AD8031B Analog Devices, AD8031B Datasheet - Page 15

• Manufacturer Part Number: AD8031B
• Description: 2.7 V/ 800 uA/ 80 MHz Rail-to-Rail I/O Amplifiers
• Manufacturer: Analog Devices

Figure 44 shows the AD8031 configured as a single supply gain-
of-2 line driver. With the output driving a back terminated 50
line, the overall gain from V
minimizing reflections, the 50
tects the transistor from damage if the cable is short circuited.
The emitter follower, which is inside the feedback loop, ensures
that the output voltage from the AD8031 stays about 700 mV
above ground. Using this circuit, very low distortion is attain-
able even when the output signal swings to within 50 mV of
ground. The circuit was tested at 500 kHz and 2 MHz. Figures
45 and 46 show the output signal swing and frequency spectrum
at 500 kHz. At this frequency, the output signal (at V
which has a peak-to-peak swing of 1.95 V (50 mV to 2 V), has a
THD of –68 dB (SFDR = –77 dB).
Figure 45. Output Signal Swing of Low Distortion Line
Driver at 500 kHz
REV. B
Figure 46. THD of Low Distortion Line Driver at 500 kHz
+9dBm
50mV
START 0Hz
2V
100
90
10
0%
0.5V
IN
to V
back termination resistor pro-
OUT
is unity. In addition to
1 s
STOP 5MHz
OUT
),
–15–
Figures 47 and 48 show the output signal swing and frequency
spectrum at 2 MHz. As expected, there is some degradation in
signal quality at the higher frequency. When the output signal
has a peak-to-peak swing of 1.45 V (swinging from 50 mV to
1.5 V), the THD is –55 dB (SFDR = –60 dB).
This circuit could also be used to drive the analog input of a
single supply high speed ADC whose input voltage range is
referenced to ground (e.g., 0 V to 2 V or 0 V to 4 V). In this
case, a back termination resistor is not necessary (assuming a
short physical distance from transistor to ADC), so the emit-
ter of the external transistor would be connected directly to the
ADC input. The available output voltage swing of the circuit
would, therefore be doubled.
Figure 47. Output Signal Swing of Low Distortion Line
Driver at 2 MHz
Figure 48. THD of Low Distortion Line Driver at 2 MHz
+7dBm
50mV
START 0Hz
1.5V
100
0%
90
10
0.2V
AD8031/AD8032
200ns
STOP 20MHz