ADS808 Datasheet, PDF (8/19 Pages) Burr-Brown (TI) – 12-Bit, 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER

APPLICATION INFORMATION
THEORY OF OPERATION
The ADS808 is a high-speed, high-performance, CMOS
ADC built with a fully differential, 9-stage pipeline architec-
ture. Each stage contains a low-resolution quantizer and
digital error-correction logic, ensuring excellent differential
linearity and no missing codes at the 12-bit level. The
conversion process is initiated by a rising edge of the
external convert clock. Once the signal is captured by the
input track-and-hold amplifier, the bits are sequentially en-
coded starting with the MSB. This process results in a data
latency of five clock cycles, after which the output data is
available as a 12-bit parallel word either coded in a straight
binary or binary two’s complement format.
The analog input of the ADS808 consists of a differential
track-and-hold circuit, as shown in Figure 1. The differential
topology produces a high level of AC-performance at high
sampling rates. It also results in a very high usable input
bandwidth that is especially important for IF, or undersampling
applications. Both inputs (IN, IN) require external biasing up
to a common-mode voltage that is typically at the mid-supply
level (+V_S/2). This is because the on-resistance of the CMOS
switches is lowest at this voltage, minimizing the effects of
the signal dependent nonlinearity of R_ON. The track-and-hold
circuit can also convert a single-ended input signal into a fully
differential signal for the quantizer. For ease of use, the
ADS808 incorporates a selectable voltage reference, a ver-
satile clock input, and a logic output driver designed to
interface to 3V or 5V logic.
ADS808
S₅
S₁
C_IN
IN
S₂
C_IN
IN
S₆
S₃
T&H
S₄
Tracking Phase: S₁, S₂, S₃, S₄ Closed; S₅, S₆ Open
Hold Phase: S₁, S₂, S₃, S₄ Open; S₅, S₆ Closed
FIGURE 1. Simplified Circuit of Input Track-and-Hold Amplifier.
DRIVING THE ANALOG INPUTS
Types of Applications
The analog input of the ADS808 can be configured in various
ways and driven with different circuits, depending on the
application and the desired level of performance. Offering a
high dynamic range at high input frequencies, the ADS808 is
particularly suited for communication systems that digitize
wideband signals. Features on the ADS808, like the input
range selector or the option of an external reference, provide
the needed flexibility to accommodate a wide range of
applications. In any case, the analog interface/driver require-
ments should be carefully examined before selecting the
appropriate circuit configuration. The circuit definition should
include considerations on the input frequency spectrum and
amplitude, single-ended versus differential driver configura-
tion, as well as the available power supplies.
Differential versus Single-Ended
The ADS808 input structure allows it to be driven either
single-ended or differentially. Differential operation of the
ADS808 requires an input signal that consists of an in-phase
and a 180° out-of-phase component simultaneously applied
to the inputs (IN, IN). Differential signals offer a number of
advantages that in many applications will be instrumental in
achieving the best harmonic performance of the ADS808:
• The signal amplitude is half of that required for the single-
ended operation and is, therefore, less demanding to
achieve while maintaining good linearity performance from
the signal source.
• The reduced signal swing allows for more headroom of
the interface circuitry and, therefore, a wider selection of
the best suitable driver amplifier.
• Even-order harmonics are minimized.
• Improves the noise immunity based on the converter’s
common-mode input rejection.
For the single-ended mode, the signal is applied to one of the
inputs while the other input is biased with a DC voltage to the
required common-mode level. Both inputs are identical in
terms of their impedance and performance except that apply-
ing the signal to the complementary input (IN) instead of the
IN-input will invert the orientation of the input signal relative
to the output code. For example, if the input driver operates
in inverting mode using IN as the signal input, it will restore
the phase of the signal to its original orientation. Time-
domain applications may benefit from a single-ended inter-
face configuration and a reduced circuit complexity. Driving
the ADS808 with a single-ended signal will result in a trade-
off of the excellent distortion performance, while maintaining
a good signal-to-noise ratio (SNR). The trade-off of the
differential input configuration over the single-ended is its
increase in circuit complexity. In either case, the selection of
the driver amplifier should be such that the amplifier’s perfor-
mance will not degrade the A/D converter’s performance.
Input Full-Scale Range versus Performance
Employing dual-supply amplifiers and AC-coupling will usually
yield the best results. DC-coupling and/or single-supply ampli-
fiers impose additional design constrains due to their head-
room requirements, especially when selecting the 2Vp-p input
range. The full-scale input range of the ADS808 is defined
either by the settings of the reference select pins (SEL1,
SEL2) or by an external reference voltage (see Table I).
8
ADS808
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