Changing converter topologies, the company’s LTC2366, a 12-bit, 3-Msample/s SAR ADC, emphasizes a small (TSOT-23) footprint and low power draw (7.2 mW at 2.6 to 3.6 V). It’s part of a five-member family that aims to give designers of portable medical devices and the like a way to trade off performance for price.
All of the devices in the family communicate via a serial SPI/QSPI/Microwire-compatible interface and offer no data latency while achieving dc specifications of ±1-LSB integral nonlinearity (INL) and differential nonlinearity (DNL). The LTC2366 measures 72-dB SNR, –80-dB total harmonic distortion (THD), and 82-dB spurious free dynamic range (SFDR) at a 1-MHz input frequency.
AMPLIFIERS TAKE CENTER STAGE One way to position analog parts with respect to performance is to focus on applications. For instance, EMI can be a headache in certain apps. An injected RF signal in a weigh scale can result in as much as 1 V of output offset, which would diminish a 10-bit ADC’s resolution (1024 codes) to a 3-bit effective number of bits (ENOB).
For use in precision weigh scales, as well as phone accessories, medical instruments, and other EMI-sensitive industrial electronic equipment, National recently introduced the LMV831 single, LMV832 dual, and LMV834 quad op amps. All of them integrate EMI filters that deliver a 120-dB EMI rejection ratio (EMIRR).
Obviously, not every designer may be familiar with that spec. To help explain it, National provides app note AN-1698 (www.national.com/an/AN/AN-1698.pdf ) and evaluation boards along with complete signal-path solutions based on the LMV83x devices and 10- and 12-bit ADCs with 1-LSB performance SPI or I2C interfaces.
Another instrumentation amp with RF filtering on the input comes from Texas Instruments. The company notes that “special filters have been integrated in series with the inputs of the INA333 to reduce RF interference” that can cause offset voltage variations in weigh scales (Fig. 2). TI goes on to explain that it focused its design efforts in the INA333 on combining “long battery life, low noise, and low offset voltage and drift.”
The chip utilizes a special zero-drift technology, which incorporates a proprietary switched-capacitor notch filter to eliminate chopping noise and provide very low input voltage noise of 50 nV/ Hz. Specifically, the amplifier is spec’d for 75-µA quiescent current at 1.8 V. Offset is 25 µV with an offset drift of 0.1 µV/°C. Input bias current is 200 pA.
Looking at 16- and 18-bit precision instrumentation applications like radar-based collision avoidance and medical instrumentation, Analog Devices came up with the low-noise ADA4898 op amp, which combines low distortion, low noise, and high speed. ADI acknowledges that it’s essentially a companion part for the company’s AD7631 and AD7634 precision data converters.
The ADA4898 uses the same commonmode linearized input ADI first used in the AD8099, a circuit designed for low noise. In the case of the new op amp, that means ultra-low broadband noise (less than 1 nV/ Hz), 1/f noise of 1.2 nV/ Hz at 10 Hz, low distortion (–110 dBc at 500 kHz), and dc precision that optimizes 16- and 18-bit ADC performance. At unity gain, the 3-dB bandwidth is 70 MHz, and the slew rate is 40 V/µs.
For MRI and digital X-ray systems, key goals are to reduce the time that patients must lie motionless during MRI examinations and reduce X-ray radiation exposure. With that in mind, Analog Devices engineers developed the 16-bit, 10-Msample/s AD7626 successive-approximation ADC. It achieves a 15-bit ENOB thanks to its 92-dB SNR, which is 8 dB better than any ADC, regardless of architecture. The AD7626 additionally wins points on size (5- by 5-mm quad flat no-lead) and power consumption (13 mW).
For driving high-performance 14- and 16-bit converters in communications infrastructure and high-speed instrumentation equipment, ADI focused on low distortion in developing its silicon-germanium (SiGe) ADA4939 differential amplifier, which consumes less than 120 mW from a single 3.3-V supply. In terms of distortion, it achieves 82-dB SFDR at 70 MHz. The new amp is available in one- and (for I/Q demodulation) two-channel versions.
The two-channel version also limits crosstalk to –80 dB at 100 MHz, while providing gain and phase matching. The amplifiers can be used in either differential-to-differential or single-ended-to-differential configurations.
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