SBAS578A May   2012  – January 2016 ADS4128

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Electrical Characteristics
    6. 7.6  Electrical Characteristics: General
    7. 7.7  Digital Characteristics
    8. 7.8  Timing Requirements: LVDS and CMOS Modes
    9. 7.9  Reset Timing Requirements
    10. 7.10 Typical Characteristics
    11. 7.11 Typical Characteristics: Contour
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Migrating From the ADS6149 Family
      2. 8.3.2 Digital Functions and Low-Latency Mode
      3. 8.3.3 Gain for SFDR and SNR Trade-Off
      4. 8.3.4 Offset Correction
      5. 8.3.5 Power Down
        1. 8.3.5.1 Global Power-Down
        2. 8.3.5.2 Standby
        3. 8.3.5.3 Output Buffer Disable
        4. 8.3.5.4 Input Clock Stop
      6. 8.3.6 Power-Supply Sequence
      7. 8.3.7 Output Data Format
    4. 8.4 Device Functional Modes
      1. 8.4.1 Output Interface Modes
        1. 8.4.1.1 Output Interface
        2. 8.4.1.2 DDR LVDS Outputs
        3. 8.4.1.3 LVDS Output Data and Clock Buffers
        4. 8.4.1.4 Parallel CMOS Interface
        5. 8.4.1.5 CMOS Interface Power Dissipation
        6. 8.4.1.6 Input Over-Voltage Indication (OVR Pin)
    5. 8.5 Programming
      1. 8.5.1 Serial Register Readout
      2. 8.5.2 Serial Interface
        1. 8.5.2.1 Register Initialization
    6. 8.6 Register Maps
      1. 8.6.1 Serial Interface Register Map
      2. 8.6.2 Register Description
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Analog Input
      2. 9.1.2 Driving Circuit
        1. 9.1.2.1 Drive Circuit Requirements
      3. 9.1.3 Analog Input
        1. 9.1.3.1 Input Common-Mode
      4. 9.1.4 Clock Input
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Analog Input
        2. 9.2.2.2 Clock Driver
        3. 9.2.2.3 Digital Interface
      3. 9.2.3 Application Curve
  10. 10Power Supply Recommendations
    1. 10.1 Sharing DRVDD and AVDD Supplies
    2. 10.2 Using DC/DC Power Supplies
    3. 10.3 Power Supply Bypassing
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Grounding
      2. 11.1.2 Supply Decoupling
      3. 11.1.3 Exposed Pad
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Device Nomenclature
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Detailed Description

8.1 Overview

The ADS4128 is a high-performance, low-power, 12-bit analog-to-digital converter (ADC) with maximum sampling rates up to 200 MSPS. The conversion process is initiated by a rising edge of the external input clock when the analog input signal is sampled. The sampled signal is sequentially converted by a series of small resolution stages, with the outputs combined in a digital correction logic block. At every clock edge the sample propagates through the pipeline, resulting in a data latency of 10 clock cycles. The output is available as 12-bit data, in DDR LVDS mode or CMOS mode, and coded in either straight offset binary or binary twos complement format.

8.2 Functional Block Diagram

ADS4128 fbd_bas578.gif

8.3 Feature Description

8.3.1 Migrating From the ADS6149 Family

The ADS4128 is pin-compatible with the previous generation ADS6149 family; this architecture enables easy migration. However, there are some important differences between the generations, as summarized in Table 6.

Table 6. Migrating From the ADS6149 Family

ADS6149 FAMILY ADS4149 FAMILY (Includes ADS4128)
PINS
Pin 21 is NC (not connected) Pin 21 is NC (not connected)
Pin 23 is MODE Pin 23 is RESERVED in the ADS4128. It is reserved as a digital control pin for an (as yet) undefined function in the next-generation ADC series.
SUPPLY
AVDD is 3.3 V AVDD is 1.8 V
DRVDD is 1.8 V No change
INPUT COMMON-MODE VOLTAGE
VCM is 1.5 V VCM is 0.95 V
SERIAL INTERFACE
Protocol: 8-bit register address and 8-bit register data No change in protocol
New serial register map
EXTERNAL REFERENCE MODE
Supported Not supported
ADS61B49 FAMILY ADS41B49 AND ADS58B18 FAMILY
PINS
Pin 21 is NC (not connected) Pin 21 is 3.3-V AVDD_BUF (supply for the analog input buffers)
Pin 23 is MODE Pin 23 is a digital control pin for the RESERVED function.
Pin 23 functions as SNR Boost enable (B18 only).
SUPPLY
AVDD is 3.3 V AVDD is 1.8 V, AVDD_BUF is 3.3 V
DRVDD is 1.8 V No change
INPUT COMMON-MODE VOLTAGE
VCM is 1.5 V VCM is 1.7 V
SERIAL INTERFACE
Protocol: 8-bit register address and 8-bit register data No change in protocol
New serial register map
EXTERNAL REFERENCE MODE
Supported Not supported

