AD9284 datasheet(13/24 Pages) AD

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AD9284

Rev. 0 | Page 13 of 24

THEORY OF OPERATION

The AD9284 is a pipeline-type converter. The input buffers are

differential, and both sets of inputs are internally biased. This

allows the use of ac or dc input modes. A sample-and-hold

amplifier is incorporated into the first stage of the multistage

pipeline converter core. The output staging block aligns the

data, carries out error correction for the pipeline stages, and

feeds that data to the output buffers. The two ADC channels

are sampled simultaneously through a single encoding clock.

All user-selected options are programmed through dedicated

digital input pins or a serial port interface (SPI).

ADC ARCHITECTURE

Each channel of the AD9284 consists of a differential input

buffer followed by a sample-and-hold amplifier (SHA). The

SHA is followed by a pipeline switched-capacitor ADC. The

quantized outputs from each stage are combined into a final

8-bit result in the digital correction logic. The pipelined

architecture permits the first stage to operate on a new input

sample, whereas the remaining stages operate on preceding

samples.

Each stage of the pipeline, excluding the last, consists of a low

resolution flash ADC connected to a switched-capacitor DAC

and interstage residue amplifier (MDAC). The residue amplifier

magnifies the difference between the reconstructed DAC output

and the flash input for the next stage in the pipeline. One bit of

redundancy is used in each stage to facilitate digital correction

of flash errors. The last stage consists of a flash ADC.

The input stage contains a differential SHA that can be ac- or

dc-coupled in differential or single-ended mode. The output

staging block aligns the data, carries out error correction, and

passes the data to the output buffers. The output buffers are

powered from a separate supply, allowing adjustment of the

output voltage swing. During power-down, the output buffers

enter a high impedance state.

ANALOG INPUT CONSIDERATIONS

The analog inputs of the AD9284 are differentially buffered.

For best dynamic performance, the source impedances driving

VIN+A, VIN+B, VIN−A, and VIN−B should be matched such

that common-mode settling errors are symmetrical. The analog

inputs are optimized to provide superior wideband performance

and must be driven differentially. SNR and SINAD performance

degrades significantly if the analog inputs are driven with a single-

ended signal.

A wideband transformer, such as Mini-Circuits® ADT1-1WT,

can provide the differential analog inputs for applications that

require a single-ended-to-differential conversion. Both analog

inputs are self-biased by an on-chip resistor divider to

a nominal 1.4 V.

Differential Input Configurations

Optimum performance is achieved when driving the AD9284

in a differential input configuration. For baseband applications,

the ADA4937-1 differential driver provides excellent performance

and a flexible interface to the ADC (see Figure 19). The output

common-mode voltage of the AD9284 is easily set to 1.4 V, and

the driver can be configured in a Sallen-Key filter topology to

provide band limiting of the input signal.

–

200Ω

227.4Ω

61.9Ω

ADA4937-1

1.2V p-p

0.1µF

200Ω

4.7pF

33Ω

VIN

–

AD9284

VCM

Figure 19. Differential Input Configuration Using the ADA4937-1

The AD9284 can also be driven passively with a differential

transformer-coupled input (see Figure 20). To bias the analog

input, the VCM voltage can be connected to the center tap of

the secondary winding of the transformer.

49.9Ω

1.2V p-p

4.7pF

0.1µF

33Ω

VIN

–

AD9284

VCM

Figure 20. Differential Transformer-Coupled Configuration

The signal characteristics must be considered when selecting a

transformer. Most RF transformers saturate at frequencies below

a few megahertz (MHz). Excessive signal power can also cause

core saturation, which leads to distortion.

VOLTAGE REFERENCE

An internal differential voltage reference creates positive and

negative reference voltages that define the 1.2 V p-p fixed span

of the ADC core. This internal voltage reference can be adjusted

by means of SPI control. It can also be driven externally with

an off-chip stable reference. See the Memory Map Register

Descriptions section for more details.

RBIAS

The AD9284 requires the user to place a 10 kΩ resistor between

the RBIAS pin and ground. This resistor, which is used to set

the master current reference of the ADC core, should have a 1%

tolerance.

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