ADS5272_14 Datasheet, PDF (20/33 Pages) Texas Instruments – 8-Channel, 12-Bit, 65MSPS ADC with Serial LVDS Interface

ADS5272
SBAS324C – JUNE 2004 – REVISED JANUARY 2009...................................................................................................................................................... www.ti.com
THEORY OF OPERATION
OVERVIEW
The ADS5272 is an 8-channel, high-speed, CMOS
ADC. It consists of a high-performance
sample-and-hold circuit at the input, followed by a
12-bit ADC. The 12 bits given out by each channel
are serialized and sent out on a single pair of pins in
LVDS format. All eight channels of the ADS5272
operate from a single clock referred to as ADCLK.
The sampling clocks for each of the eight channels
are generated from the input clock using a carefully
matched clock buffer tree. The 12x clock required for
the serializer is generated internally from ADCLK
using a phase lock loop (PLL). A 6x and a 1x clock
are also output in LVDS format along with the data to
enable easy data capture. The ADS5272 operates
from internally generated reference voltages that are
trimmed to ensure matching across multiple devices
on a board. This feature eliminates the need for
external routing of reference lines and also improves
matching of the gain across devices. The nominal
values of REF_T and REF_B are 1.95V and 0.95V,
respectively. These values imply that a differential
input of –1V corresponds to the zero code of the
ADC, and a differential input of +1V corresponds to
the full-scale code (4095 LSB). V_CM (common-mode
voltage of REF_T and REF_B) is also made available
externally through a pin, and is nominally 1.45V.
The ADC employs a pipelined converter architecture
consisting of a combination of multi-bit and single-bit
internal stages. Each stage feeds its data into the
digital error correction logic, ensuring excellent
differential linearity and no missing codes at the
12-bit level. The pipeline architecture results in a data
latency of 6.5 clock cycles.
The output of the ADC goes to a serializer that
operates from a 12x clock generated by the PLL. The
12 data bits from each channel are serialized and
sent LSB first. In addition to serializing the data, the
serializer also generates a 1x clock and a 6x clock.
These clocks are generated in the same way the
serialized data is generated, so these clocks maintain
perfect synchronization with the data. The data and
clock outputs of the serializer are buffered externally
using LVDS buffers. Using LVDS buffers to transmit
data externally has multiple advantages, such as a
reduced number of output pins (saving routing space
on the board), reduced power consumption, and
reduced effects of digital noise coupling to the analog
circuit inside the ADS5272.
The ADS5272 operates from two sets of supplies and
grounds. The analog supply/ground set is denoted as
AVDD/AVSS, while the digital set is denoted by
LVDD/LVSS.
DRIVING THE ANALOG INPUTS
The analog input biasing is shown in Figure 34. The
inputs are biased internally using two 600Ω resistors
to enable ac-coupling. A resistor greater than 20Ω is
recommended in series with each input pin.
A 4pF sampling capacitor is used to sample the
inputs. The choice of the external ac-coupling
capacitor is dictated by the attenuation at the lowest
desired input frequency of operation. The attenuation
resulting from using a 10nF ac-coupling capacitor is
0.04%.
IN+
600Ω
600Ω
IN−
V_CM
CM Buffer
ADS5272
Input
Circuitry
Internal
Voltage
Reference
NOTE: Dashed area denotes one of eight channels.
Figure 34. Analog Input Bias Circuitry
If the input is dc-coupled, then the output
common-mode voltage of the circuit driving the
ADS5272 should match the V_CM (which is provided as
an output pin) to within ±50mV. It is recommended
that the output common-mode of the driving circuit be
derived from V_CM provided by the device.
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