Figure 50 and Figure 51 show the output signal swing and
frequency spectrum at 500 kHz. At this frequency, the output
signal (at VOUT), 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 50. Output Signal Swing of Low Distortion Line Driver at 500 kHz
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 (for example, 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); therefore, the
emitter 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 52. Output Signal Swing of Low Distortion Line Driver at 2 MHz
Figure 51. THD of Low Distortion Line Driver at 500 kHz
Figure 52 and Figure 53 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).
Figure 53. THD of Low Distortion Line Driver at 2 MHz
Rev. C | Page 17 of 20