Application Note 1604
ISLA11xP50 Output Data Timing and Synchronization
Overview
Capturing data from the ISLA11xP50 ADC is easily
accomplished with current FPGA technology. The
source-synchronous LVDS interface provides DDR output
data at up to 500MHz with a 250MHz clock. The clock and
data are aligned within ±250ps providing a wide
guaranteed data-valid region of 1.5ns over the full range
of process, voltage and temperature at 500MSPS
operation.
Internally the input clock is immediately divided by two in
order to clock the two ADC cores at half the output
sample rate. Even though the 500MSPS output data
stream is generated by two interleaved ADC cores, the
output data is always delivered from a single ISLA11xP50
in a known sequence. Multiple ADCs with aligned input
clock edges may not have aligned output clock edges due
to the divide-by-two’s uncertain output phase. The
CLKOUTP signal can be high or low at the rising edge of
the input clock unless specifically forced to a known state.
The ISLA11xP50 includes synchronization features
making it easier to design systems requiring
simultaneous sampling or further interleaved sampling.
Synchronization may be as simple as using a single ADC
output data clock or the CLKDIVRST pins to force
synchronization. More complex approaches may use the
PHASE_SLIP register to adjust timing. The best method
will depend on many factors including the timing margin,
the FPGA family, the FPGA design tools and printed circuit
board (PCB) constraints. At 500MSPS operation the
CLKDIVRSTP setup and hold timing may be challenging
for some designs. These timing requirements may be
effectively relaxed by gating the ADC input clock to
provide additional margin.
4000ps
This document is intended to provide basic guidance on
the ISLA11xP50’s output timing and synchronization
methods.
Output Timing
The ISLA11xP50 input clock and data propagate through
the ISLA11xP50 with a similar delay path in order to relax
data capture timing requirements. The ADC output DATA
will transition from one sample to the next within ±250ps
of the CLKOUTP signal; leaving a wide data-valid window
of 1.5ns at 500MSPS. CLKOUTP will be delayed from
CLKP by 2.6ns to 3.3ns at 1.8V and +25°C as shown in
Figure 1 or by 2.0ns to 3.6ns over the entire
recommended operating range of 1.7V to 1.9V from
-40°C to +85°C.
Internal Operation
The interleaved operation of the ISLA11xP50 requires the
500MHz input clock to be divided by two so that each
core samples at 250MSPS. Figure 2 shows a conceptual
view of the ADCs internal clock circuitry. The clock divider
normally comes out of power-on reset in a random state
so the output clock phase (CLK_A, CLK_B in Figure 2) is
indeterminate. In normal operation with a single ADC the
unknown clock phase does not matter and the output
sample order is always correct. This may not be the case
when synchronizing multiple ADCs. The uncertainty in
CLKOUTP phase means the CLKOUTP rising edges may
not be aligned across multiple ADCs driven by the same
clock source. This possible phase difference as shown in
Figure 3 can lead to an unexpected difference in sample
time and sequence in the captured data.
5000ps
6000ps
7000ps
8000ps
2ns, 500MHz
CLKP
Min tCPD = 2.6ns
MIN
CLKOUTP
tDC =±250ps
MIN DATA
Max tCPD = 3.3ns
MAX
CLKOUTP
tDC = ±250ps
MAX DATA
FIGURE 1. 1.8V AT +25°C ADC OUTPUT TIMING
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Application Note 1604
S1
S3
REG
CLK_A
CLK
CLK/2
DATA
1 2 3 4
CLK_B
‘1’
CLKOUT
REG
S2
‘0’
S4
FIGURE 2. ADC INTERNAL DATA CLOCKING
SAMPLE CLOCK
INPUT
s1
L+td
ANALOG INPUT
(Note 1)
s2
tRSTH
CLKDIVRSTP
(Note 2)
tRSTS
tRSTRT
ADC1 OUTPUT DATA
s0
s1
s2
s3
s0
s1
s2
s3
ADC1 CLKOUTP
ADC2 OUTPUT DATA
ADC2 CLKOUTP
(PHASE 1)
(Note 3)
(Note 3)
ADC2 CLKOUTP
(PHASE 2)
NOTES:
1. Delay equals fixed pipeline latency (L cycles) plus fixed analog propagation delay td
2. CLKDIVRSTP setup and hold times are with respect to input sample clock rising edge.
CLKDIVRSTN is not shown, but must be driven, and is the compliment of CLKDIVRSTP.
