
17
FN6808.3
October 1, 2009
Theory of Operation
Functional Description
The KAD5512HP is based upon a 12-bit, 250MSPS A/D
converter core that utilizes a pipelined successive
approximation architecture (Figure
21). The input voltage is
captured by a Sample-Hold Amplifier (SHA) and converted
to a unit of charge. Proprietary charge-domain techniques
are used to successively compare the input to a series of
reference charges. Decisions made during the successive
approximation operations determine the digital code for each
input value. The converter pipeline requires six samples to
produce a result. Digital error correction is also applied,
resulting in a total latency of eight and one half clock cycles.
This is evident to the user as a time lag between the start of
a conversion and the data being available on the digital
outputs.
The KAD5512HP family offers 2.5dB improvement in SNR
over the KAD5512P by simultaneously sampling the input
signal with two ADC cores in parallel and summing the digital
result. Since the input signal is correlated between the two
cores and noise is not, an increase in SNR is achieved. As a
result of this architecture, indexed SPI operations must be
Power-On Calibration
The ADC performs a self-calibration at start-up. An internal
power-on-reset (POR) circuit detects the supply voltage
ramps and initiates the calibration when the analog and
digital supply voltages are above a threshold. The following
conditions must be adhered to for the power-on calibration to
execute successfully:
A frequency-stable conversion clock must be applied to
the CLKP/CLKN pins
DNC pins (especially 3, 4 and 18) must not be pulled up or
down
SDO (pin 66) must be high
RESETN (pin 25) must begin low
SPI communications must not be attempted
A user-initiated reset can subsequently be invoked in the
event that the previous conditions cannot be met at
power-up.
The SDO pin requires an external 4.7k
Ω pull-up to OVDD. If
the SDO pin is pulled low externally during power-up,
calibration will not be executed properly.
After the power supply has stabilized the internal POR
releases RESETN and an internal pull-up pulls it high, which
starts the calibration sequence. If a subsequent
user-initiated reset is required, the RESETN pin should be
connected to an open-drain driver with a drive strength of
less than 0.5mA.
FIGURE 21. ADC CORE BLOCK DIAGRAM
DIGITAL
ERROR
CORRECTION
SHA
1.25V
INP
INN
CLOCK
GENERATION
2.5-BIT
FLASH
6-STAGE
1.5-BIT/STAGE
3-STAGE
1-BIT/STAGE
3-BIT
FLASH
LVDS/LVCMOS
OUTPUTS
+
–
KAD5512HP