Datasheet

AD9255 Data Sheet
Rev. C | Page 26 of 44
Dither
The AD9255 has an optional dither mode that can be selected
either using the DITHER pin or using the SPI bus. 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 AD9255, 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
08505-038
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 AD9255 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 at 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 AD9255 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 AD9255 (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
200Ω
90Ω
AVDD
33Ω
33Ω
15Ω
15Ω
5pF
15pF
15pF
ADC
VIN–
VIN+
VCM
ADA4938-2
08505-039
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
R1
R1
C1
ADC
VIN+
VIN–
VCM
C2
R2
R2
C2
08505-040
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.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9255. For applications in