AD8320-EB Analog Devices Inc, AD8320-EB Datasheet - Page 13

• Manufacturer Part Number: AD8320-EB
• Description: BOARD EVAL FOR AD8320
• Manufacturer: Analog Devices Inc

Specifications of AD8320-EB

• Rohs Status: RoHS non-compliant
• Channels Per Ic: 1 - Single
• Amplifier Type: Variable Gain
• Output Type: Single-Ended
• -3db Bandwidth: 150MHz
• Operating Temperature: -40°C ~ 85°C
• Current - Supply (main Ic): 97mA
• Voltage - Supply, Single/dual (±): 5 V ~ 12 V
• Board Type: Fully Populated
• Utilized Ic / Part: AD8320
• Lead Free Status / Rohs Status: Not Compliant

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.
REV. 0
ENABLE
AGND DGND
DATA
CLK
REFIN
PD
11dBmV
PD
AD7801*
VIN
CS
5V
WR LDAC
VDD
CLR
D0–D7
CLK
D0–D7
8
74HCT164*
COMM
A B
VIN
Figure 47. Enhanced Dynamic Range Circuit
1.5
1k
AD603*
GPOS
100
GNEG
+0.5V
VPOS
VNEG
+5V
–5V
FBDK
41dBmV
VOUT
–13–
Figure 48. Output Spectrum of Enhanced Dynamic Range
Circuit (Output Level = 61 dBmV, Frequency = 42 MHz)
Figure 49. Gain Transfer Function of Enhanced Dynamic
Range Circuit
220
20pF
TO MODEM
CHANNEL
RECEIVE
0.1 F
–10
–20
–30
60
50
40
30
20
10
0
0
AD8320*
21
220
41
+12V
75
61
GAIN CONTROL WORD – Decimal
*ADDITIONAL PINS AND
DECOUPLING CAPACITORS
OMITTED FOR CLARITY
81 101 121 141
61dBmV
DIPLEXER
AD603
AD8320/AD603
AD8320
161
58dBmV
181 201 221 241
AD8320
75