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AD605BRZ-RL7

Dual/ Low Noise/ Wideband Variable Gain Amplifiers

Analog Devices 

40dB/V 30dB/V

20dB/V

35

30

25

20

15

LINEAR-IN-dB RANGE

OF AD605

10

5

0

0.5

1.0

1.5

2.0

2.5

3.0

–5

GAIN CONTROL VOLTAGE

–10

–15

–20

Figure 37. Ideal Gain Curves vs. VREF

Usable gain control voltage ranges are 0.1 V to 2.9 V for the

20 dB/V scale and 0.1 V to 1.45 V for the 40 dB/V scale. VGN

voltages of less than 0.1 V are not used for gain control because

below 50 mV the channel is powered down. This can be used to

conserve power and at the same time gate-off the signal. The

supply current for a powered-down channel is 1.9 mA, and the

response time to power the device on or off is less than 1 μs.

ACTIVE FEEDBACK AMPLIFIER (FIXED GAIN AMP)

To achieve single-supply operation and a fully differential input

to the DSX, an active feedback amplifier (AFA) was used. The

AFA is an op amp with two gm stages; one of the active stages is

used in the feedback path (therefore the name), while the other

is used as a differential input. Note that the differential input is

an open-loop gm stage that requires that it be highly linear over

the expected input signal range. In this design, the gm stage that

senses the voltages on the attenuator is a distributed one; for

example, there are as many gm stages as there are taps on the

ladder network. Only a few of them are on at any one time,

depending on the gain control voltage.

AD605

The AFA makes a differential input structure possible since one

of its inputs (G1) is fully differential; this input is made up of a

distributed gm stage. The second input (G2) is used for feedback.

The output of G1 is some function of the voltages sensed on the

attenuator taps that is applied to a high-gain amplifier (A0).

Because of negative feedback, the differential input to the high

gain amplifier is zero; this in turn implies that the differential

input voltage to G2 times gm2 (the transconductance of G2) is

equal to the differential input voltage to G1 times gm1 (the

transconductance of G1). Therefore the overall gain function

of the AFA is

VOUT = gm1 × R1× R2

(7)

VATTEN gm2

R2

where:

VOUT is the output voltage.

VATTEN is the effective voltage sensed on the attenuator.

(R1 + R2)/R2 = 42.

gm1/gm2 = 1.25; the overall gain is therefore 52.5 (34.4 dB).

The AFA has additional features: inverting the output signal by

switching the positive and negative input to the ladder network;

the possibility of using the −IN input as a second signal input;

and independent control of the DSX common-mode voltage.

Under normal operating conditions, it is best to connect a

decoupling capacitor to Pin VOCM, in which case, the common-

mode voltage of the DSX is half of the supply voltage; this allows

for maximum signal swing. Nevertheless, the common-mode

voltage can be shifted up or down by directly applying a voltage

to VOCM. It can also be used as another signal input, the only

limitation being the rather low slew rate of the VOCM buffer.

If the dc level of the output signal is not critical, another

coupling capacitor is normally used at the output of the DSX;

again, this is done for level shifting and to eliminate any dc

offsets contributed by the DSX (see the AC Coupling section).

The gain range of the DSX is programmable by a resistor

connected between Pin FBK and Pin OUT. The possible ranges

are −14 dB to +34.4 dB when the pins are shorted together, or

0 dB to +48.4 dB when FBK is left open. Note that for the

higher gain range, the bandwidth of the amplifier is reduced by

a factor of five to about 8 MHz because the gain increased by

14 dB. This is the case for any constant gain bandwidth product

amplifier that includes the active feedback amplifier.

Rev. D | Page 15 of 20

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