Dual, Ultralow Noise Variable Gain Amplifier
The AD604 is an ultralow noise, very accurate, dual-channel, linear-in-dB variable gain amplifier (VGA) optimized for time-based variable gain control in ultrasound applications; however, it supports any application requiring low noise, wide bandwidth, variable gain control. Each channel of the AD604 provides a 300 kΩ input resistance and unipolar gain control for ease of use. User determined gain ranges, gain scaling (dB/V), and dc level shifting of output further optimize performance.
Each channel of the AD604 utilizes a high performance preamplifier that provides an input referred noise voltage of 0.8 nV/√Hz. The very accurate linear-in-dB response of the AD604 is achieved with the differential input exponential amplifier (DSX-AMP) architecture. Each of the DSX-AMPs comprise a variable attenuator of 0 dB to 48.36 dB followed by a high speed fixed gain amplifier. The attenuator is a 7-stage R-1.5R ladder network. The attenuation between tap points is 6.908 dB and 48.36 dB for the ladder network.
The equation for the linear-in-dB gain response is
G(dB) = (Gain Scaling (dB/V) × VGN (V))
+ (Preamp Gain (dB) – 19 dB)
Preamplifier gains between 5 and 10 (14 dB and 20 dB) provide overall gain ranges per channel of 0 dB through 48 dB and 6 dB through 54 dB. The two channels of the AD604 can be cascaded to provide greater levels of gain range by bypassing the second channel’s preamplifier. However, in multiple channel systems, cascading the AD604 with other devices in the AD60x VGA family that do not include a preamplifier may provide a more efficient solution. The AD604 provides access to the output of the preamplifier, allowing for external filtering between the preamplifier and the differential attenuator stage.
Note that scale factors up to 40 dB/V are achievable with reduced accuracy for scales above 30 dB/V. The gain scales linearly-in-dB with control voltages of 0.4 V to 2.4 V with the 20 dB/V scale. Below and above this gain control range, the gain begins to deviate from the ideal linear-in-dB control law. The gain control region below 0.1 V is not used for gain control. In fact when the gain control voltage is <50 mV, the amplifier channel is powered down to 1.9 mA.
The AD604 is available in 24-lead SSOP, SOIC, and PDIP packages and is guaranteed for operation over the −40°C to +85°C temperature range.
- Ultrasound and sonar time gain control
- High performance AGC systems
- Signal measurement
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At least one model within this product family is in production and available for purchase. The product is appropriate for new designs but newer alternatives may exist.
The evaluation board requires only a dual 5 V supply capable of 200 mA or higher to operate both channels. Prior to shipment, the evaluation board is fully tested. Users need only attach power supply leads and the appropriate test equipment to the board. See AD604 data sheet on-line for details.
Product Selection Guide
Variable Gain Amplifier Selection Table
Ultrasound System Considerations and their Impact on Front-End Components
(Analog Dialogue, Vol 36, No. 3)
Choosing High-Speed Signal Processing Components for Ultrasound Systems
(Analog Dialogue, Vol. 29, No. 2, 1995)
ADI has always placed the highest emphasis on delivering products that meet the maximum levels of quality and reliability. We achieve this by incorporating quality and reliability checks in every scope of product and process design, and in the manufacturing process as well. "Zero defects" for shipped products is always our goal.
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The USA list pricing shown is for BUDGETARY USE ONLY, shown in United States dollars (FOB USA per unit for the stated volume), and is subject to change. International prices may differ due to local duties, taxes, fees and exchange rates. For volume-specific price or delivery quotes, please contact your local Analog Devices, Inc. sales office or authorized distributor. Pricing displayed for Evaluation Boards and Kits is based on 1-piece pricing.