AD8031/AD8032

–15–

REV. B

Figure 44 shows the AD8031 configured as a single supply gain-

of-2 line driver. With the output driving a back terminated 50

Ω

minimizing reflections, the 50

Ω back termination resistor pro-

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

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).

2V

50mV

10

0%

100

90

1 s

0.5V

Figure 45. Output Signal Swing of Low Distortion Line

Driver at 500 kHz

STOP 5MHz

START 0Hz

+9dBm

Figure 46. THD of Low Distortion Line Driver at 500 kHz

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.

50mV

10

0%

100

90

200ns

0.2V

1.5V

Figure 47. Output Signal Swing of Low Distortion Line

Driver at 2 MHz

START 0Hz

STOP 20MHz

+7dBm

Figure 48. THD of Low Distortion Line Driver at 2 MHz