12
FN6818.0
December 5, 2008
Functional Description
The KAD2710 is a ten bit, 275MSPS A/D converter in a
pipelined architecture. The input voltage is captured by a
sample and hold circuit and converted to a unit of charge.
Proprietary charge-domain techniques are used to compare
the input to a series of reference charges. These
comparisons determine the digital code for each input value.
The converter pipeline requires 24 sample clocks to produce
a result. Digital error correction is also applied, resulting in a
total latency of 28 clock cycles. This is evident to the user as
a latency between the start of a conversion and the data
being available on the digital outputs.
At power-up, a self-calibration is performed to minimize gain
and offset errors. The reset pin (RST) is held low internally at
power-up and will remain in that state until the calibration is
complete. The clock frequency should remain fixed during
this time.
Calibration accuracy is maintained for the sample rate at
which it is performed, and therefore should be repeated if the
clock frequency is changed by more than 10%. Recalibration
can be initiated via the RST pin, or power cycling, at any
time.
Reset
Recalibration of the ADC can be initiated at any time by
driving the RST pin low for a minimum of one clock cycle. An
open-drain driver is recommended.
The calibration sequence is initiated on the rising edge of
RST, as shown in Figure 21. The over-range output (ORP) is
set high once RST is pulled low, and remains in that state
until calibration is complete. The ORP output returns to
normal operation at that time, so it is important that the
analog input be within the converter’s full-scale range in
order to observe the transition. If the input is in an over-
range state the ORP pin will stay high and it will not be
possible to detect the end of the calibration cycle.
While RST is low, the output clock (CLKOUTP/CLKOUTN)
stops toggling and is set low. Normal operation of the output
clock resumes at the next input clock edge (CLKP/CLKN)
after RST is deasserted. At 275MSPS the nominal
calibration time is ~240ms.
Voltage Reference
The VREF pin is the reference voltage which sets the
full-scale input voltage for the chip. This pin requires a
bypass capacitor of 0.1µF at a minimum. The internally
generated bandgap reference voltage is provided by an on-
chip voltage buffer. This buffer can sink or source up to 50µA
externally.
An external voltage may be applied to this pin to provide a
more accurate reference than the internally generated
bandgap voltage, or to match the full-scale reference for
multiple KAD2710L chips.One option in the latter
configuration is to use one KAD2710L's internally generated
reference as the external reference voltage for the other
chips in the system. Additionally, an externally provided
reference can be changed from the nominal value to adjust
the full-scale input voltage within a limited range.
To select whether the full-scale reference is internally
generated or externally provided, the digital input VREFSEL
is set low for internal, or high for external.This pin has
internal pull-up.use the internally generated reference
VREFSEL can be tied directly to AVSS, and to use an
external reference VREFSEL can be left unconnected.
Analog Input
The ADC core contains a fully differential input (INP/INN) to
the sample and hold circuit. The ideal full-scale input voltage
is 1.50V, centered at the VCM voltage of 0.86V as shown in
Figure 22.
Best performance is obtained when the analog inputs are
driven differentially. The common-mode output voltage,
VCM, should be used to properly bias the inputs as shown in
Figures 23 and 24. An RF transformer will give the best
noise and distortion performance for wideband and/or high
intermediate frequency (IF) inputs. Two different transformer
input schemes are shown in Figures 23 and 24.
FIGURE 21. CALIBRATION TIMING
CLKP
CLKN
CLKOUTP
RST
ORP
Calibration Begins
Calibration Complete
Calibration Time
FIGURE 22. ANALOG INPUT RANGE
1.0
1.8
0.6
0.2
1.4
INP
INN
VCM
0.86V
0.75V
-0.75V
V
t
KAD2710L
