Targeting Stability: Designing a very high-speed op amp is more challenging than just getting wider bandwidth and higher slew rates. One problem is the need to ensure stability under all operating conditions. Designers have to consider such electrical details as phase margin and compensation, as well as the end use. TI didn't want external feedback nodes available because the parasitic loading from the feedback could make the system unstable. Between that and the very high frequencies involved, the company decided to simplify the application of this gain function by creating a family of fixed-gain parts.
At gigahertz frequencies, the bonding wires and package pads combine to form a low-pass filter that's potentially well below the desired bandwidth of the parts. Normal package impedance characteristics are due to the bond pads, lead wires, and packaging. A 0.1-in. long, 2-mil diameter wire has an inductance between 2 and 3 nH, depending on the loop size. This inductance is a part of a pi-section low-pass filter comprising the bonding pad capacitance, the wire inductance, the lead bonding area, the package lead, and the pc-board pad. Depending on the actual parasitic values, the 3-dB roll-off frequency can be well below 1 GHz.
To solve these problems of package parasitics and op amp stability, TI developed new packaging and pin-outs for the amplifiers (see the figure). The new packaging is a leadless MSOP, a four-sided package with four pins on each side. The pins on the top and bottom are power-supply connections. Signal inputs and the power-down input are on the right, while outputs are on the left. This new package reduces inductance from bond wires and minimizes the package parasitics by putting four connections on the output and the supply leads. These parallel leads reduce lead and package inductance and resistance by a factor of four.
By developing a new package and keeping the feedback internal, TI created a device with much greater stability for easier use in systems. Potential applications include high-speed buffers and drivers for high-speed, high-precision data converters and DSP systems. These functions require the low distortion and noise characteristics of the THS430x parts for 16-bit or higher resolution.
The speed and low-noise characteristics should work well in medical imaging systems where the data accuracy requirements are very high. Communications systems could benefit from the devices' capabilities, especially in applications like basestations and other high-frequency signal-processing functions. The combination of speed and low noise will enable many new applications for these amplifiers.
One potential new application is in IF stages in communications receivers. Previously, designers had to use RF-type amplifiers or design special purpose IF stages for the multiple conversion receivers. The IF stages needed these specialized amplifiers because they were the only components available with appropriate gain and linearity. The new amplifiers open up the possibility for new architectures in software radios. By amplifying input signals and feeding them directly to a bank of fast converters, a DSP-based radio could capture and demodulate a wide range of signal bands and modulation types with very few RF components.
TI is starting to characterize the op amps with inductive and capacitive loads for this application. The company also is going to create application notes because this change in components will change the architecture of the RF and IF stages.
Price & Availability
The THS430x op amps are specified and priced at fairly competitive levels. For 1000-piece orders, the THS4302 costs $1.97 each. Samples and evaluation module assemblies are available now.
Texas Instruments Inc., www.ti.com; (800) 477-8924, ext. 4500.