AD15700SLASHPCB datasheet(41/44 Pages) AD

41 / 44 page

REV. A

AD15700

–41–

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.

VOUT

VIN

49.9

200

2.49

49.9

10 F

0.1 F

49.9

2N3904

Figure 43. Low Distortion Line Driver for Single-

Supply Ground Referenced Signals

50mV

100

1µs

0.5V

Figure 44. Output Signal Swing of Low Distortion

Line Driver at 500 kHz

+9dBm

START 0Hz

STOP 5MHz

Figure 45. THD of Low Distortion Line Driver at 500 kHz

Figure 43 shows the amplifier configured as a single-supply

gain-of-two line driver. With the output driving a back terminated

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.

1.5V

50mV

100

200ns

0.2V

Figure 46. Output Signal Swing of Low Distortion

Line Driver at 2 MHz

START 0Hz

STOP 20MHz

+7dBm

Figure 47. THD of Low Distortion Line Driver at 2 MHz

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