AD810 datasheet(15/22 Pages) AD

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Data Sheet

AD810

Rev. B | Page 15 of 22

THEORY OF OPERATION

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. As shown in Table 5 and

Table 6, the closed-loop bandwidth is not a strong function of

gain, as it is for a voltage feedback amplifier. 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. The

characteristic curves in Figure 23 and Figure 24 illustrate that

bandwidths of over 100 MHz on 30 V total supplies and over

50 MHz on 5 V total supplies can be achieved.

Table 5. −3 dB Bandwidth vs. Closed-Loop Gain and

Resistance Values (RL = 150 Ω), VS = ±15 V

Closed-Loop Gain

−3 dB Bandwidth (MHz)

1 kΩ

Open

715 Ω

+10

270 Ω

30 Ω

–1

681 Ω

–10

249 Ω

24.9 Ω

Table 6. −3 dB Bandwidth vs. Closed-Loop Gain and

Resistance Values (RL = 150 Ω), VS = ±5 V

Closed-Loop Gain

−3 dB Bandwidth (MHz)

910 Ω

Open

715 Ω

+10

270 Ω

30 Ω

–1

620 Ω

–10

249 Ω

24.9 Ω

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.

QUALITY OF COAXIAL CABLE

Optimum flatness when driving a coaxial cable is possible only

when the driven cable is terminated at each end with a resistor

matching its characteristic impedance. With an ideal coaxial

cable, the resulting flatness is not affected by the length of the

cable. Although outstanding results can be achieved using

inexpensive cables, some variation in flatness due to varying

cable lengths is to be expected.

POWER SUPPLY BYPASSING

Adequate power supply bypassing can be critical when

optimizing the performance of a high frequency circuit.

Inductance in the power supply leads can contribute to resonant

circuits that produce peaking in the amplifier response.

Although the recommended 0.1 µF power supply bypass

capacitors are sufficient in most applications, more elaborate

bypassing (such as using two paralleled capacitors) may be

required in some cases. In addition, if large current transients

must be delivered to the load, bypass capacitors (typically

greater than 1 µF) are required to optimize settling time and

lowest distortion.

POWER SUPPLY OPERATING RANGE

The AD810 operates with supplies from ±18 V down to about

±2.5 V. On ±2.5 V supplies, the low distortion output voltage

swing is greater than 1 V p-p. Single-supply operation can be

achieved by biasing the input common-mode voltage at the

supply midpoint.

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