
AD9627
Rev. B | Page 29 of 76
External Reference Operation
The use of an external reference may be necessary to enhance
the gain accuracy of the ADC or improve thermal drift charac-
internal reference in 1.0 V mode.
2.5
–2.5
–40
TEMPERATURE (°C)
R
E
F
E
RE
NC
E
V
O
L
T
AG
E
RRO
R
(
m
V
)
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–20
0
204060
80
06
57
1-
05
4
Figure 54. Typical VREF Drift
When the SENSE pin is tied to AVDD, the internal reference is
disabled, allowing the use of an external reference. An internal
reference buffer loads the external reference with an equivalent
6 kΩ load (se
e Figure 15). The internal buffer generates the
positive and negative full-scale references for the ADC core.
Therefore, the external reference must be limited to a maximum
of 1.0 V.
CLOCK INPUT CONSIDERATIONS
For optimum performance, the AD9627 sample clock inputs,
CLK+ and CLK, should be clocked with a differential signal.
The signal is typically ac-coupled into the CLK+ and CLK pins
via a transformer or capacitors. These pins are biased internally
1.2V
AVDD
2pF
CLK–
CLK+
0
65
71-
0
55
Figure 55. Equivalent Clock Input Circuit
Clock Input Options
The AD9627 has a very flexible clock input structure. Clock input
can be a CMOS, LVDS, LVPECL, or sine wave signal. Regardless of
the type of signal being used, clock source jitter is of the most
the AD9627 (at clock rates up to 625 MHz). A low jitter clock
source is converted from a single-ended signal to a differential
signal using either an RF balun or an RF transformer.
The RF balun configuration is recommended for clock
frequencies between 125 MHz and 625 MHz, and the RF
transformer is recommended for clock frequencies from 10 MHz
to 200 MHz. The back-to-back Schottky diodes across the
transformer/balun secondary limit clock excursions into the
AD9627 to approximately 0.8 V p-p differential.
This helps prevent the large voltage swings of the clock from
feeding through to other portions of the AD9627 while preserving
the fast rise and fall times of the signal that are critical to a low
jitter performance.
0.1F
SCHOTTKY
DIODES:
HSMS2822
CLOCK
INPUT
50
100
CLK–
CLK+
ADC
AD9627
Mini-Circuits
ADT1–1WT, 1:1Z
XFMR
065
71
-05
6
Figure 56. Transformer-Coupled Differential Clock (Up to 200 MHz)
0.1F
1nF
CLOCK
INPUT
1nF
50
CLK–
CLK+
ADC
AD9627
SCHOTTKY
DIODES:
HSMS2822
06
57
1-
0
57
Figure 57. Balun-Coupled Differential Clock (Up to 625 MHz)
If a low jitter clock source is not available, another option is to
ac couple a differential PECL signal to the sample clock input
jitter performance.
100
0.1F
240
240
PECL DRIVER
50k
50k
CLK–
CLK+
ADC
AD9627
CLOCK
INPUT
CLOCK
INPUT
AD951x
06
571-
0
58
Figure 58. Differential PECL Sample Clock (Up to 625 MHz)
A third option is to ac-couple a differential LVDS signal to the
drivers offer excellent jitter performance.
100
0.1F
50k
50k
CLK–
CLK+
ADC
AD9627
CLOCK
INPUT
CLOCK
INPUT
AD951x
LVDS DRIVER
06
571-
0
59
Figure 59. Differential LVDS Sample Clock (Up to 625 MHz)