Analog Devices AD604 Manuel D’Utilisation

 

Rev. E | Page 16 of 32 

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 (+DSXx and −DSXx), 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. 

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 decibels 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) 

00

54

0-

04

2

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 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, 
V

REF

 can be calculated as shown in Equation 7. 

dB/V

20

V

500

.

2

×

=

 (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. 

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 g

m

 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 g

m

 stage that requires it to be highly linear over 

the expected input signal range. In this design, the g

m

 stage that 

senses the voltages on the attenuator is a distributed one; for 
example, there are as many g

m

 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 g

m

 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 g

m2

 (the 

transconductance of G2) has to be equal to the differential  
input voltage to G1 times g

m1

 (the transconductance of G1).  

Therefore, the overall gain function of the AFA is 

+

×

=

 (8) 

where: 
V

 is the output voltage.  

 is the effective voltage sensed on the attenuator.  

(R1 + R2)/R2 = 42 
g

/g

 = 1.25 

The overall gain is thus 52.5 (34.4 dB). 

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