Analog Devices AD604 Car Amplifier User Manual

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AD604

Rev. F | Page 13 of 32

THEORY OF OPERATION

The AD604 is a dual-channel VGA with an ultralow noise

preamplifier. Figure 37 shows the simplified block diagram of

one channel. Each identical channel consists of a preamplifier

with gain setting resistors (R5, R6, and R7) and a single-supply

X-AMP® (hereafter called DSX, differential single-supply X-AMP)

made up of the following:

• A precision passive attenuator (differential ladder).

• A gain control block.

• A VOCM buffer with supply splitting resistors

(R3 and R4).

• An active feedback amplifier (AFA) with gain setting

resistors (R1 and R2). To understand the active-feedback

amplifier topology, refer to the AD830 data sheet. The

AD830 is a practical implementation of the idea.

The preamplifier is powered by a ±5 V supply, while the DSX

uses a single +5 V supply. The linear-in-dB gain response of the

AD604 can generally be described by

G (dB) = Gain Scaling (dB/V) × Gain Control (V) +

(Preamp Gain (dB) − 19 dB) (1)

Each channel provides between 0 dB to 48.4 dB and 6 dB to 54.4

dB of gain, depending on the user-determined preamplifier

gain. 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

of the preamplifier is typically either 14 dB or 20 dB but can be

set to intermediate values by a single external resistor (see the

Preamplifier section for details). The gain of the DSX can vary

from −14 dB to +34.4 dB, as determined 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 the preamp gain is set to 14 dB 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) – 5 dB

The desired gain can then be achieved by setting the unipolar

gain control (VGN) to a voltage within its nominal operating

range of 0.25 V to 2.65 V (for 20 dB/V gain scaling). The gain is

monotonic for a complete gain control voltage range of 0.1 V to

2.9 V. Maximum gain can be achieved at a VGN of 2.9 V.

The inputs VREF and VOCM are common to both channels.

They are decoupled to ground, minimizing interchannel

crosstalk. For the highest gain scaling accuracy, VREF should

have an external low impedance voltage source. For low accuracy

20 dB/V applications, the VREF input can be decoupled with a

capacitor to ground. In this mode, the gain scaling is determined

by the midpoint between VPOS and GND; therefore, care

should be taken to control the supply voltage to 5 V. The input

resistance looking into the VREF pin is 10 kΩ ± 20%.

The DSX portion of the AD604 is a single-supply circuit, and

the VOCM pin is used to establish the dc level of the midpoint

of this portion of the circuit. The VOCM pin only needs an

external decoupling capacitor to ground to center the midpoint

between the supply voltages (5 V, GND); however, the VOCM

can be adjusted to other voltage levels if the dc common-mode

level of the output is important to the user (for example, see the

section entitled Medical Ultrasound TGC Driving the AD9050,

a 10-Bit, 40 MSPS ADC). The input resistance looking into the

VOCM pin is 45 kΩ ± 20%.

FBKx

VGNx

PAIx

–DSXx

EXT.

COMx

VPOS

OCM

OUT

175Ω

EXT.

175Ω

200kΩ

VREF

DISTRIBUTED G

GAIN

CONTROL

DIFFERENTIAL

ATTENUATOR

820Ω

20Ω

+DSXxPAOx

40Ω

32Ω

8Ω

00540-037

Figure 37. Simplified Block Diagram of a Single Channel of the AD604

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Analog Devices AD604 Car Amplifier User Manual