For price, delivery and to place orders: Hittite Microwave Corporation, 2 Elizabeth Drive, Chelmsford, MA 01824
978-250-3343 tel • 978-250-3373 fax • Order On-line at www.hittite.com
Application Support: apps@hittite.com
A / D CONVERTERS - SMT
0
0 - 10
HMCAD1051-80
v01.0411
SINGLE 13/12-BIT 65/80 MSPS
A/D CONVERTER
Figure 8: Alternative input network
Clock Input and Jitter considerations
Typically high-speed ADCs use both clock edges to
generate internal timing signals. In the HMCAD1051-
80 only the rising edge of the clock is used. Hence,
input clock duty cycles between 20% and 80% are
acceptable.
The input clock can be supplied in a variety of formats.
The clock pins are AC-coupled internally. Hence a
wide common mode voltage range is accepted. Differ-
ential clock sources as LVDS, LVPECL or differential
sine wave can be connected directly to the input pins.
For CMOS inputs, the CKN pin should be connected
to ground, and the CMOS clock signal should be con-
nected to CKP. For differential sine wave clock, the
input amplitude must be at least ± 800 mVpp.
The quality of the input clock is extremely important
for high-speed, high-resolution ADCs. The contribu-
tion to SNR from clock jitter with a full scale signal at a
given frequency is shown in equation 1,
SNRjitter = 20 · log (2 · π · ƒIN · єt) (1)
where fIN is the signal frequency, and εt is the total
rms jitter measured in seconds. The rms jitter is the
total of all jitter sources including the clock generation
circuitry, clock distribution and internal ADC circuitry.
For applications where jitter may limit the obtainable
performance, it is of utmost importance to limit the
clock jitter. This can be obtained by using precise and
stable clock references (e.g. crystal oscillators with
good jitter specications) and make sure the clock dis-
tribution is well controlled. It might be advantageous
to use analog power and ground planes to ensure
low noise on the supplies to all circuitry in the clock
distribution. It is of utmost importance to avoid cross-
talk between the ADC output bits and the clock and
between the analog input signal and the clock since
such crosstalk often results in harmonic distortion.
The jitter performance is improved with reduced rise
and fall times of the input clock. Hence, optimum jitter
performance is obtained with LVDS or LVPECL clock
with fast edges. CMOS and sine wave clock inputs will
result in slightly degraded jitter performance.
If the clock is generated by other circuitry, it should
be re-timed with a low jitter master clock as the last
operation before it is applied to the ADC clock input.
Digital Outputs
Digital output data are presented on parallel CMOS
form. The voltage on the OVDD pin set the levels of the
CMOS outputs. The output drivers are dimensioned to
drive a wide range of loads for OVDD above 2.25V,
but it is recommended to minimize the load to ensure
as low transient switching currents and resulting noise
as possible. In applications with a large fanout or large
capacitive loads, it is recommended to add external
buffers located close to the ADC chip.
The timing is described in the Timing Diagram section.
Note that the load or equivalent delay on CK_EXT
always should be lower than the load on data outputs
to ensure sufficient timing margins.
The digital outputs can be set in tristate mode by set-
ting the OE_N signal high.
The HMCAD1051-80 employs digital offset correc-
tion. This means that the output code will be 4096 with
shorted inputs. However, small mismatches in para-
sitics at the input can cause this to alter slightly. The
offset correction also results in possible loss of codes
at the edges of the full scale range. With no offset
correction, the ADC would clip in one end before the
other, in practice resulting in code loss at the oppo-
site end. With the output being centered digitally, the
output will clip, and the out of range ags will be set,
before max code is reached. When out of range ags
are set, the code is forced to all ones for over range
and all zeros for under range.
Note that the out of range ags (ORNG) will behave
differently for 12 bit and 13 bit output. For 13 bit output
ORNG will be set when digital output data are all ones
or all zeros. For 12-bit output the ORNG ags will be
set when all twelve bits are zeros or ones and when
the thirteenth bit is equal to the rest of the bits.
Data Format Selection
The output data are presented on offset binary form
when DFRMT is low (connect to OVSS). Setting
DFRMT high (connect to OVDD) results in 2’s comple-
ment output format. Details are shown in table 3.