AD9268 datasheet(28/45 Pages) AD

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AD9268

Rev. A | Page 27 of 44

Common-Mode Voltage Servo

In applications where there may be a voltage loss between the VCM

output of the AD9268 and the analog inputs, the common-mode

voltage servo can be enabled. When the inputs are ac-coupled and

a resistance of >100 Ω is placed between the VCM output and the

analog inputs, a significant voltage drop can occur and the

common-mode voltage servo should be enabled. Setting Bit 0 in

this mode, the AD9268 monitors the common-mode input level

at the analog inputs and adjusts the VCM output level to keep the

common-mode input voltage at an optimal level. If both channels

are operational, Channel A is monitored. However, if Channel A

is in power-down or standby mode, then the Channel B input is

monitored.

Dither

The AD9268 has an optional dither mode that can be selected

for one or both channels. 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 AD9268, 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

AD9268

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 AD9268 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 AD9268

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 ADA4938-2 is easily

set with the VCM pin of the AD9268 (see Figure 66), and the

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

provide band limiting of the input signal.

VIN

76.8

Ω

120

Ω

0.1µF

200

Ω

200

Ω

AVDD

Ω

5pF

15pF

AD9268

VIN–

VIN+

VCM

ADA4938-2

Figure 66. Differential Input Configuration Using the ADA4938-2

For baseband applications in which 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

49.9

Ω

0.1µF

AD9268

VIN+

VIN–

VCM

Figure 67. Differential Transformer-Coupled Configuration

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