8.3.2 Digital Functions and Low-Latency Mode

The device has several useful digital functions such as test patterns, gain, and offset correction. All of these functions require extra clock cycles for operation and increase the overall latency and power of the device. Alternately, the device has a low-latency mode in which the raw ADC output is routed to the output data pins with a latency of 10 clock cycles. In this mode, the digital functions are bypassed. Figure 36 shows more details of the processing after the ADC.

The device is in low-latency mode after reset. In order to use any digital functions, low-latency mode must first be disabled by setting the DIS LOW LATENCY register bit to 1. Afterwards, the respective register bits must be programmed as described in the following sections and in the Register Maps section.

ADS4128 ai_digi_process_fbd_bas578.gif Figure 36. Digital Processing Block Diagram

8.3.3 Gain for SFDR and SNR Trade-Off

The ADS4128 includes gain settings that can be used to get improved SFDR performance. Gain is programmable from 0 dB to 6 dB (in 0.5-dB steps) using the GAIN register bits. For each gain setting, the analog input full-scale range scales proportionally, as shown in Table 7.

The SFDR improvement is achieved at the expense of SNR; for each gain setting, SNR degrades approximately between 0.5 dB and 1 dB. SNR degradation is reduced at high input frequencies. As a result, gain is very useful at high input frequencies because SFDR improvement is significant with marginal SNR degradation. Therefore, gain can be used to trade-off between SFDR and SNR.

After a reset, the device is in low-latency mode and the gain function is disabled. To use gain:

  • First, disable low-latency mode (DIS LOW LATENCY = 1).
  • This setting enables the gain and puts the device in a 0-dB gain mode.
  • For other gain settings, program the GAIN bits.

Table 7. Full-Scale Range Across Gains

GAIN (dB) TYPE FULL-SCALE (VPP)
0 Default after reset 2
1 Programmable gain 1.78
2 Programmable gain 1.59
3 Programmable gain 1.42
4 Programmable gain 1.26
5 Programmable gain 1.12
6 Programmable gain 1.00

8.3.4 Offset Correction

The ADS4128 has an internal offset correction algorithm that estimates and corrects dc offset up to ±10 mV. The correction can be enabled using the EN OFFSET CORR serial register bit. Once enabled, the algorithm estimates the channel offset and applies the correction every clock cycle. The correction loop time constant is a function of the sampling clock frequency. The time constant can be controlled using the OFFSET CORR TIME CONSTANT register bits, as described in Table 8.

Table 8. Offset Correction Loop Time Constant

OFFSET CORR TIME CONSTANT TIME CONSTANT, TCCLK
(Number of Clock Cycles)		 TIME CONSTANT, TCCLK × 1/fS (sec)(1)
0000 1 M 4 ms
0001 2 M 8 ms
0010 4 M 16.7 ms
0011 8 M 33.5 ms
0100 16 M 67 ms
0101 32 M 134 ms
0110 64 M 268 ms
0111 128 M 537 ms
1000 256 M 1.1 s
1001 512 M 2.15 s
1010 1 G 4.3 s
1011 2 G 8.6 s
1100 Reserved —
1101 Reserved —
1110 Reserved —
1111 Reserved —
(1) Sampling frequency, fS = 200 MSPS.

After the offset is estimated, the correction can be frozen by setting FREEZE OFFSET CORR = 1. Once frozen, the last estimated value is used for every clock cycle offset correction. Note that offset correction is disabled by default after reset.

After a reset, the device is in low-latency mode and offset correction is disabled. To use offset correction:

  • First, disable low-latency mode (DIS LOW LATENCY = 1).
  • Then set EN OFFSET CORR to 1 and program the required time constant.

