AD605 Просмотр технического описания (PDF) - Analog Devices

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AD605

Dual, Low Noise, Single-Supply Variable Gain Amplifier

Analog Devices 

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 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))

(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 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

(2)

AD605

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 Information 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%.

1 To understand the active-feedback amplifier topology, refer to the AD830

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

VREF

VGN

+IN

–IN

VOCM

C3

EXT

C1

EXT

C2

VPOS

R3

200kΩ

+

R4

200kΩ

175Ω

GAIN

CONTROL

DIFFERENTIAL

ATTENUATOR

175Ω

R2

20Ω

DISTRIBUTED gm

+

G1

+

Ao

G2

+

R1

820Ω

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

OUT

3.36kΩ

FBK

Rev. F | Page 13 of 24

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