AD9265-80EBZ datasheet(27/45 Pages) AD

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AD9265

Data Sheet

Rev. C | Page 26 of 44

Dither

The AD9265 has an optional dither mode that can be selected

either through the SPI bus or by using the DITHER pin. Dithering

is the act of injecting a known but random amount of white noise,

commonly referred to as dither, into the input of the ADC.

Dithering has the effect of improving the local linearity at

various points along the ADC transfer function. Dithering

can significantly improve the SFDR when quantizing small

signal inputs, typically when the input level is below −6 dBFS.

As shown in Figure 65, the dither that is added to the input of

the ADC through the dither DAC is precisely subtracted out

digitally to minimize SNR degradation. When dithering is enabled,

the dither DAC is driven by a pseudorandom number generator

(PN gen). In the AD9265, the dither DAC is precisely calibrated

to result in only a very small degradation in SNR and SINAD.

The typical SNR and SINAD degradation values, with dithering

enabled, are only 1 dB and 0.8 dB, respectively.

ADC CORE

DITHER

DAC

PN GEN

DITHER ENABLE

VIN

DOUT

Figure 65. Dither Block Diagram

Large Signal FFT

In most cases, dithering does not improve SFDR for large signal

inputs close to full scale, for example, with a −1 dBFS input. For

large signal inputs, the SFDR is typically limited by front-end

sampling distortion, which dithering cannot improve. However,

even for such large signal inputs, dithering may be useful for

certain applications because it makes the noise floor whiter.

As is common in pipeline ADCs, the AD9265 contains small

DNL errors caused by random component mismatches that

produce spurs or tones that make the noise floor somewhat

randomly colored part-to-part. Although these tones are

typically at very low levels and do not limit SFDR when the

ADC is quantizing large signal inputs, dithering converts these

tones to noise and produces a whiter noise floor.

Small Signal FFT

For small signal inputs, the front-end sampling circuit typically

contributes very little distortion, and, therefore, the SFDR is likely

to be limited by tones caused by DNL errors due to random com-

ponent mismatches. Therefore, for small signal inputs (typically,

those below −6 dBFS), dithering can significantly improve

SFDR by converting these DNL tones to white noise.

Static Linearity

Dithering also removes sharp local discontinuities in the INL

transfer function of the ADC and reduces the overall peak-to-

peak INL.

In receiver applications, utilizing dither helps to reduce DNL errors

that cause small signal gain errors. Often this issue is overcome

by setting the input noise 5 dB to 10 dB above the converter

noise. By utilizing dither within the converter to correct the

DNL errors, the input noise requirement can be reduced.

Differential Input Configurations

Optimum performance is achieved while driving the AD9265 in a

differential input configuration. For baseband applications, the

AD8138, ADA4937-2, and ADA4938-2 differential drivers provide

excellent performance and a flexible interface to the ADC.

The output common-mode voltage of the ADA4938 is easily set

with the VCM pin of the AD9265 (see Figure 66), and the

driver can be configured in the filter topology shown to provide

band limiting of the input signal.

VIN

76.8Ω

120Ω

0.1µF

200Ω

90Ω

AVDD

33Ω

15Ω

5pF

15pF

ADC

VIN–

VIN+

VCM

ADA4938-2

Figure 66. Differential Input Configuration Using the ADA4938-2

For baseband applications where SNR is a key parameter,

differential transformer coupling is the recommended input

configuration. An example is shown in Figure 67. To bias the

analog input, the VCM voltage can be connected to the center

tap of the secondary winding of the transformer.

2V p-p

49.9Ω

0.1µF

ADC

VIN+

VIN–

VCM

Figure 67. 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.

AD9265-80EBZ Datasheet(PDF) 27 Page - Analog Devices

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