AD604

REV. 0

–13–

APPLICATIONS

The most basic circuit in Figure 41 shows the connections for

one channel of the AD604. The signal is applied at Pin 5. RGN

is normally zero, in which case the preamplifier is set to a gain-

of-five (14 dB). When Pin FBK1 is left open, the preamplifier is

set to a gain-of-ten (20 dB) and the gain range shifts up by

6 dB. The ac coupling capacitors before pins –DSX1 and

+DSX1 should be selected according to the required lower cut-

off frequency. In this example the 0.1

µF capacitors together

with the 175

Ω seen looking into each of the DSX input pins,

provides a –3 dB high pass corner of about 9.1 kHz. The upper

cutoff frequency is determined by the bandwidth of the channel

which is 40 MHz. Note, the signal can be simply inverted by

connecting the output of the preamplifier to pin –DSX1 instead

of +DSX1, this is due to the fully differential input of the DSX.

12

11

10

9

8

1

2

3

4

7

6

5

13

16

15

14

24

23

22

21

20

19

18

17

AD604

VGN

RGN

0.1µF

0.1µF

2.500V

0.1µF

0.1µF

500

Ω

+5V

–5V

OUT

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

Figure 41. Basic Connections for a Single Channel

As shown here, the output is ac coupled for optimum perfor-

mance. In the case of connecting to the AD9050, ac coupling

can be eliminated as long as pin VOCM is biased by the same

3.3 V common-mode voltage as the AD9050 (see Figure 50).

40 dB/V and 20 dB/V gain scaling respectively. Voltage VGN

controls the gain; its nominal operating range is from 0.25 V to

2.65 V for 20 dB/V gain scaling, and 0.125 V to 1.325 V for

40 dB/V scaling. When this pin is taken to ground, the chan-

nel will power down and disable its output.

Pin COM1 is the main signal ground for the preamplifier and

needs to be connected with as short a connection as possible to

the input ground. Since the internal feedback resistors of the

preamplifier are very small for noise reasons (8

Ω and 32 Ω

nominally), it is of utmost importance to keep the resistance in

this connection to a minimum! Furthermore, excessive induc-

tance in this connection may lead to oscillations.

As a consequence of the AD604’s ultralow noise and wide band-

width, large dynamic currents will be flowing to and from the

power supply. To insure the stability of the part, extreme atten-

tion to supply decoupling is required. A large storage capacitor

in parallel with a smaller high frequency capacitor connected

right at the supply pins, together with a ferrite bead coming from

the supply should be used to insure high frequency stability.

To provide for additional flexibility, Pin COM1 can be used to

depower the preamplifier. When COM1 is connected to VP,

the preamplifier will be off, yet the DSX portion can be used

independently. This may be of value when one desires to cas-

cade the two DSX stages in the AD604. In this case the first

DSX output signal with respect to noise will be large and using

the second preamplifier at this point would be a waste of power

(see AGC Amplifier Application).

An Ultralow Noise AGC Amplifier with 82 to 96 dB Gain

Range

Figure 42 shows an implementation of an AGC amplifier with

82 dB of gain range using a single AD604. First, the connec-

tions for the two channels of the AD604 will be discussed, and

second, how the detector circuitry that closes the loop works.

VG

13

16

15

14

19

18

17

24

23

22

21

20

12

11

10

9

8

1

2

3

4

7

6

5

AD604

–5V

+5V

C3

0.1µF

C11

1µF

VIN

VREF

C4

0.1µF

C7

0.1µF

R2

453

Ω

C2

0.1µF

RF OUT

C1

0.1µF

R1

49.9

Ω

–5V

+5V

(MAX

800mV p-p)

8

7

6

5

1

2

3

4

AD711

OFFS

NULL

–VS

NC

+VS

OFFS

NULL

OUT

C6

0.56µF

C7

0.33µF

R3

1k

Ω

8765

1

234

Y1

Y2

VN

Z

X1

X2

VP

W

AD835

R4

2k

Ω

C8

0.33µF

+5V

1V

–5V

R5

2k

Ω

C9

0.33µF

R6

2k

Ω

R7

1k

Ω

C10

1µF

LOW

PASS

FILTER

–5V

+5V

2

IF V1 = A*cos (wt)

R8

2k

Ω

VSET (

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

C12

0.1µF

FB

FB

C13

0.1µF

+5V

–5V

ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.

Figure 42. AGC Amplifier with 82 dB of Gain Range