AD604ARSZ-R7 datasheet(16/32 Pages) AD

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AD604

Rev. D | Page 16 of 32

AC COUPLING

The DSX portion of the AD604 is a single-supply circuit and,

therefore, its inputs need to be ac-coupled to accommodate

ground-based signals. External capacitors C1 and C2 in Figure 37

level shift the ground referenced preamplifier output from

ground to the dc value established by VOCM (nominal 2.5 V).

C1 and C2, together with the 175 Ω looking into each of the

DSX inputs (+DSX and −DSX), act as high-pass filters with

corner frequencies depending on the values chosen for C1 and

C2. As an example, for values of 0.1 μF at C1 and C2, combined

with the 175 Ω input resistance at each side of the differential

ladder of the DSX, the −3 dB high-pass corner is 9.1 kHz.

If the AD604 output needs to be ground referenced, another ac

coupling capacitor is required for level shifting. This capacitor

also eliminates any dc offsets contributed by the DSX. With a

nominal load of 500 Ω and a 0.1 μF coupling capacitor, this adds

a high-pass filter with −3 dB corner frequency at about 3.2 kHz.

The choice for all three of these coupling capacitors depends on

the application. They should allow the signals of interest to pass

unattenuated, while at the same time, they can be used to limit

the low frequency noise in the system.

GAIN CONTROL INTERFACE

The gain control interface provides an input resistance of

approximately 2 MΩ at 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, which for VGN is 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 42

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, VREF = 2.500 V

(4)

G (20 dB/V) = 30 × VGN – 5, VREF = 1.666 V

(5)

G (20 dB/V) = 40 × VGN – 5, VREF = 1.250 V

(6)

GAIN CONTROL VOLTAGE (VGN)

–5

30dB/V

40dB/V

20dB/V

0.5

1.0

1.5

2.0

2.5

3.0

LINEAR-IN-dB RANGE

OF AD604 WITH

PREAMPLIFIER

SET TO 14dB

Figure 42. Ideal Gain Curves vs. VGN

From these equations, it can be seen that all gain curves intercept at

the same −5 dB point; this intercept is +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 gain scaling,

VREF can be calculated as shown in Equation 7.

Scale

Gain

VREF

dB/V

500

(7)

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 (preamplifier and DSX) is powered

down. This can be used to conserve power and, at the same

time, to 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 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 it to 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.

The AFA makes a differential input structure possible because

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, 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

implies that the differential input voltage to G2 times gm2 (the

transconductance of G2) has to be equal to the differential

input voltage to G1 times gm1 (the transconductance of G1).

Therefore, the overall gain function of the AFA is

ATTEN

OUT

(8)

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 thus 52.5 (34.4 dB).

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