AD8011AN(2000) 查看數據表（PDF） - Analog Devices

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AD8011AN
(Rev.:2000)

300 MHz, 1 mA Current Feedback Amplifier

Analog Devices 

AD8011

INCREASING BW AT HIGH GAINS

As presented above, for a fixed RF (feedback gain setting resis-

tor) the AD8011 CLBW will decrease as RN is reduced (in-

creased G). This effect can be minimized by simply reducing RF

and thus partially restoring the devices optimized BW for gains

greater than +2/–1. Note that the AD8011 is ac optimized (high

BW and low peaking) for AV =+2/–1 and RF equal to 1 kΩ.

Using this optimized G as a reference and the VO(s) equations

above, the following relationships results:

RF = 1k + 2 – G/2 gm for G = 1+ RF/RN

(noninverting) or:

RF = 1k + G + 1/2 gm for G = –RF/RN

(inverting)

Using 1/2 gm equal to 120 Ω results in a RF of 500 Ω for

G = 5/–4 and a corresponding RN of 125 Ω. This will extend the

AD8011’s BW to near its optimum design value of typically 180

MHz at RL = 1 kΩ. In general, for gains greater than +7/–6, RF

should not be reduced to values much below 400 Ω else ac

peaking can result. Using this RF value as the a lower limit, will

result in BW restoration near its optimized value to the upper G

values specified. Gains greater than about +7/–6 will result

in CLBW reduction. Again, the derivations above are just

approximations.

DRIVING A SINGLE-SUPPLY A/D CONVERTER

New CMOS A/D converters are placing greater demands on the

amplifiers that drive them. Higher resolutions, faster conversion

rates and input switching irregularities require superior settling

characteristics. In addition, these devices run off a single +5 V

supply and consume little power, so good single-supply operation

with low power consumption are very important. The AD8011

is well positioned for driving this new class of A/D converters.

Figure 36 shows a circuit that uses an AD8011 to drive an

AD876, a single supply, 10-bit, 20 MSPS A/D converter that

requires only 140 mW. Using the AD8011 for level shifting and

driving, the A/D exhibits no degradation in performance com-

pared to when it is driven from a signal generator.

The analog input of the AD876 spans 2 V centered at about

2.6 V. The resistor network and bias voltages provide the level

shifting and gain required to convert the 0 V to 1 V input signal

to a 3.6 V to 1.6 V range that the AD876 wants to see.

Biasing the noninverting input of the AD8011 at 1.6 V dc forces

the inverting input to be at 1.6 V dc for linear operation of the

amplifier. When the input is at 0 V, there is 3.2 mA flowing out

of the summing junction via R1 (1.6 V/499 Ω). R3 has a current

of 1.2 mA flowing into the summing junction (3.6 V–1.6 V)/

1.65 kΩ. The difference of these two currents (2 mA) must flow

through R2. This current flows toward the summing junction

and requires that the output be 2 V higher than the summing

junction or at 3.6 V.

When the input is at 1 V, there is 1.2 mA flowing into the sum-

ming junction through R3 and 1.2 mA flowing out through R1.

These currents balance and leave no current to flow through

R2. Thus the output is at the same potential as the inverting

input or 1.6 V.

The input of the AD876 has a series MOSFET switch that turns

on and off at the sampling rate. This MOSFET is connected to

a hold capacitor internal to the device. The on impedance of the

MOSFET is about 50 Ω, while the hold capacitor is about 5 pF.

In a worst case condition, the input voltage to the AD876 will

change by a full-scale value (2 V) in one sampling cycle. When

the input MOSFET turns on, the output of the op amp will be

connected to the charged hold capacitor through the series

resistance of the MOSFET. Without any other series resistance,

the instantaneous current that flows would be 40 mA. This

would cause settling problems for the op amp.

The series 100 Ω resistor limits the current that flows instanta-

neously after the MOSFET turns on to about 13 mA. This

resistor cannot be made too large or the high frequency perfor-

mance will be affected.

The sampling MOSFET of the AD876 is closed for only half of

each cycle or for 25 ns. Approximately 7 time constants are

required for settling to 10 bits. The series 100 Ω resistor along

with the 50 Ω on resistance and the hold capacitor, create a

750 ps time constant. These values leave a comfortable margin

for settling. Obtaining the same results with the op amp A/D

combination as compared to driving with a signal generator

indicates that the op amp is settling fast enough.

Overall the AD8011 provides adequate buffering for the AD876

A/D converter without introducing distortion greater than that

of the A/D converter by itself.

+5V

3.6V

R3

1.65k⍀

R2

1k⍀

0.1␮F

10␮F

0.1␮F

R1

1V

VIN

499k⍀

+3.6V

REFT

0V

50⍀

AD8011

100⍀

AD876

0.1␮F

3.6V

1.6V

REFB

+1.6V

1.6V

Figure 36. AD8011 Driving the AD876

REV. B

–13–

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