AD9637 Datasheet, PDF (20/40 Pages) Analog Devices – Octal, 12-Bit, 40/80 MSPS, Serial LVDS

AD9637
If the internal reference of the AD9637 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 48 shows
how the internal reference voltage is affected by loading.
0
–0.5
–1.0
–1.5
INTERNAL V_REF = 1V
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
0
0.5
1.0
1.5
2.0
2.5
3.0
LOAD CURRENT (mA)
Figure 48. VREF Error vs. Load Current
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-
teristics. Figure 49 shows the typical drift characteristics of the
internal reference in 1.0 V mode.
4
2
0
–2
–4
–6
–8
–40
–15
10
35
60
85
TEMPERATURE (°C)
Figure 49. 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
7.5 kΩ load (see Figure 42). 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. It is not recommended to leave the SENSE pin floating.
Data Sheet
CLOCK INPUT CONSIDERATIONS
For optimum performance, clock the AD9637 sample clock inputs,
CLK+ and CLK−, 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 (see Figure 36) and
require no external bias.
Clock Input Options
The AD9637 has a very flexible clock input structure. The 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 utmost concern, as described in the Jitter Considerations
section.
Figure 50 and Figure 51 show two preferred methods for clocking
the AD9637 (at clock rates of up to 640 MHz prior to the internal
CLK divider). A low jitter clock source is converted from a single-
ended signal to a differential signal using either an RF transformer
or an RF balun.
The RF balun configuration is recommended for clock frequencies
between 80 MHz and 640 MHz, and the RF transformer is recom-
mended for clock frequencies from 10 MHz to 200 MHz. The
back-to-back Schottky diodes across the transformer/balun
secondary winding limit clock excursions into the AD9637 to
approximately 0.8 V p-p differential.
This limit helps prevent the large voltage swings of the clock
from feeding through to other portions of the AD9637 while
preserving the fast rise and fall times of the signal that are critical
to a low jitter performance. However, the diode capacitance comes
into play at frequencies above 500 MHz. Care must be taken in
choosing the appropriate signal limiting diode.
CLOCK
INPUT
0.1µF
Mini-Circuits^®
ADT1-1WT, 1:1 Z
0.1µF
XFMR
50Ω ^100Ω
0.1µF
0.1µF
SCHOTTKY
DIODES:
HSMS2822
CLK+
ADC
CLK–
Figure 50. Transformer Coupled Differential Clock (Up to 200 MHz)
CLOCK
INPUT
0.1µF
50Ω
0.1µF
0.1µF
0.1µF
SCHOTTKY
DIODES:
HSMS2822
CLK+
ADC
CLK–
Figure 51. Balun Coupled Differential Clock (80 MHz to 640 MHz)
Rev. 0 | Page 20 of 40