Datasheet AD9224 (Analog Devices) - 8

AD9224
converter. Specifically, the input to the A/D core is the difference of the voltages applied at the VINA and VINB input pins.
Therefore, the equation, INTRODUCTION The AD9224 is a high performance, complete single-supply 12bit ADC. The analog input range of the AD9224 is highly flexible allowing for both single-ended or differential inputs of
varying amplitudes that can be ac or dc coupled. VCORE = VINA – VINB (1) defines the output of the differential input stage and provides
the input to the A/D core. It utilizes a four-stage pipeline architecture with a wideband
input sample-and-hold amplifier (SHA) implemented on a costeffective CMOS process. Each stage of the pipeline, excluding
the last stage, consists of a low resolution flash A/D connected
to a switched capacitor DAC and interstage residue amplifier
(MDAC). The residue amplifier amplifies the difference between the reconstructed DAC output and the flash input for the
next stage in the pipeline. One bit of redundancy is used in each
of the stages to facilitate digital correction of flash errors. The
last stage simply consists of a flash A/D. The voltage, VCORE, must satisfy the condition,
–VREF ≤ VCORE ≤ VREF (2) where VREF is the voltage at the VREF pin.
While an infinite combination of VINA and VINB inputs exist
that satisfy Equation 2, an additional limitation is placed on the
inputs by the power supply voltages of the AD9224. The power
supplies bound the valid operating range for VINA and VINB.
The condition, The pipeline architecture allows a greater throughput rate at the
expense of pipeline delay or latency. This means that while the
converter is capable of capturing a new input sample every clock
cycle, it actually takes three clock cycles for the conversion to be
fully processed and appear at the output. This latency is not a
concern in most applications. The digital output, together with
the out-of-range indicator (OTR), is latched into an output
buffer to drive the output pins. The output drivers of the
AD9224 can be configured to interface with +5 V or +3.3 V
logic families. AVSS – 0.3 V < VINA < AVDD + 0.3 V
AVSS – 0.3 V < VINB < AVDD + 0.3 V (3) where AVSS is nominally 0 V and AVDD is nominally +5 V,
defines this requirement. The range of valid inputs for VINA
and VINB is any combination that satisfies both Equations 2
and 3.
For additional information showing the relationship between
VINA, VINB, VREF and the digital output of the AD9224, see
Table IV. The AD9224 uses both edges of the clock in its internal timing
circuitry (see Figure 1 and specification page for exact timing
requirements). The A/D samples the analog input on the rising
edge of the clock input. During the clock low time (between the
falling edge and rising edge of the clock), the input SHA is in
the sample mode; during the clock high time it is in hold. System disturbances just prior to the rising edge of the clock and/or
excessive clock jitter may cause the input SHA to acquire the
wrong value, and should be minimized. Refer to Table I and Table II at the end of this section for a
summary of both the various analog input and reference
configurations.
ANALOG INPUT OPERATION Figure 14 shows the equivalent analog input of the AD9224
which consists of a differential sample-and-hold amplifier
(SHA). The differential input structure of the SHA is highly
flexible, allowing the devices to be easily configured for either a
differential or single-ended input. The dc offset, or commonmode voltage, of the input(s) can be set to accommodate either
single-supply or dual-supply systems. Note also, that the analog
inputs, VINA and VINB, are interchangeable, with the exception that reversing the inputs to the VINA and VINB pins results in a polarity inversion. ANALOG INPUT AND REFERENCE OVERVIEW Figure 13 is a simplified model of the AD9224. It highlights the
relationship between the analog inputs, VINA, VINB, and the
reference voltage, VREF. Like the voltage applied to the top of
the resistor ladder in a flash A/D converter, the value VREF
defines the maximum input voltage to the A/D core. The minimum input voltage to the A/D core is automatically defined to
be –VREF. CH
QS2 AD9224
+VREF VINA
VCORE VINB A/D
CORE VINA CPIN+
CPAR QS1 12 VINB –VREF CS QS1
QH1 CS CPIN–
CPAR QS2
CH Figure 13. Equivalent Functional Input Circuit Figure 14. Simplified Input Circuit The addition of a differential input structure gives the user an
additional level of flexibility that is not possible with traditional
flash converters. The input stage allows the user to easily configure the inputs for either single-ended operation or differential
operation. The A/D’s input structure allows the dc offset of the
input signal to be varied independently of the input span of the The AD9224 has a wide input range. The input peaks may be
moved to AVDD or AVSS before performance is compromised.
This allows for much greater flexibility when selecting singleended drive schemes. Op amps and ac coupling clamps can be
set to available reference levels rather than be dictated by what
the ADC “needs.” –8– REV. A