8.3.5 Power Down

The ADS4128 has three power-down modes: power-down global, standby, and output buffer disable.

8.3.5.1 Global Power-Down

In this mode, the entire chip (including the ADC, internal reference, and the output buffers) are powered down, resulting in reduced total power dissipation of approximately 10 mW. The output buffers are in a high-impedance state. The wake-up time from the global power-down to data becoming valid in normal mode is typically 100 µs. To enter the global power-down mode, set the PDN GLOBAL register bit.

8.3.5.2 Standby

In this mode, only the ADC is powered down and the internal references are active, resulting in a fast wake-up time of 5 µs. The total power dissipation in standby mode is approximately 185 mW. To enter standby mode, set the STBY register bit.

8.3.5.3 Output Buffer Disable

The output buffers can be disabled and put in a high-impedance state; wakeup time from this mode is fast, approximately 100 ns. This mode can be controlled by using the PDN OBUF register bit or the OE pin.

8.3.5.4 Input Clock Stop

In addition, the converter enters low-power mode when the input clock frequency falls below 1 MSPS. Power dissipation is approximately 80 mW.

8.3.6 Power-Supply Sequence

During power-up, the AVDD and DRVDD supplies can come up in any sequence. The two supplies are separated in the device. Externally, they can be driven from separate supplies or from a single supply.

8.3.7 Output Data Format

Two output data formats are supported: binary twos complement and offset binary. These formats can be selected by using the DATA FORMAT serial interface register bit or controlling the DFS pin in parallel configuration mode. In the event of an input voltage overdrive, the digital outputs go to the appropriate full-scale level.

8.4 Device Functional Modes

The ADS4128 has several modes that can be configured using a serial programming interface, as described in Table 9, Table 10, and Table 11. In addition, the device has two dedicated parallel pins for quickly configuring commonly used functions. The parallel pins are DFS (analog 4-level control pin) and OE (digital control pin). The analog control pins can be easily configured using a simple resistor divider (with 10% tolerance resistors).

Table 9. DFS: Analog Control Pin

VOLTAGE APPLIED ON DFS DESCRIPTION
(DATA FORMAT AND OUTPUT INTERFACE)
0, 100 mV/–0 mV Twos complement and DDR LVDS
(3/8) AVDD ± 100 mV Twos complement and parallel CMOS
(5/8) AVDD ± 100 mV Offset binary and parallel CMOS
AVDD, 0 mV/–100 mV Offset binary and DDR LVDS

Table 10. OE: Digital Control Pin

VOLTAGE APPLIED ON OE DESCRIPTION
0 Output data buffers disabled
AVDD Output data buffers enabled

When the serial interface is not used, the SDATA pin can also be used as a digital control pin to place the device in standby mode. To enable this, the RESET pin must be tied high. In this mode, SEN and SCLK do not have any alternative functions. Keep SEN tied high and SCLK tied low on the board.

Table 11. SDATA: Digital Control Pin

VOLTAGE APPLIED ON SDATA DESCRIPTION
0 Normal operation
Logic high Device enters standby
ADS4128 config_par_sen_sclk_bas483.gif Figure 37. Simplified Diagram to Configure DFS Pin

Table 12. High Performance Modes(3)(1)(2)

MODE DESCRIPTION
Mode 1 Set the MODE 1 register bits to get best performance across sample clock and input signal frequencies.
Register address = 03h, register data = 03h
Mode 2 Set the MODE 2 register bit to get best performance at high input signal frequencies.
Register address = 4Ah, register data = 01h
(1) See the Serial Interface section for details on register programming.
(2) Note that these modes cannot be set when the serial interface is not used (when the RESET pin is tied high); see the Programming section.
(3) See the Serial Interface section for details on low-speed mode.

8.4.1 Output Interface Modes

The ADS4128 provides 12-bit data and an output clock synchronized with the data.

8.4.1.1 Output Interface

Two output interface options are available: double data rate (DDR) LVDS and parallel CMOS. These modes can be selected by using the LVDS CMOS serial interface register bit or the DFS pin.

8.4.1.2 DDR LVDS Outputs

In this mode, the data bits and clock are output using low voltage differential signal (LVDS) levels. Two data bits are multiplexed and output on each LVDS differential pair, as shown in Figure 38.