3. Either Output Clock Phase (phase 1 or phase 2 ) equally likely prior to synchronization.
FIGURE 3. MULTIPLE ADC CLKOUT PHASE UNCERTAINTY
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Application Note 1604
CLKP1
CLKDIVRSTP1
ADC1
CLKOUTP1
CK1
CK2
CK3
FPGA
CLKP2
CLKDIVRSTP2
CLOCK
SOURCE
DATA1
CLKOUTP2
DATA2
ADC2
FIGURE 4. MULTIPLE ADC SYNCHRONIZATION CIRCUITRY
Synchronization
Multiple ADC Timing Margin
The ISLA11xP50 provides two mechanisms to control the
output clock phase:
Device-to-device timing becomes important for two
reasons when multiple devices are to be synchronized.
1. The CLKDIVRSTP pins offer the simplest method to
synchronize multiple ADCs. When CLKDIVRSTP is
set high within the datasheet setup and hold times
the CLKOUTP signal will always be forced to a known
phase. Routing CLKP and CLKDIVRSTP to multiple
ADCs with equal PCB delays allows all ADCs to be
simultaneously set to the same sample phase.
Assertion of CLKDIVRSTP may cause the internal
DLL to lose lock for up to 52µs. Valid data may be
captured after this 52µs period. This process must
be completed after each power cycle or ADC reset.
2. The PHASE_SLIP register (0x71) can be written to
effectively invert the CLKOUTP signal. The User Test
Mode allows a pair of known values to be output but
using these values to identify the clock phase
relationship requires more FPGA code than using
CLKDIVRSTP. After synchronization with
CLKDIVRSTP the PHASE_SLIP register can be used
to delay the output data to further interleave
multiple ADCs.
If relaxed setup and hold times are required for
CLKDIVRSTP the input clock can be gated off,
CLKDIVRSTP set high then the clock re-enabled.
Glitchless clock gating circuitry must be used to assure
reliable operation.
1. In some cases it may be possible to synchronize
multiple ADCs by only using a single clock. This
eliminates the clock phase uncertainty since all the
data is captured by a single clock. The downside is
reduced timing margin since each ADC will have
slightly different timing. The dtCPD specification can
be used to quantify this uncertainty.
2. The FPGA must be able to manage the difference in
clock delay through the ADCs in order to correctly
capture the data. Managing timing where the clock
may slip one or more cycles across the ADCs is
simplified by current FPGA technology’s ability to
automatically align input data streams with a known
pattern. Design examples are available from Xilinx
(XAPP1064) and Altera (AN236). Search their web
pages for ‘source-synchronous ddr’ to find more
examples.
Each ADCs output data will transition within ±250ps of
the CLKOUTP signal. CLKOUTP will be delayed from CLKP
by 2.6ns to 3.3ns at 1.8V and +25°C as shown in
Figure 5. When more than one ADCs output data is to be
captured using a single CLKOUTP signal the valid data
window is a minimum of 800ps (1.8V, +25°C).
Figure 4 illustrates the connections necessary to
synchronize two ADCs with CLKDIVRSTP. Note that any
mismatch in the CLKP and CLKDIVRSTP traces must be
accounted for in the timing budget. Additional ADCs may
be synchronized by extending the circuitry in Figure 4 for
as many ADCs as necessary.
