ADDS-21160M-EZLITE View Datasheet(PDF) - Analog Devices
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ADDS-21160M-EZLITE Datasheet PDF : 44 Pages
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AD15700
High Performance Single-Supply Line Driver
Even though the amplifier swings close to both rails, the amplifier
has optimum distortion performance when the signal has a common-
mode level halfway between the supplies and when there is about
500 mV of headroom to each rail. If low distortion is required in
single-supply applications for signals that swing close to ground,
an emitter follower circuit can be used at the amplifier output.
5V
10F
0.1F
VIN
49.9⍀
2N3904
2.49⍀
2.49⍀
49.9⍀
200⍀
VOUT
49.9⍀
Figure 43. Low Distortion Line Driver for Single-
Supply Ground Referenced Signals
Figure 43 shows the amplifier configured as a single-supply
gain-of-two line driver. With the output driving a back terminated
50 W line, the overall gain from VIN to VOUT is unity. In addition
to minimizing reflections, the 50 W 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 amplifier 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 44
and 45 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).
Figures 46 and 47 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 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.
100
90
2V
1.5V
100
90
10
0%
50mV
0.5V
1µs
Figure 44. Output Signal Swing of Low Distortion
Line Driver at 500 kHz
+9dBm
10
0%
50mV
0.2V
200ns
Figure 46. Output Signal Swing of Low Distortion
Line Driver at 2 MHz
+7dBm
START 0Hz
STOP 5MHz
Figure 45. THD of Low Distortion Line Driver at 500 kHz
START 0Hz
STOP 20MHz
Figure 47. THD of Low Distortion Line Driver at 2 MHz
REV. A
–41–
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