AD9734/AD9735/AD9736
Rev. A | Page 54 of 72
INPUT DATA TIMING
The AD973x is intended to operate with the LVDS and sync
controllers running to compensate for timing drift due to
voltage and temperature variations. In this mode, the key to
correct data capture is to present valid data for a minimum
amount of time. The AD973x minimum valid data time is
measured by increasing the input data rate to the point of
failure. The nominal supply voltages are used and the
temperature is set to the worst case of 85°C. The input
data is verified via the BIST signature registers, because the
DAC output does not run as fast as the input data logic. The
following example explains how the minimum data valid
period is calculated for the typical performance case.
These factors must be considered in determining the minimum
valid data window at the receiver input:
Data rise and fall times: 100 ps (rise + fall)
Internal clock jitter: 10 ps
(DATACLK_OUT + DATACLK_IN)
Bit-to-bit skew: 50 ps
Bit-to-DATACLK_IN skew: 50 ps
Internal data sampling signal resolution: 80 ps
For nominal silicon, the BIST typically indicates failure at
2.15 GSPS or a DACCLK period of 465 ps. The valid data
window is calculated by subtracting all the other variables
from the total data period:
Minimum Data Valid Time = DACCLK Period Data Rise
Data Fall Jitter Bit-to-Bit Skew Bit-to-DATACLK_IN Skew
Data Sampling Signal Resolution
For the 400 mV p-p LVDS signal case:
Minimum Data Valid = 465 ps 100 ps 10 ps 50 ps
80 ps = 465 ps 240 ps = 225 ps
For correct data capture, the input data must be valid for 225 ps.
Slower edges, more jitter, or more skew require an increase in
the clock period to maintain the minimum data valid period.
Table 27 shows the typical minimum data valid period (tMDE) for 400 mV p-p differential and 250 mV p-p differential LVDS swings.
The ability of the AD973x to capture incoming data is
dependent on the speed of the silicon, which varies from lot to
lot. The typical (or average) silicon speed operates with data
that is valid for 225 ps at 85°C. Statistically, the worst extreme
for slow silicon may require up to a 344 ps valid data period, as
Table 27. Typical Minimum Data Valid Times
Differential Input
Voltage
BIST
Max fCLK
Min Clock
Period
Typ Min Data
Valid at Receiver
400 mV
2.15 GHz
465 ps
225 ps
250 mV
2.00 GHz
500 ps
260 ps
At 1.2 GHz, the typical 400 mV p-p minimum data valid period
of 225 ps leaves 608 ps for external factors. Under the same
conditions, the worst expected minimum data valid period of
344 ps leaves 489 ps for external data uncertainty.
The 100 mV LVDS VOD threshold test is a dc test to verify that
the input logic state changes. It does not indicate the operating
speed. The ability of the receiver to recover the data depends on
the input signal overdrive. With a 250 mV input, there is a
150 mV overdrive, and with a 400 mV signal, there is a 300 mV
overdrive. The relationship between overdrive level and timing
is very nonlinear. Higher levels of overdrive result in smaller
minimum valid data windows.
For typical silicon, decreasing the LVDS swing from 400 mV p-p
to 250 mV p-p requires the minimum data valid period to
225ps
400mV
260ps
250mV
04862-098
Figure 100. Typical Minimum Valid Data Time (tMDE) vs. LVDS Swing
The minimum valid data window changes with temperature,
voltage, and process. The maximum value presented in the
specification table was determined from a 6σ distribution in the
worst-case conditions.