specifically designed to provide ultra-high input impedance buffering for high-impedance signal sources. Unlike traditional operational amplifiers configured as buffers, the AD8244 is purpose-built as a unity-gain buffer, eliminating the need for external feedback components and optimizing the design for the buffer function. The device features N-channel JFET input transistors that provide extremely low input bias current of 2 pA maximum at 25°C, near-zero current noise of 0.8 fA/√Hz, and an exceptional input impedance of 10 TΩ. This makes the AD8244 ideal for interfacing with high-impedance sources such as photodiodes, piezoelectric sensors, biopotential electrodes, and high-value resistive sensors where even tiny input currents would introduce significant errors. The B-grade AD8244BRMZ offers high DC accuracy with a maximum input offset voltage of 350 µV at 25°C and a maximum offset temperature coefficient of 3 µV/°C over the full operating temperature range. The rail-to-rail output stage swings to within 25 mV of the supply rails with a 10 kΩ load, maximizing the output dynamic range, especially in low-voltage single-supply systems. A unique and important design feature of the AD8244 is its innovative pinout. Unlike traditional quad op-amp packages where power supply pins are positioned next to high-impedance input pins (creating leakage paths that can swamp the fA-level bias currents), the AD8244’s pinout physically separates the high-impedance inputs from the low-impedance supplies and outputs of other buffers. This dramatically simplifies PCB guard ring design, allowing compact board layouts while maintaining the ultra-high impedance performance. The 10-lead MSOP package is significantly smaller than a conventional 14-pin quad op-amp package, providing higher channel density. The AD8244 operates from a wide supply range of 3V to 36V single supply (or ±1.5V to ±18V dual supply), making it compatible with a broad range of system voltages. With only 250 µA supply current per channel, it is well-suited for low-power and battery-powered applications. The 3 MHz bandwidth and 0.8 V/µs slew rate provide adequate speed for many precision signal conditioning applications. An interesting application technique enabled by the quad configuration is paralleling all four buffers to reduce the voltage noise by half (from 13 nV/√Hz to approximately 6.5 nV/√Hz), since uncorrelated noise sources add in an RMS fashion. The AD8244BRMZ is specified over the industrial temperature range of -40°C to +85°C and is ideal for medical instrumentation, high-impedance sensor interfaces, photodiode amplification, active filters, and general-purpose high-impedance buffering applications.

The AD8244 is a purpose-built unity-gain buffer amplifier that uses JFET input transistors to achieve ultra-high input impedance and very low input bias current. Each buffer channel consists of a JFET differential input stage followed by additional gain stages and a rail-to-rail output stage, all configured for unity gain with internal feedback. The JFET (junction field-effect transistor) input stage is what gives the device its extremely high input impedance and low bias current characteristics. JFETs operate by controlling current through a channel using an electric field, and the gate terminal is essentially insulated by a reverse-biased PN junction, resulting in gate currents that are measured in picoamps or femtoamps rather than microamps as in BJT input amplifiers. This makes JFET inputs ideal for buffering very high impedance signal sources. Because the AD8244 is configured as a unity-gain buffer (voltage follower), the output is connected directly back to the inverting input internally, creating a feedback loop that forces the output voltage to precisely follow the input voltage. The unity-gain configuration provides the widest possible bandwidth, fastest settling, and best stability for a given amplifier. The rail-to-rail output stage uses complementary transistors that can swing very close to both supply rails, maximizing the output voltage range. The input common-mode range extends down to the negative supply (ground in single-supply operation), which is particularly useful for AC-coupled or ground-referenced signals. A key innovation of the AD8244 is the physical layout that separates high-impedance input pins from power supply and output pins. In conventional op-amp packages, the proximity of supply pins to input pins creates parasitic leakage paths through surface contamination and PCB material. For JFET-input amplifiers with pA-level bias currents, even minute leakage (nanoamps or even hundreds of picoamps) can severely degrade performance. By placing the high-impedance inputs on one side of the package and the low-impedance supply/output pins on the other, the AD8244 allows the PCB designer to easily implement a guard ring around the input pins that follows the input voltage, preventing leakage currents from reaching the sensitive input node. The channel-to-channel matching of the AD8244 is also carefully controlled, enabling the four channels to be used in differential signal chains with minimal error. Additionally, paralleling multiple buffer outputs (with small balancing resistors) can reduce the total voltage noise by a factor equal to the square root of the number of paralleled channels.

Note: The unique pinout separates high-impedance input pins from supply and output pins, simplifying PCB guard ring design for high-impedance applications. Input pins (1, 3, 6, 8) are interleaved with output pins (2, 4, 7, 9) on one side, while supply pins (5, 10) are on opposite sides and far from input pins.

• Biopotential electrode buffering for ECG, EEG, EMG, and other medical instrumentation
• High-impedance sensor signal conditioning: piezoelectric, piezoresistive, MEMS sensors
• Photodiode preamplifiers and optical receiver frontends
• Active filter input stages and high-impedance filter networks
• pH electrodes and electrochemical/ion-selective electrode interfaces
• DAC output buffering for driving resistive or capacitive loads
• Data acquisition system input isolation and impedance matching
• Low-noise precision signal chains where high input Z is critical
• Portable and battery-powered test and measurement instruments
• Charge amplifier input stages for piezoelectric sensors
• Instrumentation amplifier input protection and buffering