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Application Note 1604
4000ps
5000ps
6000ps
7000ps
8000ps
2ns, 500MHz
CLKP
Min tCPD = 2.6ns
(1.8V +25°C)
CLKOUTP1
tDC = ±250ps
DATA1
SAMPLE 1
Min tCPD = 3.3ns
(1.8V, +25°C)
CLKOUTP2
0.8/1.8ns
DATA2
tDC = ±250ps
SAMPLE 2
FIGURE 5. 1.8V AT +25°C MULTIPLE ADC TIMING SKEW
4000ps
5000ps
6000ps
7000ps
8000ps
2ns, 500MHz
CLKP
Min tCPD = 2ns
(1.7V TO 1.9V, -40°C to +85°C)
CLKOUTP1
tDC = ±250ps
SAMPLE 1
DATA1
Min tCPD = 3.6ns
(1.7V TO 1.9V, -40°C to +85°C)
CLKOUTP2
-100ps/+900ps
tDC =±250ps
SAMPLE 2
DATA2
FIGURE 6. 1.7V TO 1.9V AT -40°C TO +85°C MULTIPLE ADC TIMING SKEW
If the ADCs are at the opposite extremes of voltage and
temperature (i.e. ADC1 sees 1.9V @ -40°C and ADC2
sees 1.7V at +85°C) the timing will be at the worst
possible specified skew as shown in Figure 6. In this
case, operation at 500MHz will most likely result in
negative timing margin. This means a single ADCs output
clock cannot be used to capture data from multiple ADCs
without adjusting the FPGA capture for proper data
sequence. Even when using each ADCs output clock the
FPGA may have to tolerate a possible one cycle clock slip
between devices.
Relative Output Clock Delay Matching
(dtCPD)
A more realistic design will operate all the synchronized
ADCs at the same voltage and temperature. This is the
condition covered by the dtCPD specification. This
specification says CLKOUTP from all devices at the same
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voltage and temperature will fall with ±450ps of the
nominal timing (CLKOUT_NOM) of CLKOUTP from any
other ADC as shown in Figure 7. This means that both
devices’ CLKOUTP will be within a 900ps window
bounded by the tCPD specification over the full voltage
and temperature range. Either device can lead or lag the
nominal by up to 450ps so the FPGA must be able to
manage up to a ±900ps delta between ADCs even when
they are synchronized with CLKDIVRSTP. The
ISLA11xP50’s fixed pattern capability works with the
FPGA’s DDR capture circuitry to automatically enable
robust data capture over a range of more than one clock
cycle as described in application notes from Xilinx
(XAPP1064) and Altera (AN236). The minimum valid
data window for this condition is 600ps. The variation in
the ±250ps data transition relative to the output clock is
dominated by matching and is not correlated to the
overall input to output clock delay.
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Application Note 1604
4000ps
5000ps
6000ps
7000ps
2ns, 500MHz
CLKP
tCPD = 2.6ns
(nominal)
CLKOUT_NOM
dtCPD = -0.45 TO 0.45
CLKOUTP1
-0.7, 0.7
DATA1
{-0.9, 0.9}
dtCPD = -0.45 TO 0.45
CLKOUTP2
-1.4, 1.4
0.6, 3.4
DATA2
FIGURE 7. PART-TO-PART TIMING FOR ADCS, EQUAL VOLTAGE AND TEMPERATURE
FPGA
DDR RECEIVER
ADC1_OUTPUT_DATA
B
B
B1
ADC1_FPGA_DATA
ADC 1
RECONSTRUCTION
/2
A
A
TO FPGA
FABRIC
A1
ADC1_OUTPUT_CLK
500MHz
ADC 2
ADC1_FPGA_CLK
/2
0° CLOCK
ADC 3
/2
DDR CLOCK RECEIVER
(DLL OR PLL)
180° CLOCK
ADC 4
/2
THE RECEIVER
ABOVE DUPLICATED
FOR EACH ADC
FIGURE 8. FPGA DATA CAPTURE
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Application Note 1604
FPGA Data Capture
One possible FPGA data capture implementation is
shown in Figure 8. The DDR clock receiver is used to
generate the 180° DDR clock. The incoming data and
clock are aligned within ±250ps from the ISLA11xP50 so
the FPGA must also shift the clock relative to the data for
optimum data capture. The data from any one ADC may
be offset up to 900ps from any other ADC when they
operate at the same voltage and temperature. Care must
be taken to provide sufficient timing margin since the
900ps offset can be in either direction possibly affecting
the DDR receiver’s setup and hold time. Current FPGA
data capture circuitry includes features that can simplify
recovery of source synchronous DDR data streams.
Note
CLKP, CLKOUTP and CLKDIVRSTP all refer to the positive
LVDS signals which also include the CLKN, CLKOUTN and
CLKDIVRSTN complement signals. All LVDS signals must
be routed as differential pairs. All timing numbers are for
LVDS signals and are copied from the datasheet for
convenience. The datasheet timing specifications take
precedence in all cases since this document does not
guarantee performance limits.
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reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding.
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