Datasheet AD847 (Analog Devices) - 12

AD847 VIDEO LINE DRIVER
Figure 24 shows the AD847 driving 100 pF and 1000 pF loads. The AD847 functions very well as a low cost, high speed line driver for either terminated or unterminated cables. Figure 23 shows the AD847 driving a doubly terminated cable in a fol- lower configuration. The termination resistor, RT, (when equal to the cable’s charac- teristic impedance) minimizes reflections from the far end of the cable. While operating from ± 5 V supplies, the AD847 main- tains a typical slew rate of 200 V/µs, which means it can drive a ±1 V, 30 MHz signal into a terminated cable.
+VS 75
Ω
R IN 0.1
µ
F COAX
Figure 24. AD847 Driving Capacitive Loads
100
Ω
75
Ω
VIN COAX V 75
Ω
AD847 OUT FLASH ADC INPUT BUFFER RBT R T 75
Ω The 35 MHz unity gain bandwidth of the AD847 makes it an
75
Ω excellent choice for buffering the input of high speed flash A/D
0.1
µ
F
converters, such as the AD9048.
500
Ω
–VS
Figure 25 shows the AD847 as a unity inverter for the input to the AD9048.
C 500
Ω
C 0.1 SEE TABLE I –5.2V AD589 2k 10k
Ω Figure 23. Video Line Driver
27 Table I. Video Line Driver Performance Chart 100 0.1 2N3906 AD741 Over- 5 1k 1k VIN* VSUPPLY CC –3 dB BW shoot 1.5k
Ω
0.1
µ
F
0 dB or ± 500 mV Step ±15 20 pF 23 MHz 4%
ANALOG 1.5k
Ω
RB RT
0 dB or ± 500 mV Step ±15 15 pF 21 MHz 0%
INPUT 43
Ω 0 dB or ± 500 mV Step ±15 0 pF 13 MHz 0%
(0V TO +2V) D1 50
Ω
V AD847 IN (MSB)
0 dB or ± 500 mV Step ±5 20 pF 18 MHz 2% 0 dB or ± 500 mV Step ±5 15 pF 16 MHz 0%
AD9048
0 dB or ± 500 mV Step ±5 0 pF 11 MHz 0%
TTL D8 CONVERT CONVERT (LSB) SIGNAL
*–3 dB bandwidth numbers are for the 0 dBm signal input. Overshoot numbers
V V EE CC
are the percent overshoot of the 1 volt step input.
0.1
µ
F 0.1
µ
F
A back-termination resistor (RBT, also equal to the characteristic impedance of the cable) may be placed between the AD847 out-
–5.2V +5.0V
put and the cable input, in order to damp any reflected signals Figure 25. Flash ADC Input Buffer caused by a mismatch between RT and the cable’s characteristic impedance. This will result in a flatter frequency response, al- though this requires that the op amp supply ± 2 V to the output in order to achieve a ± 1 V swing at resistor RT. –10– REV. F