New Designs Cut Op Amp Noise

Sep 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

This year's op amps offer significant improvement over past designs — particularly in terms of noise output. Three op amps described here all feature low noise.

Texas Instruments'(TI) THS4271/5 family of high-speed op amps combines ultralow distortion with ultralow noise. Based on a voltage-feedback design, these low distortion op amps offer unity gain stability with 1.4-GHz unity-gain bandwidth, along with 900V/µs slew rate and a 2.8nV/Hz noise density. With a 2V peak-to-peak output signal, the THS4271/5 offers the low distortion of -84 dB below the carrier (dBc) at 30 MHz. This combination of features enables wideband signal acquisition, yielding greater dynamic range and resolution.

A reference pin allows designers to control the logic levels of the power-down pin. This pin is mainly intended for systems with supply voltages higher than 5V; however, it works with any voltage within the amplifier's voltage range (for example, using a 3V microcontroller to control the power-down pin of an amplifier operating from a 5V split supply). Using the traditional power-down circuit, the designer would have to buffer/boost the 3V signal from the microcontroller to a higher level to ensure the proper voltage level for the amplifier to toggle between enable and disable. With the reference circuit, the designer can connect the 3V signal directly to the power-down pin.

The THS4271/5 op amp series leverages the company's second generation of complementary bipolar (BiCom II) process technology to provide both speed and output signal quality. This design topology, coupled with the BiCom II process, enables the op amps to achieve the conflicting design goals of reducing distortion and noise while increasing slew rate — providing higher resolution at both high and low frequencies. The THS4275 includes a power-down pin for the power conscious.

The THS4271/5 family can drive high-speed 14-bit data converters, which reduces system costs by eliminating intermediate signal stages. They can also be used in phase look loop filter circuits. Filtering, gain stages, and signal conditioning in automatic test equipment (ATE) and measurement instrumentation benefit from the speed, low noise, and low distortion of the devices. In mixed-signal tester equipment, the op amps enable ATE to match the operating speed for testing next-generation ICs.

To enhance performance characteristics, the THS4271/5 will be the first high-speed op amp from TI available in a leadless miniature small outline package (MSOP). The leadless packaging technology enables it to push the speed envelope through the minimization of parasitic inductance and capacitance typically associated with leaded packages.

The THS4271/5 op amp is sampling now, with volume production scheduled during September. All devices will be available in a leadless MSOP and MSOP. Planned pricing per unit in quantities of 1,000 units is $2.69.

Dual Op Amp

National Semiconductor's LMH6628 high-speed dual op amp offers a traditional voltage feedback topology, featuring unity gain stability and slew-enhanced circuitry. Its input supply range is ±2.5V to ±6V, allowing it to operate from a single (5V to 12V) or dual (±5V) power supply.

Using ±5V supplies, it can provide a typical output swing of ±3.8V under a no-load condition. Additional output swing is possible with slightly higher supply voltages. For loads of less than 50Ω, its output current capability — typically 85mA — will limit the output swing. You can improve output settling time when driving capacitive loads by using a series output resistor.

Each of its closely matched channels provides a 300-MHz unity gain bandwidth. Fig.1 shows the gain and phase characteristics for an inverting amplifier with gains of -1, -2, and -5 and 200mV peak-to-peak output.

Ultimate low noise performance requires proper selection of external resistors. By selecting appropriate low valued resistors for RF and RG, amplifier circuits using the LMH6628 can achieve output noise that is approximately the equivalent voltage input noise of 2nV/Hz multiplied by the desired gain (A_V). Low second/third harmonic distortion (-65/-74dBc at 10 MHz) makes it a perfect wide dynamic range amplifier for matched I/Q channels.

Cancellation of the output-offset voltage due to input bias currents is possible by making the resistance seen from the inverting and noninverting inputs equal. Once done, the residual output offset voltage will be the input offset voltage (V_OS) multiplied by the desired gain (A_V).

Fast and accurate settling (12ns to 0.1%) makes the LMH6228 an excellent choice for wide dynamic range, anti-aliasing filters to buffer the inputs of high-resolution analog-to-digital converters.

Proper power supply bypassing is critical to insure good high frequency performance and low noise. You should place decoupling capacitors of 0.1 µF as close as possible to the power supply pins. The use of surface- mounted capacitors is recommended due to their low series inductance.

A good high frequency layout keeps power supply and ground traces away from the inverting input and output pins. Parasitic capacitance from these nodes to ground causes frequency response peaking and possible circuit oscillation.

Combining the LMH6628's two tightly matched amplifiers in a single 8-pin SOIC package reduces cost and board space for many composite amplifier applications such as active filters, differential line drivers/receivers, fast peak detectors, and instrumentation amplifiers.

The LMH6628 is fabricated using National's VIP10^™ complimentary bipolar process.

Five-Member Op Amp Family

The TSH11x family of low-cost, wide band, low-noise, low-power op amps from STMicroelectronics meets the requirement of high-volume consumer applications that need high-performance amplification and conditioning. The single-channel TSH110 and TSH111, the dual-channel TSH112, the triple-channel TSH113, and the quad-channel TSH114 are current-feedback operational amplifiers. They all feature a 490V/µs slew rate, 100-MHz bandwidth, and 3.2mA quiescent supply current. The TSH111 and TSH113 also feature a standby function for each channel.

Because these op amps employ current feedback, their bandwidth depends on feedback component values and supply voltage. A good choice of these components is necessary to achieve gain flatness and stability. For example, a 510 and ±6V supply provides 46 MHz, -3 dB bandwidth with a closed-loop gain of 10. The same supply with a 2.2 kΩ feedback resistor paralleled with a 2-pF capacitor provides a -3 dB bandwidth of 170 MHz and a gain of 1.

Distortion is -65 dB at 2 MHz for an output dynamic of 2V peak on a 100Ω load. Bandwidth is 100 MHz with a good gain flatness of 25 MHz at 6 dB gain. The large bandwidth, high slew rate, low consumption ratio, very low noise of 3nV/Hz and intermodulation distortion of -85 dB (180 kHz and 280 kHz, 4Vpeak output dynamic, 12V power supply). Harmonic and intermodulation distortion is very low, making them ideal for applications requiring wide bandwidth with multiple carriers.

For board space and weight saving, the TSH110 comes in a miniature SOT23-5L package, the TSH111 comes in SO-8 and TSSOP8 packages, the TSH112 comes in SO-8 and TSSOP8 packages, and the TSH113 and TSH114 come in SO-14 and TSSOP14 packages.

The TSH11x operates from 12V down to 5V power supplies, achieved with a dual power supply of ±6V and ±2.5V, or a single supply of 12V and 5V referenced to ground. For asymmetrical supplies, biasing is necessary to achieve a positive output dynamic range between 0V and the +V_CC supply rails. Considering the values of V_OH and V_OL, the amplifier provides an output from 1.35V to 10.75V with a 12V supply, from 0.6V to 4.5V for a 5V supply. Fig. 2, on page 72, shows the circuit for an op amp with a single +5V supply.

Texas Instruments, Dallas
CIRCLE 346 on Reader Service Card

National Semiconductor, Santa Clara, Calif.
CIRCLE 347 on Reader Service Card

STMicroelectronics, Schaumburg, Ill.
CIRCLE 348 on Reader Service Card