High frequency AD8331 VGA interconnected with ADC AD9215

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Circuit function and advantage

Variable gain amplifiers (VGAs) can play an important role when converting analog signals with a wide dynamic range to digital format without adequate ADC resolution to capture all useful information. For example, a 10-bit converter with a 2 V pp input range has an LSB size of 2 ÷ 1024, which is slightly less than 2 mV. The VGA amplifies the input signal by less than the lowest resolution and attenuates the large signal to avoid saturation of the ADC. Ultrasonic receivers with signal strengths in the range of a few microvolts to several volts, as well as intermediate frequency (IF) amplifiers used in almost all receivers, are examples of such applications. For DC or low frequency analog signals, Σ-Δ ADCs with resolutions up to 24 bits are economical and versatile, but the sampling frequency is usually limited to hundreds of kHz. The resolution of existing advanced ADCs decreases as the sampling frequency increases, making it extremely difficult to accurately digitize high frequency, low amplitude signals using standard ADCs. A variable gain amplifier can easily solve this problem. Figure 1 shows a typical application of a VGA-driven ADC.

Figure 1: AD8331 VGA is interconnected with the AD9215 ADC (schematic diagram, all connections/decoupling are not shown)

The AD8331/AD8332/AD8334 are single/dual/quad, ultralow noise, linear dB variable gain amplifiers (VGAs), optimized for ultrasound systems and can be used as low noise variable gain components with the highest operating frequency Up to 120 MHz.

Each channel contains an ultra-low noise preamplifier (LNA), a 48-dB gain range X-AMP® VGA, and an optional gain post-amplifier with adjustable output limiting. The LNA has a gain of 19 dB and features single-ended inputs and differential outputs. The LNA input impedance can be adjusted with a resistor to match the source without affecting noise performance.

The VGA's 48 dB gain range makes these devices suitable for a wide variety of applications. Bandwidth maintains excellent consistency over the entire gain range. The gain control interface provides accurate 50 dB/V linear dB adjustment for control voltages from 40 mV to 1 V. Factory adjustments ensure excellent gain matching between devices and channels.