Power Electronics

Op Amps Keep on Coming

Two high-speed, voltage-feedback op amps from Micrel — MIC920 and MIC921 — provide an order of magnitude improvement in the ratio of slew rate-to-power consumption, compared with competing devices (Fig. 1). They are decompensated derivatives of the recently introduced MIC918 and MIC919. Decompensation enables the MIC920 and MIC921 to achieve an even higher slew rate and gain-bandwidth product for the same power consumption, while maintaining unity-gain stability. The table, on page 62, compares the characteristics of the two devices.

The MIC920 and MIC921 are stable driving any capacitive load, making them much easier to use than most conventional high-speed devices. The ability to drive these loads also makes it possible to drive long coaxial cables. Most high-speed op amps can only drive limited capacitance. Increasing load capacitance reduces the speed of the device. In applications where the load capacitance reduces the speed of the op amp to an unacceptable level, the effect of the load capacitance can be reduced by adding a small resistor (<100Ω) in series with the output.

Conventional op amp gain configurations and resistor selection apply, the MIC920 and MIC921 are not current feedback devices. Also, for minimum peaking, the feedback resistor should have low parasitic capacitance, usually 470Ω is ideal. To use the part as a follower, the output should be connected to input via a short wire.

All high-speed devices require careful p. c. board layout. It's important to recognize that capacitance, particularly on the two input pins, will degrade performance. Also, avoid large copper traces to the inputs, keep the output signal away from the inputs, and use a ground plane. Ensure that supply bypassing capacitors are located close to the device.

Regular supply bypassing techniques are recommended. A 10µF capacitor in parallel with a 0.1µF capacitor on the positive and negative supplies are ideal. For best performance, all bypass capacitors should be located as close to the op amp as possible and all capacitors should be low ESL and ESR. Surface-mount ceramic capacitors are ideal.

The SC70 package and the SOT-23 package, like all small packages, have a high thermal resistance. It's important the lC does not exceed the maximum operating junction (die) temperature of 85°C. The part can be operated up to the absolute maximum temperature rating of 125°C, but between 85°C and 125°C performance will degrade — in particular, CMRR will reduce.

The MIC920 and MIC921 are specified for a broad supply voltage range, allowing their use in low-voltage circuits or applications requiring large dynamic range.

Devices are available in 5-pin SOT-23 and SC70 packages. Pricing for the MIC920 and MIC921 in SC70-5 packaging is $1.25 in 1,000-piece quantities. Samples are available immediately, and production quantities are available eight weeks ARO.

Ultra-Low Offset/Drift, Low-Noise Op Amps

Among Maxim Integrated Products newest op amp introductions are the MAX4238/MAX4239 ultra-precision op amps (Fig. 2). Offset is 0.1µV (typ), 2µV (max); drift is 10nV/°C and 50nV per 1,000 hr (equivalent to 5µV over 10 yr). Open-loop gain is 150 dB, PSRR is 140 dB, CMRR is 140 dB, and noise is 1µV peak-to-peak from dc to 10 Hz.

Typical applications include amplifiers for thermocouples, strain gauges, and portable medical instruments that can take advantage of device performance. Fig. 3 shows an application circuit employing a strain gauge.

Devices use a patented auto-zero technique to achieve nearly ideal op amp specifications and still manage to squeeze into a 6-pin SOT-23 package. The auto-correlating zeroing method measures and compensates the input offset voltage, eliminating drift over time and temperature and the effect of 1/f noise without introducing fixed-frequency “chopper” noise.

The MAX4238 and MAX4239 feature 1pA input bias current, ground-sensing input, *Rail-to-Rail® outputs. They operate from a single 2.7V to 5.5V supply, and consume only 600µA quiescent supply current. An active-low shutdown mode decreases supply current to 0.1µA.

The MAX4238 is unity-gain stable with a gain-bandwidth product of 1 MHz. The MAX4239 is stable with an AV of 10V/V, but extends its gain-bandwidth to 6.5 MHz. All devices are specified for operation over the automotive temperature range (-40°C to 125°C). The MAX4238/MAX4239 are available in 8-pin narrow SO and 6-pin SOT-23 packages. Prices start at 85 cents (2,500-up).

Micropower, Rail-to-Rail I/0 Op Amps

Also from Maxim are the MAX4162/MAX4163/MAX4164 single/dual/quad micropower op amps that combine an excellent bandwidth-to-power consumption ratio with true Rail-to-Rail inputs and outputs (Fig. 4, on page 63). They consume 25µA quiescent current per amplifier, yet achieve 200-kHz gain-bandwidth product and operate from a single supply (2.7V to 10V) or dual supplies (±1.35V to ±5V).

Input common-mode range extends 250mV beyond either of the supply rails, providing a substantial increase in dynamic range compared with other amps. Although the minimum specified operation is 2.7V, the devices typically provide full Rail-to-Rail operation below 2V.

These amplifiers don't suffer from midswing common-mode-reject ion degradation or crossover nonlinearity, often encountered in Rail-to-Rail op amps. Extremely low, 1pA input bias current makes these devices ideal for special applications. They're also protected against phase reversal (inferred from CMRR test) and latch-up for input signals extending beyond the supply rails. The output stage achieves a lower output impedance than traditional Rail-to-Rail output stages, providing an output voltage range that typically swings within 150mV of the supply rails for 1mA loads while consuming only 25µA of quiescent current. This architecture also maintains high open-loop gain and output swing while driving substantial loads. Internal compensation allows these amplifiers to remain unity-gain stable while driving any capacitive load.

The combination of bandwidth/power performance, single-supply operation, and miniature footprint makes these op amps ideal for portable equipment and other low-power, single-supply applications. The single MAX4162 is available in 8-pin SO and space-saving 5-pin SOT23 packages. The MAX4163 is available in an 8-pin ultra chip-scale package (UCSP) and an 8-pin µMAX or SO package. The MAX4164 is available in a 14-pin SO package.

*Rail-to-Rail is a registered trademark of Nippon Motorola Ltd.

Micrel, San Jose, Calif.
CIRCLE 347 on Reader Service Card

Maxim, Sunnyvale, Calif.
CIRCLE 348 on Reader Service Card

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