Electronic Design

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 fT 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 fTs 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.

Texas Instruments Inc.

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