The circuit shown in Figure 1 uses the AD8331
low noise wideband VGA as a driver to extend the dynamic range of the AD9215
10-bit ADC. The gain range of the AD8331 is 48 dB and is linear-in-dB with respect to the control voltage. The overall dynamic range of the combined VGA and ADC is greater than 100 dB.
Variable gain amplifiers (VGAs) serve a critical function when an analog signal with wide dynamic range is converted to digital format, and the ADC resolution is insufficient to capture all useful information. For example, a 10-bit converter with a 2 V p-p input range has an LSB weight of 2 ÷ 1024, or just under 2 mV.
Figure 1: Interconnecting an AD8331 VGA and an AD9215 ADC (Simplified Schematic, All Connections/Decoupling Not Shown)
This represents a dynamic range of approximately 60 dB. A wideband VGA can be used ahead of the ADC to amplify input signals with amplitudes less than the minimum resolution and attenuate large signals that would otherwise saturate the ADC.
Examples of such applications are ultrasound receivers where the signal strength ranges from microvolts to several volts, and the intermediate frequency (IF) amplifier used in virtually all receivers. For dc or low frequency analog signals, Σ-Δ ADCs with resolutions up to 24 bits are economical and plentiful, but typically limited in sampling frequency to a few hundred kilohertz. State-of-the-art, available ADC resolution decreases as sampling frequency increases. This makes accurate digitization of high frequency, low amplitude signals extremely difficult using standard ADCs.
Variable gain amplifiers are a convenient solution to this problem. The AD8331/AD8332/AD8334 are single-, dual-, and quad-channel, ultralow noise, linear-in-dB, variable gain amplifiers (VGAs). Optimized for ultrasound systems, they are usable as a low noise variable gain element at frequencies up to 120 MHz.
Included in each channel are an ultralow noise preamp (LNA), an X-AMP® VGA with 48 dB of gain range, and a selectable gain postamp with adjustable output limiting. The LNA gain is 19 dB with a single-ended input and differential outputs. Using a single resistor, the LNA input impedance can be adjusted to match a signal source without compromising noise performance.
The 48 dB gain range of the VGA makes these devices suitable for a variety of applications. Excellent bandwidth uniformity is maintained across the entire range. The gain control interface provides precise linear-in-dB scaling of 50 dB/V for control voltages between 40 mV and 1 V. Factory trim ensures excellent part-to-part and channel-to-channel gain matching.