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Dual, Ultralow Noise Variable Gain Amplifier AD604

a FEATURES Ultralow Input Noise at Maximum Gain: 0.80 nV/√Hz, 3.0 pA/√Hz Two Independent Linear-in-dB Channels Absolute Gain Range per Channel Programmable: 0 dB to +48 dB (Preamp Gain = +14 dB), through +6 dB to +54 dB (Preamp Gain = +20 dB) ؎1.0 dB Gain Accuracy Bandwidth: 40 MHz (–3 dB) 300 k⍀ Input Resistance Variable Gain Scaling: 20 dB/V through 40 dB/V Stable Gain with Temperature and Supply Variations Single-Ended Unipolar Gain Control Power Shutdown at Lower End of Gain Control Can Drive A/D Converters Directly

FUNCTIONAL BLOCK DIAGRAM PAO

–DSX

+DSX

VGN

GAIN CONTROL AND SCALING

VREF

DIFFERENTIAL ATTENUATOR R-1.5R LADDER NETWORK

PAI

AFA

OUT

0 TO –48.4dB VOCM PROGRAMMABLE ULTRALOW NOISE PREAMPLIFIER G = 14–20dB PRECISION PASSIVE INPUT ATTENUATOR

FIXED GAIN AMPLIFIER +34.4dB

APPLICATIONS Ultrasound and Sonar Time-Gain Control High Performance AGC Systems Signal Measurement

PRODUCT DESCRIPTION

The AD604 is an ultralow noise, very accurate, dual channel, linear-in-dB variable gain amplifier (VGA) optimized for timebased variable gain control in ultrasound applications; however it will support 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 application 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 based on a seven stage R-1.5R ladder network. The attenuation between tap points is 6.908 dB and 48.36 dB for the ladder network. Each independent channel of the AD604 provides a gain range of 48 dB which can be optimized for the application by programming the preamplifier with a single external resistor in the preamp feedback path. The linear-in-dB gain response of the AD604 can be described by the equation: 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 2nd channel’s preamplifier. However, in multiple channel systems, cascading the AD604 with other devices in the AD60x VGA family, which 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. The gain control interface provides an input resistance of approximately 2 MΩ and scale factors from 20 dB/V to 30 dB/V for a VREF input voltage of 2.5 V to 1.67 V respectively. Note that scale factors up to 40 dB/V are achievable with reduced accuracy for scales above 30 dB/V. The gain scales linear-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 a 24-pin plastic SSOP, SOIC and DIP, and is guaranteed for operation over the –40°C to +85°C temperature range.

REV. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 617/329-4700 World Wide Web Site: http://www.analog.com Fax: 617/326-8703 © Analog Devices, Inc., 1996


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