REV. C

AD8011

–13–

INCREASING BW AT HIGH GAINS

(increased G). This effect can be minimized by simply reducing

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

Ω. Using this

Ω + 2 – G/2 gm for

Ω + G + 1/2 gm for

Using 1/2 gm equal to 120

Ω results in a R

Ω for G =

Ω. This will extend the

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

Ω. In general, for gains greater than +7/–6,

Ω or else ac

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. 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 re solutions, 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 14 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 compared to

when it is driven from a signal generator.

3.6V

1.6V

AD8011

+5V

10 F

R2

1k

R3

1.65k

R1

499k

3.6V

50

0.1 F

1.6V

1V

0V

100

AD876

+1.6V

+3.6V

REFT

REFB

0.1 F

0.1 F

Figure 14. AD8011 Driving the AD876

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 k

Ω.

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 resis-

tance 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 instantane-

ously to about 13 mA after the MOSFET turns on. This resistor

cannot be made too large or the high frequency performance

will be affected.

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

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

required for settling to 10 bits. The series 100

Ω resistor, 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.