Analog Devices AD604 Manuel D’Utilisation

 

Rev. E | Page 13 of 32 

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 

 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%. 

 

 

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