Differential Amps Match ADCs With Complementary-Bipolar SiGe

Analog-to-digital-converter (ADC) resolution and sampling rates keep pushing the envelope. And every time they push further, it gets harder to match the ADC to an input amplifier in high-end test and measurement, medical imaging, wireless infrastructure, and industrial applications. So to meet the needs of ADCs reaching 100-MHz speeds, Texas Instruments came up with a third fully differential amplifier that uses the company's complementary-bipolar Bicom-III silicon-germanium (SiGe) process technology.

Bicom-III is the only production SiGe technology with pnp and npn transistors whose f_T characteristics are matched--or matched within a factor of two. TI's 2002 white paper on the process lists cutoff frequencies of 20 and 15 GHz for npns and pnps, respectively. For the new parts, a TI spokesperson says both f_Ts were "roughly 18 GHz."

The process is being refined. The new amplifier outperforms its predecessors in all characteristics (see the figure). With 1.9 GHz of bandwidth at a gain of 10 dB into a 200-Ω load, the THS 4509 delivers 2.0-nV/(check)Hz input referred voltage noise. It also provides ­80-dBc and ­87-dBc second- and third-order harmonic distortion at 70 MHz (2 V p-p into a 200-Ω load) and a 2-ns 1% settling time with a 2-V output step. The amp slews a 2-V step (25% to 75%) at 6600 V/µs. Offset is 0.5 mV, and input bias is 6 µA. There's a 0.65-mA power-down mode as well. (All datasheet values are typical.)

In contrast, earlier Bicom-III fully differential amplifiers were rated for 550 MHz at a gain of 10. Other specs are hard to compare because the parts are characterized at lower frequencies.

The THS 4509's architecture uniquely decouples the gain, output common-mode voltage, and output impedance-matching issues from each other so they can be set independently. In addition, the device will perform single-ended input-to-differential output conversion to enable dc-coupled data-acquisition systems.

The part works with ADCs like TI's 14-bit ADS5500. Driving that chip at 125 Msamples/s with a 70-MHz input frequency, spurious-free dynamic range shows 81 dBc for a single tone and 89 dBc for two tones with 5-MHz spacing. Signal-to-noise ratio is 70.1 dBFS.

The THS4509 is sampling now in a 16-pin quad flat no-lead (QFN) package, priced at $3.75 in 1000-unit quantities. An evaluation module also is available.