Data Sheet
The quality of the input clock is extremely important for
high-speed, high-resolution ADCs. The contribution to SNR
from clock jitter with a full scale signal at a given frequency
is shown in equation 1.
SNR
jitter
= 20
•
log (2
•
π
•
F
IN
•
ε
t
)
where F
IN
is the signal frequency, and
ε
t
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 per-
formance, it is of utmost importance to limit the clock jit-
ter. This can be obtained by using precise and stable clock
references (e.g. crystal oscillators with good jitter specifi-
cations) and make sure the clock distribution is well con-
trolled. 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 crosstalk 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 per-
formance 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
retimed with a low jitter master clock as the last operation
before it is applied to the ADC clock input.
The digital outputs can be set in tristate mode by setting
the OE_N signal high.
Note that the out of range flags (ORNG) will behave differ-
ently 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 flags will be set when
all twelve bits are zeros or ones and when the thirteenth
bit is equal to the rest of the bits.
The CDK2307 employs digital offset correction. This means
that the output code will be 4096 with the positive and
negative inputs shorted together(zero differential). How-
ever, small mismatches in parasitics 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 opposite end. With the output being centered digitally,
the output will clip, and the out of range flags will be set,
before max code is reached. When out of range flags are
set, the code is forced to all ones for over-range and all
zeros for under-range.
CDK2307
Dual, 20/40/65/80MSPS, 12/13-bit Analog-to-Digital Converters
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 complement output
format. Details are shown in Table 1 on page 14.
The data outputs can be used in three different configurations.
Normal mode:
All 13-bits are used. MSB is Dx_12 and LSB is Dx_0. This
mode gives optimum performance due to reduced quanti-
zation noise.
12-bit mode:
The LSB is left unconnected such that only 12 bits are used.
MSB is Dx_12 and LSB is Dx_1. This mode gives slightly
reduced performance, due to increased quantization noise.
Reduced full scale range mode:
The full scale range is reduced from 2V
pp
to 1V
pp
which is
equivalent to 6dB gain in the ADC frontend. MSB is Dx_11
and LSB is Dx_0. Note that the codes will wrap around
when exceeding the full scale range, and that out of range
bits should be used to clamp output data. See section
Reference Voltages for details. This mode gives slightly
reduced performance.
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Digital Outputs
Digital output data are presented in a parallel CMOS form.
The voltage on the OVDD pin sets 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 rec-
ommended to minimize the load to ensure as low tran-
sient 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 CLK_EXT always
should be lower than the load on data outputs to ensure
sufficient timing margins.
©2009 CADEKA Microcircuits LLC
Rev 2A
13
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
