AD8320

–13–

REV. 0

Enhanced Dynamic Range Application

The AD8320 can be combined with the AD603 to give addi-

tional dynamic range as shown in Figure 47. The AD603 is a

voltage controlled variable gain amplifier. The gain of the AD603 is

determined by the difference in voltage between the GPOS and

GNEG pins. This differential voltage has a range of

±0.5 V. In

this example, the voltage on GNEG is tied to +0.5 V. As the

voltage on GPOS is varied from 0 V to 1 V, the gain of the AD603

changes from –10 dB to +30 dB with a slope of 25 mV/dB (i.e.,

linear in dB). The gain control voltage is supplied by the AD7801

DAC. The output voltage of the DAC (0 V to +2.5 V) is divided

down to fit the 0 V to 1 V range of the AD603 using a resistor

attenuator network.

In order that the same gain control word can be used for both

the AD603 and the AD8320, the serial data stream is converted

to the parallel format of the AD7801 DAC using a serial-to-

parallel shift register. The rising edge of the enable pulse simul-

taneously updates both amplifiers.

As the control word is varied from 00Dec to 255Dec, the gain of

the signal chain varies from –26 dB to +50 dB (there is 6 dB of

attenuation between AD603 and AD8320). In practice, this

circuit is not usable at the lower end of the gain range due to the

small input signal (11 dBmV or about 10 mV p-p). Figure 48

shows the spectrum of the output signal at a frequency of

42 MHz and an output level of 61 dBmV (3.1 V p-p, max gain).

The gain vs. code transfer function of the two amplifiers along

with the overall gain is shown in Figure 49. The overall gain

transfer function combines a linear in dB transfer function with

a linear in Volts/Volt transfer function. It is clear from Figure 49

that the overall gain transfer function can be considered to be

approximately linear in dB over the top 50 dB of its range.

Figure 48. Output Spectrum of Enhanced Dynamic Range

Circuit (Output Level = 61 dBmV, Frequency = 42 MHz)

0

GAIN CONTROL WORD – Decimal

10

0

161

21

41

61

81 101 121 141

50

30

20

60

40

181 201 221 241

–10

–20

–30

AD8320/AD603

AD8320

AD603

Figure 49. Gain Transfer Function of Enhanced Dynamic

Range Circuit

75

TO MODEM

RECEIVE

CHANNEL

AD7801*

DIPLEXER

+12V

VIN

220

220

75

41dBmV

61dBmV

58dBmV

VIN

COMM

1.5

1k

+5V

–5V

+0.5V

11dBmV

DATA

CLK

ENABLE

PD

D0–D7

CLK

100

GPOS

GNEG

VNEG

FBDK

VOUT

VPOS

AD8320*

CS

D0–D7

WR LDAC

AD603*

74HCT164*

8

AB

20pF

0.1 F

*ADDITIONAL PINS AND

DECOUPLING CAPACITORS

OMITTED FOR CLARITY

PD

CLR

VDD

REFIN

5V

AGND DGND

Figure 47. Enhanced Dynamic Range Circuit