AD605

Rev. E | Page 13 of 20

THEORY OF OPERATION

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:

• 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))

(1)

where:

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 were shorted and VREF were set to

2.50 V (to establish a gain scaling of 20 dB/V), the gain

equation would simplify to

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

(2)

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 range of 0.1 V to 2.9 V.

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

Because the two channels are identical, only Channel 1 is used

to describe their operation. VREF and VOCM are the only

inputs that are shared by the two channels, and because they are

normally ac grounds, crosstalk between the two channels is

minimized. 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 +VCC 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 AD605 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. VOCM needs only an external decoupling capacitor

to ground to center the midpoint between the supply voltages (5

V, GND). However, if the dc level of the output is important to

the user (see the Applications section of the AD9050 data sheet

for an example), VOCM can be specifically set. The input

resistance looking into the VOCM pin is 45 kΩ ± 20%.

data sheet. The AD830 is a practical implementation of the idea.

R1

820Ω

VREF

VGN

VPOS

VOCM

R3

200kΩ

C3

OUT

175Ω

175Ω

G1

GAIN

CONTROL

Ao

G2

R2

20Ω

R4

200kΩ

EXT

+IN

–IN

FBK

3.36kΩ

DIFFERENTIAL

ATTENUATOR

EXT

C2

C1

+

+

+

+

Figure 35. Simplified Block Diagram of a Single Channel of the AD605