ISLA118P50_11 Datasheet, PDF (31/34 Pages) Intersil Corporation – 8-Bit, 500MSPS A/D Converter

Equivalent Circuits (Continued)
ISLA118P50
AVDD
+
0.535V
–
VCM
FIGURE 53. VCM_OUT OUTPUT
A/D Evaluation Platform
Intersil offers an A/D Evaluation platform which can be used to
evaluate any of Intersil’s high speed A/D products. The platform
consists of a FPGA based data capture motherboard and a family
of A/D daughter cards. This USB based platform allows a user to
quickly evaluate the A/D’s performance at a user’s specific
application frequency requirements. More information is
available at
http://www.intersil.com/converters/adc_eval_platform/
Layout Considerations
Split Ground and Power Planes
Data converters operating at high sampling frequencies require
extra care in PC board layout. Many complex board designs
benefit from isolating the analog and digital sections. Analog
supply and ground planes should be laid out under signal and
clock inputs. Locate the digital planes under outputs and logic
pins. Grounds should be joined under the chip.
Clock Input Considerations
Use matched transmission lines to the transformer inputs for the
analog input and clock signals. Locate transformers and
terminations as close to the chip as possible.
Exposed Paddle
The exposed paddle must be electrically connected to analog
ground (AVSS) and should be connected to a large copper plane
using numerous vias for optimal thermal performance.
Bypass and Filtering
Bulk capacitors should have low equivalent series resistance.
Tantalum is a good choice. For best performance, keep ceramic
bypass capacitors very close to device pins. Longer traces will
increase inductance, resulting in diminished dynamic
performance and accuracy. Make sure that connections to
ground are direct and low impedance. Avoid forming ground
loops.
LVDS Outputs
Output traces and connections must be designed for 50Ω (100Ω
differential) characteristic impedance. Keep traces direct and
minimize bends where possible. Avoid crossing ground and
power-plane breaks with signal traces.
LVCMOS Outputs
Output traces and connections must be designed for 50Ω
characteristic impedance.
Unused Inputs
Standard logic inputs (RESETN, CSB, SCLK, SDIO, SDO) which will
not be operated do not require connection to ensure optimal A/D
performance. These inputs can be left floating if they are not
used. Tri-level inputs (NAPSLP, OUTMODE, OUTFMT) accept a
floating input as a valid state, and therefore should be biased
according to the desired functionality.
Definitions
Analog Input Bandwidth is the analog input frequency at which
the spectral output power at the fundamental frequency (as
determined by FFT analysis) is reduced by 3dB from its full-scale
low-frequency value. This is also referred to as Full Power
Bandwidth.
Aperture Delay or Sampling Delay is the time required after the
rise of the clock input for the sampling switch to open, at which
time the signal is held for conversion.
Aperture Jitter is the RMS variation in aperture delay for a set of
samples.
Clock Duty Cycle is the ratio of the time the clock wave is at logic
high to the total time of one clock period.
Differential Non-Linearity (DNL) is the deviation of any code width
from an ideal 1 LSB step.
Effective Number of Bits (ENOB) is an alternate method of
specifying Signal to Noise-and-Distortion Ratio (SINAD). In dB, it
is calculated as: ENOB = (SINAD - 1.76)/6.02
Gain Error is the ratio of the difference between the voltages that
cause the lowest and highest code transitions to the full-scale
voltage less 2 LSB. It is typically expressed in percent.
I2E The Intersil Interleave Engine. This highly configurable
circuitry performs estimates of offset, gain, and sample time
skew mismatches between the core converters, and updates
analog adjustments for each to minimize interleave spurs.
Integral Non-Linearity (INL) is the maximum deviation of the
A/D’s transfer function from a best fit line determined by a least
squares curve fit of that transfer function, measured in units of
LSBs.
31
FN7565.2
July 25, 2011