AD605

Rev. E | Page 15 of 20

35

30

25

20

15

10

5

0

–5

–10

–15

–20

40dB/V

30dB/V

20dB/V

LINEAR-IN-dB RANGE

OF AD605

1.0

0.5

1.5

2.0

2.5

3.0

GAIN CONTROL VOLTAGE

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.

FIXED GAIN AMPLIFIER AND INTERPOLATOR

CIRCUITS—APPLYING AN ACTIVE FEEDBACK

AMPLIFIER

A typical X-amp architecture is powered by a dual polarity

power supply. Because the AD605 operates from a single-

supply, a supply-common equal to half the value of the supply

voltage is required. An active feedback amplifier (AFA) is used

to provide a differential input and to implement the feedback

one is used in the feedback path and the other is used as a

highly linear differential input.

ladder network, one stage for each of the ladder nodes. Only a

few of the stages are active at any time and are dependent on the

gain control voltage.

The AFA makes a differential input structure possible because

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

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 transconductance of G1). Therefore, the overall gain

function of the AFA is

R2

R2

R1

g

g

V

V

m

m

ATTEN

OUT

×

×

=

2

1

(7)

where:

(

R1 + R2)/R2 = 42.

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.