Analog Devices, Inc. (ADI), a global leader in high-performance semiconductors for signal-processing applications, introduced the ADA4898, the latest addition to its portfolio of low-noise operational amplifiers (op amps). Following the success of the ADA4899 and AD8099 operational amplifiers, the ADA4898 offers designers a unique blend of low distortion, low noise, and high speed for applications that also require high precision. Designed to work with Analog Devices’ precision data converters, including the AD7631 and AD7634, the ADA4898 is especially suited for radar-based collision avoidance, medical instrumentation and other 16-bit and 18-bit precision instrumentation applications.

The ADA4898 unity gain stable op amp offers design engineers ultra-low broadband noise (<1 nV/rt Hz), 1/f noise of 1.2 nV/rt Hz at 10 Hz, low distortion (¬–110 dBc at 500 kHz), and dc precision that optimizes the performance of analog-to-digital converters (ADCs). Particularly suited for 16- and 18-bit systems, the ADA4898 features high speed (–3 dB bandwidth of 70 MHz, at G=+1) and high slew rate (40 V/µs), providing the optimal combination of amplifier speed and precision. The ADA4898 has a wide supply range of from ±5 V to ±15 V. The ADA4898-1 is an ultralow noise and distortion, unity gain stable, voltage feedback op amp that is ideal for use in 16-bit and 18-bit systems with power supplies from ±5 V to ±16 V. The ADA4898-1 features a linear, low noise input stage and internal compensation that achieves high slew rates and low noise.

With the wide supply voltage range, low offset voltage, and wide bandwidth, the ADA4898-1 is designed to work in the most demanding applications. The ADA4898-1 also features an input bias current cancellation mode that reduces input bias current by a factor of 60. The ADA4898-1 is available in an 8-lead SOIC package that features an exposed metal paddle on its underside that improves heat transfer to the ground plane. This is a significant improvement over traditional plastic packages. The ADA4898-1 is rated to work over the extended automotive temperature range of -40°C to +105°C.

The ADA4898-1 is a voltage feedback op amp that combines unity-gain stability with 0.9 nV/√Hz input noise. It employs a highly linear input stage that can maintain greater than -90 dBc (@ 2 V p-p) distortion out to 500 kHz while in a unity-gain configuration. This rare combination of low gain stability, low input referred noise, and extremely low distortion is the result of Analog Devices, Inc., proprietary op amp architecture and high speed complementary bipolar processing technology. The simplified ADA4898-1 topology is a single gain stage with a unity-gain output buffer. It has over 100 dB of open-loop gain and maintains precision specifications such as CMRR, PSRR and offset to levels that are normally associated with topologies having two or more gain stages.

The PD (power down) pin saves power by decreasing the quiescent power dissipated in the device. It is very useful when power is an issue and the device does not need to be turned on at all times. The response of the device is rapid when going from a power down mode to full power operation mode. Note that PD does not put the output in a high-Z state, which means that the ADA4898-1 is not recommended for use as a multiplexer.

To measure the very low (2.4 pA/√Hz) input current noise of the ADA4898-1, 10 kΩ resistors were used on both inputs of the amplifier. Figure 41 shows the noise measurement circuit used. The 10 kΩ resistors are used on both inputs to balance the input impedance and cancel the common-mode noise. In addition, a high gain configuration is used to increase the total output noise and bring it above the noise floor of the instrument. The ADA4898-1 schematic for the noninverting gain configuration is nearly a textbook example (see Figure 42). The only exception is the feedback capacitor in parallel with the feedback resistor, RF, but this capacitor is recommended only when using a large RF value (>300 Ω).

To analyze the noise performance of an amplifier circuit, identify the noise sources, and then determine if each source has a significant contribution to the overall noise performance of the amplifier. To simplify the noise calculations, noise spectral densities were used rather than actual voltages to leave bandwidth out of the expressions (noise spectral density, which is generally expressed in nV/√Hz, is equivalent to the noise in a 1 Hz bandwidth).

Because the ADA4898-1 can operate up to 65 MHz, it is essential that RF board layout techniques be employed. All ground and power planes under the pins of the ADA4898-1 should be cleared of copper to prevent the formation of parasitic capacitance between the input pins to ground and the output pins to ground. A single mounting pad on a SOIC footprint can add as much as 0.2 pF of capacitance to ground if the ground plane is not cleared from under the mounting pads.

Power supply bypassing for the ADA4898-1 has been optimized for frequency response and distortion performance. Power supply bypassing is critical for stability, frequency response, distortion, and PSR performance.

Related products

• 3G power amplifier for dual-band handsets for EGSM network
• Industry’s smallest video filter amplifier