Analog Devices AD604 - Manual (Page 13)

AD604
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) x 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-dei|rmined-pr’eamplifie
gain. The c|n|err40 dBrofgain is exactly liBear-in-dB while the
gain error increasetri/th|top and bottpmffffheiangE. The
p ain
of the preamplifierisiypically e
i
ther 1* dB or2odB out canbe
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) x 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 kO ± 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 supplyvoltageG (5 VlGNp);rtowtver, the VOCM
canpe adjlste! to fthpr yDltage4Jvell if the dl common-mode
levetrofthereutput is i^jportant to-lhJuser (ferexample, 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 kO ± 20%.
Figure 37. Simplified Block Diagram of a Single Channel of the AD604
Rev. E | Page 13 of 32
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