AD9220ARSZ datasheet(16/32 Pages) AD

16 / 32 page

REV. E

–16–

AD9221/AD9223/AD9220

AC COUPLING AND INTERFACE ISSUES

For applications where ac coupling is appropriate, the op amp’s

output can be easily level shifted to the common-mode voltage,

VCM, of the AD9221/AD9223/AD9220 via a coupling capacitor.

This has the advantage of allowing the op amp’s common-mode

level to be symmetrically biased to its midsupply level (i.e.,

(VCC + VEE)/2). Op amps that operate symmetrically with respect

to their power supplies typically provide the best ac performance

as well as the greatest input/output span. Thus, various high

speed/performance amplifiers that are restricted to +5 V/–5 V

operation and/or specified for 5 V single-supply operation can be

easily configured for the 5 V or 2 V input span of the AD9221/

AD9223/AD9220. The best ac distortion performance is achieved

when the A/D is configured for a 2 V input span and common-

mode voltage of 2.5 V. Note that differential transformer coupling,

which is another form of ac coupling, should be considered for

optimum ac performance.

Simple AC Interface

Figure 15 shows a typical example of an ac-coupled, single-ended

configuration. The bias voltage shifts the bipolar, ground-refer-

enced input signal to approximately VREF. The value for C1

and C2 will depend on the size of the resistor, R. The capacitors,

C1 and C2, are typically a 0.1

µF ceramic and 10 µF tanta-

lum capacitor in parallel to achieve a low cutoff frequency

while maintaining a low impedance over a wide frequency

range. The combination of the capacitor and the resistor form a

high-pass filter with a high-pass –3 dB frequency determined

by the equation,

=× × ×

()

The low impedance VREF voltage source both biases the VINB

input and provides the bias voltage for the VINA input. Figure 15

shows the VREF configured for 2.5 V; thus the input range

VINA

VINB

SENSE

+5V

–5V

VREF

+VREF

–VREF

VIN

AD9221/

AD9223/

AD9220

Figure 15. AC-Coupled Input

of the A/D is 0 V to 5 V. Other input ranges could be selected

by changing VREF, but the A/D’s distortion performance will

degrade slightly as the input common-mode voltage deviates

from its optimum level of 2.5 V.

Alternative AC Interface

Figure 16 shows a flexible ac-coupled circuit that can be config-

ured for different input spans. Since the common-mode voltage

of VINA and VINB are biased to midsupply independent of

VREF, VREF can be pin-strapped or reconfigured to achieve

input spans between 2 V and 5 V p-p. The AD9221/AD9223/

AD9220’s CMRR along with the symmetrical coupling R-C

networks will reject both power supply variations and noise. The

resistors, R, establish the common-mode voltage. They may

have a high value (e.g., 5 k

Ω) to minimize power consumption

and establish a low cutoff frequency. The capacitors, C1 and

C2, are typically 0.1

µF ceramic and 10 µF tantalum capacitors

in parallel to achieve a low cutoff frequency while maintaining a

low impedance over a wide frequency range. RS isolates the

buffer amplifier from the A/D input. The optimum performance

is achieved when VINA and VINB are driven via «Immetrical

networks. The f–3 dB point can be approximated by the equation,

–

=× ×

()

VINA

VINB

+5V

–5V

VIN

+5V

AD9221/

AD9223/

AD9220

Figure 16. AC-Coupled Input-Flexible Input Span,

VCM = 2 V

Op Amp Selection Guide

Op amp selection for the AD9221/AD9223/AD9220 is highly

dependent on a particular application. In general, the performance

requirements of any given application can be characterized by

either time domain or frequency domain parameters. In either

case, one should carefully select an op amp that preserves the

performance of the A/D. This task becomes challenging when

one considers the AD9221/AD9223/AD9220’s high perfor-

mance capabilities coupled with other extraneous system level

requirements such as power consumption and cost.

The ability to select the optimal op amp may be further compli-

cated by either limited power supply availability and/or limited

acceptable supplies for a desired op amp. Newer, high perfor-

mance op amps typically have input and output range limitations

in accordance with their lower supply voltages. As a result, some

op amps will be more appropriate in systems where ac-coupling

is allowable. When dc-coupling is required, op amps without

headroom constraints, such as rail-to-rail op amps or ones

where larger supplies can be used, should be considered. The

following section describes some op amps currently available

from Analog Devices. The system designer is always encouraged

to contact the factory or local sales office to be updated on Analog

Devices’ latest amplifier product offerings. Highlights of the

areas where the op amps excel and where they may limit the

performance of the AD9221/AD9223/AD9220 is also included.

AD817:

50 MHz Unity GBW, 70 ns Settling to 0.01%, +5 V

±15 V Supplies

Best Applications: Sample Rates < 7 MSPS, Low

Noise, 5 V p-p Input Range

Limits: THD above 100 kHz

AD826:

Dual Version of AD817

Best Applications: Differential and/or Low Imped-

ance Input

Drivers, Low Noise

Limits: THD above 100 kHz

AD818:

130 MHz @ G = +2 BW, 80 ns Settling to 0.01%,

+5 V to

±15 V Supplies

Best Applications: Sample Rates < 7 MSPS, Low

Noise, 5 V p-p Input Range, Gains

≥ +2

Limits: THD above 100 kHz

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