DESCRIPTION GÉNÉRALE

The AD810 is a composite and HDTV-compatible, current feedback, video operational amplifier, ideal for use in systems such as multimedia, digital tape recorders, and video cameras. The 0.1 dB flatness specification at a bandwidth of 30 MHz  (G = +2) and the differential gain and phase of 0.02% and 0.04° (NTSC) make the AD810 ideal for any broadcast quality video system. All these specifications are under load conditions of 150 Ω (one 75 Ω back terminated cable).  The AD810 is ideal for power sensitive applications such as video cameras, offering a low power supply current of 8.0 mA maximum. The disable feature reduces the power supply current to only 2.1 mA, while the amplifier is not in use, to conserve power. Furthermore, the AD810 is specified over a power supply range of ±5 V to ±15 V. The AD810 works well as an ADC or DAC buffer in video systems due to its unity gain (G = +1) −3 dB bandwidth of 80 MHz. Because the AD810 is a transimpedance amplifier, this bandwidth can be maintained over a wide range of gains while featuring a low noise of 2.9 nV/√Hz for wide dynamic range applications.

 

CARACTÉRISTIQUES

High speed

80 MHz typical −3 dB bandwidth (G = +1)

75 MHz typical −3 dB bandwidth (G = +2)

1000 V/µs typical slew rate

50 ns typical settling time to 0.1% (VOUT = 10 V step)

Ideal for video applications

30 MHz typical 0.1 dB bandwidth (G = +2, VS = ±15 V)

0.02% typical differential gain (VS = ±15 V)

0.04° typical differential phase (VS = ±15 V)

Faible bruit

2.9 nV/√Hz typical input voltage noise

13 pA/√Hz typical inverting input current noise

Faible consommation

8.0 mA maximum supply current (quiescent)

2.1 mA typical supply current (power-down mode)

High performance disable function

Turn off time: 100 ns typical

Break before make guaranteed

Input to output isolation of 64 dB (off state)

Flexible operation

Specified for ±5 V and ±15 V operation

±2.9 V typical output swing into a 150 Ω load (VS = ±5 V)

 

CANDIDATURES

Professional video cameras

Multimedia systems

NTSC-, PAL-, and SECAM-compatible systems

Video line drivers

ADC or DAC buffers

DC restoration circuits

 

THÉORIE DU FONCTIONNEMENT

GENERAL DESIGN CONSIDERATIONS

The AD810 is a current feedback amplifier optimized for use in high performance video and data acquisition systems. Because the AD810 uses a current feedback architecture, its closed-loop bandwidth depends on the value of the feedback resistor. Table 5 and Table 6 list recommended resistor values for some useful closed-loop gains and supply voltages. The recommended resistor values results in maximum bandwidths with less than 0.1 dB of peaking in the gain vs. frequency response. The −3 dB bandwidth is also somewhat dependent on the power supply voltage. Lowering the supplies increases the values of internal capacitances, reducing the bandwidth. To compensate for this reduction, smaller values of feedback resistor are sometimes used at lower supply voltages.

ACHIEVING VERY FLAT GAIN RESPONSE AT HIGH FREQUENCY

Achieving and maintaining gain flatness of less than 0.1 dB above 10 MHz is not difficult if the recommended resistor values are used. Additionally, consider feedback resistor selection, PCB layout, coaxial cable quality, power supply bypassing and operating range, and offset nulling to ensure consistently optimal results.

CHOICE OF FEEDBACK RESISTOR

Because the 3 dB bandwidth depends on the feedback resistor, the fine scale flatness varies to some extent with feedback resistor tolerance. It is recommended that resistors with a 1% tolerance be used to maintain exceptional flatness through high volume production.

PRINTED CIRCUIT BOARD LAYOUT

As with all wideband amplifiers, PCB parasitics can affect the overall closed-loop performance. Most important are stray capacitances at the output and inverting input nodes. (An added capacitance of 2 pF between the inverting input and ground adds about 0.2 dB of peaking in the gain of 2 response, and increase the bandwidth to 105 MHz). Leave space (3/16 inches is sufficient) around the signal lines to minimize coupling. Also, keep signal lines connecting the feedback and gain resistors short enough so that their associated inductance does not cause high frequency gain errors. Line lengths less than ¼ inches are recommended.