AD604

REV. 0

–12–

Gain Control Interface

The gain-control interface provides an input resistance of ap-

proximately 2 M

Ω at Pin VGN1 and gain scaling factors from

20 dB/V to 40 dB/V for VREF input voltages of 2.5 V to 1.25 V

respectively. The gain scales linearly-in-dB for the center 40 dB

of gain range, that is for VGN equal to 0.4 V to 2.4 V for the 20

dB/V scale, and 0.2 V to 1.2 V for the 40 dB/V scale. Figure 40

shows the ideal gain curves for a nominal preamplifier gain of

14 dB which are described by the following equations:

G (20 dB/V) = 20

× VGN – 5, V

(4)

G (30 dB/V) = 30

× VGN – 5, V

(5)

G (40 dB/V) = 40

× VGN – 5, V

(6)

GAIN CONTROL VOLTAGE – VGN

20

40

35

30

25

15

10

5

50

45

0

–5

LINEAR-IN-dB RANGE

OF AD604 WITH

PREAMPLIFIER

SET TO 14dB

0.5

1.0

2.5

1.5

2.0

3.0

30dB/V

40dB/V

20dB/V

From these equations you can see that all gain curves intercept

at the same –5 dB point; this intercept will be 6 dB higher

(+1 dB) if the preamplifier gain is set to +20 dB or 14 dB,

lower (–19 dB) if the preamplifier is not used at all. Outside of

the central linear range, the gain starts to deviate from the ideal

control law but still provides another 8.4 dB of range. For a given

V

2.500 V

× 20 dB / V

Gain Scale

(7)

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

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 since

below 50 mV the channel (preamp and DSX) 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, 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) is utilized. The

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

taps on the ladder network. Only a few of them are on at any

one time, depending on the gain-control voltage.

The AFA makes a differential input structure possible since one

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

back. The output of G1 will be some function of the voltages

sensed on the attenuator taps which is applied to a high gain

amplifier (A0). Because of negative feedback, the differential

input to the high gain amplifier has to be zero; this in turn

transconductance of G2) has to be equal to the differential input

fore the overall gain function of the AFA is:

=

×

R1

+ R2

R2

(8)

1.25; the overall gain is thus 52.5 (34.4 dB).

The AFA has additional features: (1) inverting the signal by

switching the positive and negative input to the ladder network,

(2) the possibility of using the DSX1 input as a second signal

input, (3) fully differential high impedance inputs when both

preamplifiers are used with one DSX (the other DSX could still

be used alone), and (4) 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 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 off-

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