概要

The AD605 is a low noise, accurate, dual-channel, linear-in-dB variable gain amplifier (VGA), optimized for any application requiring high performance, wide bandwidth variable gain control. Operating from a single 5 V supply, the AD605 provides differential inputs and unipolar gain control for ease of use. Added flexibility is achieved with a user-determined gain range and an external reference input that provide user-determined gain scaling (dB/V).

The high performance linear-in-dB response of the AD605 is achieved with the differential input, single-supply, exponential amplifier (DSX-AMP) architecture. Each of the DSX-AMPs comprises a variable attenuator of 0 dB to −48.4 dB followed by a high speed, fixed-gain amplifier. The attenuator is based on a 7-stage R-1.5R ladder network. The attenuation between tap points is 6.908 dB, and 48.360 dB for the entire ladder network. The DSX-AMP architecture results in 1.8 nV/√Hz input noise spectral density and accepts a ±2.0 V input signal when VOCM is biased at VP/2.

Each independent channel of the AD605 provides a gain range of 48 dB that can be optimized for the application. Gain ranges between −14 dB to +34 dB and 0 dB to +48 dB can be selected by a single resistor between Pin FBK and Pin OUT. The lower and upper gain ranges are determined by shorting Pin FBK to Pin OUT or leaving Pin FBK unconnected, respectively. The two channels of the AD605 can be cascaded to provide 96 dB of very accurate gain range in a monolithic package.

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 linearly in dB with control voltages (VGN) of 0.4 V to 2.4 V for the 20 dB/V scale and 0.20 V to 1.20 V for the 40 dB/V scale.When VGN is<50 mV, the amplifier is powered down to draw 1.9 mA. Under normal operation, the quiescent supply current of each amplifier channel is only 18 mA.

The AD605 is available in a 16-lead PDIP and a 16-lead SOIC_N package and is guaranteed for operation over the −40°C to +85°C temperature range.

 

特徴

2 independent linear-in-dB channels

Input noise at maximum gain: 1.8 nV/√Hz, 2.7 pA/√Hz

Bandwidth: 40 MHz (−3 dB)

Differential input

Absolute gain range programmable 14 dB to +34 dB (FBK shorted to OUT) through 0 dB to 48 dB (FBK open)

Variable gain scaling: 20 dB/V through 40 dB/V Stable gain with temperature and supply variations Single-ended unipolar gain control

Output common mode independently set Power shutdown at lower end of gain control Single 5 V supply

Low power: 90 mW/channel Drives ADCs directly

 

アプリケーション

Ultrasound and sonar time-gain controls

High performance AGC systems

Signal measurement

 

動作理論

The AD605 is a dual-channel, low noise VGA. Figure 35 shows the simplified block diagram of one channel. Each channel consists of a single-supply X-AMP® (hereafter called DSX, differential single-supply X-AMP) comprising the following:

  • Precision passive attenuator (differential ladder)
  • Gain control block
  • VOCM buffer with supply splitting resistors R3 and R4
  • Active feedback amplifier1 (AFA) with gain setting resistors R1 and R2 The linear-in-dB gain response of the AD605 can generally be described

by Equation 1.

G (dB) = (Gain Scaling (dB/V)) × (Gain Control (V)) − (19 dB − (14 dB) × (FB))

どこだ？

FB = 0, if FBK to OUT is shorted.

FB = 1, if FBK to OUT is open.

Each channel provides between −14 dB to +34.4 dB through 0 dB to +48.4 dB of gain, depending on the value of the resistance connected between Pin FBK and Pin OUT. The center 40 dB of gain is exactly linear-in-dB while the gain error increases at the top and bottom of the range. The gain is set by the gain control voltage (VGN). The VREF input establishes the gain scaling. The useful gain scaling range is between 20 dB/V and 40 dB/V for a VREF voltage of 2.5 V and 1.25 V, respectively. For example, if FBK to OUT is shorted and VREF is set to 2.50 V (to establish a gain scaling of 20 dB/V), the gain equation simplifies to

G (dB) = (20 (dB/V)) × (VGN (V)) – 19 dB