5962-0924001VXC Datasheet, PDF (31/46 Pages) Texas Instruments – 12-Bit, 1-GSPS Analog-to-Digital Converter

ADS5400-SP
www.ti.com
SLAS669C – SEPTEMBER 2010 – REVISED AUGUST 2012
APPLICATION INFORMATION
Theory of Operation
The ADS5400 is a 12-bit, 1-GSPS, monolithic pipeline ADC. Its bipolar transistor analog core operates from 5-V
and 3.3-V supplies, while the output uses a 3.3-V supply to provide LVDS-compatible digital outputs. The
conversion process is initiated by the falling edge of the external input clock. At the sampling instant, the
differential input signal is captured by the input track-and-hold (T&H), and the input sample is sequentially
converted by a series of lower resolution stages, with the outputs combined in a digital correction logic block.
Both the rising and the falling clock edges are used to propagate the sample through the pipeline every half clock
cycle. This process results in a data latency of 7 - 8.5 clock cycles (output mode dependent), after which the
output data is available as a 12-bit parallel word, coded in offset binary or two's complement format.
The user can select to accept the data at the full sample rate using one bus (bus A, latency 7 cycles), or
demultiplex the data into two buses (bus A and B, latency 7.5 or 8.5 cycles) at half rate. A serial peripheral
interface (SPI) is provided for adjusting operational modes, as well as for calibrations of analog gain, analog
offset and clock phase for inter-leaving multiple ADS5400. Die temperature readout using the SPI is provided.
SYNC and RESET modes exist for synchronizing output data across multiple ADS5400.
Input Configuration
The analog input for the ADS5400 consists of an analog pseudo-differential buffer followed by a bipolar transistor
track-and-hold (see Figure 27). The integrated analog buffer isolates the source driving the input of the ADC from
sampling glitches on the T&H and allows for the integration of a 100-Ω differential input resistor. The input
common mode is set internally through a 500-Ω resistor connected from half of the AVDD5 supply voltage to
each of the inputs. The parasitic package capacitance shown is with the package unsoldered. Once soldered,
depending on the board characteristics, one can expect another ~1pF at the analog input pins, which is board
dependent.
AINP
Analog
Inputs
AINN
ADS5400
~5.25 nH Bond Wire
AVDD5
~0.75 pF ~0.2 pF
Package Bondpad
AGND
~5.25 nH Bond Wire
AVDD5
~0.75 pF ~0.2 pF
Package Bondpad
AGND
Bipolar
Transistor
Buffer
112 W
500 W
0.3 pF
2.5 V
500 W
AGND
0.3 pF
Sample and
Hold
st
1 Stage
Of Pipeline
Bipolar
Transistor
Buffer
Figure 27. Analog Input Equivalent Circuit
For a full-scale differential input, each of the differential lines of the input signal swing symmetrically between 2.5
V + 0.5 V and 2.5 V – 0.5 V. This means that each input has a maximum signal swing of 1 V_PP for a total
differential input signal swing of 2 V_PP. The maximum fullscale range can be programmed from 1.5-2Vpp using
the SPI. The maximum swing is determined by the internal reference voltage generator and the fullscale range
set using the SPI, eliminating the need for any external circuitry for this purpose. The analog gain adjustment has
a resolution of 12-bits across the 1.5-2V_PP range, providing for fine calibration of analog gain mismatches across
multiple ADS5400 signal chains, primarily for interleaving.
Copyright © 2010–2012, Texas Instruments Incorporated
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Product Folder Links: ADS5400-SP