ISLA224S
36
FN7911.2
April 25, 2013
ADC Evaluation Platform
Intersil offers ADC Evaluation platforms which can be used to
evaluate any of Intersil’s high speed ADC products. Each platform
consists of a FPGA based data capture motherboard and a family
of ADC daughtercards. The USB interface and evaluation
platform control software allow a user to quickly evaluate the
ADC’s performance at a user’s specific application frequency
requirements. More information is available at
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, as longer traces
between the ceramic bypass capacitors and the device pins will
increase inductance, which can result in diminished dynamic
performance. Best practices bypassing is especially important on
the AVDD and OVDD(PLL) power supply pins. Whenever possible,
each supply pin should have its own 0.1uF bypass capacitor.
Make sure that connections to ground are direct and low
impedance. Avoid forming ground loops.
CML Outputs
Output traces and connections must be designed for 50
Ω (100Ω
differential) characteristic impedance. Keep traces direct and
short, and minimize bends and vias where possible. Avoid
crossing ground and power-plane breaks with signal traces. Keep
good clearance (at least 5 trace widths) between the SERDES
traces and other signals. Given the speed of these outputs and
importance of maintaining an open eye to achieve low BER,
signal integrity simulations are recommended, especially when
the data lane rate exceeds 3Gbps and/or the trace or cable
length between the ADC and the reciever gets larger than 20cm.
Unused Inputs
Standard logic inputs (RESETN, CSB, SCLK, SDIO, SDO) which will
not be operated do not require connection to ensure optimal ADC
performance. These inputs can be left floating if they are not
used. Tri-level inputs (NAPSLP) 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 than 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
ADC’s transfer function from a best fit line determined by a least
squares curve fit of that transfer function, measured in units of
LSBs.
Least Significant Bit (LSB) is the bit that has the smallest value or
weight in a digital word. Its value in terms of input voltage is
VFS/(2N - 1) where N is the resolution in bits.
Missing Codes are output codes that are skipped and will never
appear at the ADC output. These codes cannot be reached with
any input value.
Most Significant Bit (MSB) is the bit that has the largest value or
weight.
Pipeline Delay is the number of clock cycles between the
initiation of a conversion and the appearance at the output pins
of the data.
Power Supply Rejection Ratio (PSRR) is the ratio of the observed
magnitude of a spur in the ADC FFT, caused by an AC signal
superimposed on the power supply voltage.
Signal to Noise-and-Distortion (SINAD) is the ratio of the RMS
signal amplitude to the RMS sum of all other spectral
components below one half the clock frequency, including
harmonics but excluding DC.