AD604ARSZ-R7 datasheet(19/32 Pages) AD

19 / 32 page

AD604

Rev. D | Page 19 of 32

AD604

–5V

+5V

VREF

RF OUT

–5V

+5V

AD711

OUT

AD835

+5V

–5V

+5V

VSET (<0V)

–DSX1

+DSX1

PAI1

FBK1

PAO1

COM1

COM2

PAI2

FBK2

PAO2

+DSX2

–DSX2

VGN1

VREF

VPOS

GND1

OUT1

VNEG

VPOS

GND2

OUT2

VOCM

VGN2

+5V

–5V

ALL SUPPLY PINS ARE DECOUPLED AS SHOWN.

OFFS

NULL

0.1µF

VIN

(MAX

800mV p-p)

49.9Ω

0.1µF

C12

0.1µF

C13

0.1µF

0.56µF

453Ω

1kΩ

0.33µF

V1 = VIN ×G

2kΩ

0.33µF

234

0.33µF

2kΩ

–(A)2

IF V1 = A × cos (wt)

–(V1)2

LOW-

PASS

FILTER

1kΩ

C10

1µF

C11

1µF

2kΩ

+VS

–VS

OFFS

NULL

Figure 44. AGC Amplifier with 82 dB of Gain Range

AN ULTRALOW NOISE AGC AMPLIFIER WITH

82 dB TO 96 dB GAIN RANGE

Figure 44 shows an implementation of an AGC amplifier with

82 dB of gain range using a single AD604. The signal is applied

to connector VIN, and because the signal source is 50 Ω, a

terminating resistor (R1) of 49.9 Ω is added. The signal is then

amplified by 14 dB (Pin FBK1 shorted to PAO1) through the

Channel 1 preamplifier and is further processed by the Channel 1

DSX. Next, the signal is applied directly to the Channel 2 DSX. The

second preamplifier is powered down by connecting its COM2 pin

to the positive supply as explained in the Preamplifier section. C1

and C2 level shift the signal from the preamplifier into the first

DSX and at the same time eliminate any offset contribution of the

preamplifier. C3 and C4 have the same offset cancellation purpose

for the second DSX. Each set of capacitors, combined with the

175 Ω input resistance of the corresponding DSX, provides a

high-pass filter with a −3 dB corner frequency of about 9.1 kHz.

VOCM is decoupled to ground by a 0.1 μF capacitor, while

VREF can be externally provided; in this application, the gain

scale is set to 20 dB/V by applying 2.500 V. Because each of the

DSX amplifiers operates from a single 5 V supply, the output is

ac-coupled via C6 and C7. The output signal can be monitored

at the connector labeled RF OUT.

Figure 45 and Figure 46 show the gain range and gain error for

the AD604 connected as shown. The gain range is −14 dB to

+82 dB; the useful range is 0 dB to +82 dB if the RF output

amplitude is controlled to ±400 mV (+2 dBm). The main limitation

on the lower end of the signal range is the input capability of

the preamplifier. This limitation can be overcome by adding an

attenuator in front of the preamplifier, but that would defeat the

advantage of the ultralow noise preamplifier. It should be noted

that the second preamplifier is not used because its ultralow

noise and the associated high power consumption are overkill

after the first DSX stage. It is disabled in this application by

connecting the COM2 pin to the positive supply. Nevertheless,

the second preamplifier can be used, if so desired, and the

useful gain range increases by 14 dB to encompass 0 dB to

96 dB of gain. For the same +2 dBm output, this allows signals

as small as −94 dBm to be measured.

To achieve the highest gains, the input signal must be band-

limited to reduce the noise; this is especially true if the second

preamplifier is used. If the maximum signal at OUT2 of the AD604

is limited to ±400 mV (+2 dBm), the input signal level at the

AGC threshold is +25 μV rms (−79 dBm). The circuit as shown in

Figure 44 has about 40 MHz of noise bandwidth; the 0.8 nV/√Hz

of input referred voltage noise spectral density of the AD604

results in an rms noise of 5.05 μV in the 40 MHz bandwidth.

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