Even data bits (D0, D2, D4, and so on) are output at the CLKOUTP falling edge and the odd data bits (D1, D3, D5, and so on) are output at the CLKOUTP rising edge. Both the CLKOUTP rising and falling edges must be used to capture all 12 data bits, as shown in Figure 39.

ADS4128 ai_ddr_bas578.gif Figure 38. LVDS Data Outputs
ADS4128 ai_tim_ddr_lvds_iface_bas578.gif Figure 39. DDR LVDS Interface

8.4.1.3 LVDS Output Data and Clock Buffers

The equivalent circuit of each LVDS output buffer is shown in Figure 40. After reset, the buffer presents a 100-Ω output impedance to match the external 100-Ω termination.

VDIFF voltage is nominally 350 mV, resulting in a ±350-mV output swing with a 100-Ω external termination. VDIFF voltage is programmable using the LVDS SWING register bits from ±125 mV to ±570 mV.

Additionally, a mode exists to double the LVDS buffer strength to support 50-Ω differential termination. This mode can be used when the output LVDS signal is routed to two separate receiver chips, each using a 100-Ω termination. This mode can be enabled using the LVDS DATA STRENGTH and LVDS CLKOUT STRENGTH register bits for data and output clock buffers, respectively.

The buffer output impedance behaves in the same way as a source-side series termination. By absorbing reflections from the receiver end, it helps to improve signal integrity.

ADS4128 ai_lvds_buf_equiv_cir_bas578.gif
NOTE: Use the default buffer strength to match the 100-Ω external termination (ROUT = 100 Ω). To match with a 50-Ω external termination, set the LVDS STRENGTH bit (ROUT = 50 Ω).
Figure 40. LVDS Buffer Equivalent Circuit

8.4.1.4 Parallel CMOS Interface

In CMOS mode, each data bit is output on a separate pin as the CMOS voltage level, for every clock cycle. The output clock CLKOUT rising edge can be used to latch data in the receiver. Figure 41 depicts the CMOS output interface.

Switching noise (caused by CMOS output data transitions) can couple into the analog inputs and degrade SNR. The coupling and SNR degradation increases as the output buffer drive is made stronger. To minimize this degradation, the CMOS output buffers are designed with controlled drive strength. The default drive strength ensures a wide data stable window (even at 200 MSPS) is provided so the data outputs have minimal load capacitance. It is recommended to use short traces (one to two inches or 2,54 cm to 5,08 cm) terminated with less than 5-pF load capacitance; see Figure 42.

In some high-speed applications using CMOS interface, it may be required to use an external clock to capture data. For such cases, delay from the input clock to output data and the data valid times are specified for higher sampling frequencies. These timings can be used to delay the input clock appropriately and use it to capture data.

ADS4128 ai_cmos_out_iface_bas578.gif Figure 41. CMOS Output Interface
ADS4128 ai_cmos_dout_bas578.gif Figure 42. Using the CMOS Data Outputs

8.4.1.5 CMOS Interface Power Dissipation

With CMOS outputs, the DRVDD current scales with the sampling frequency and load capacitance on every output pin. The maximum DRVDD current occurs when each output bit toggles between 0 and 1 every clock cycle. In actual applications, this condition is unlikely to occur. The actual DRVDD current is determined by the average number of output bits switching, which is a function of the sampling frequency and the nature of the analog input signal.

Equation 1. Digital Current as a Result of CMOS Output Switching = CL × DRVDD × (N × fAVG)

where

shows the current across sampling frequencies at a 2-MHz analog input frequency.

8.4.1.6 Input Over-Voltage Indication (OVR Pin)

The device has an OVR pin that provides information about analog input overload. At any clock cycle, if the sampled input voltage exceeds the positive or negative full-scale range, the OVR pin goes high. OVR remains high as long as the overload condition persists. The OVR pin is a CMOS output buffer (running off of a DRVDD supply), independent of the output data interface (DDR LVDS or CMOS).

For a positive overload, the D[11:0] output data bits are FFFh in offset binary output format and 7FFh in twos complement output format. For a negative input overload, the output code is 000h in offset binary output format and 800h in twos complement output format.

8.5 Programming

8.5.1 Serial Register Readout

The serial register readout function allows the contents of the internal registers to be read back on the OVR_SDOUT pin. This readback may be useful as a diagnostic check to verify the serial interface communication between the external controller and the ADC.

After power-up and device reset, the OVR_SDOUT pin functions as an over-range indicator pin by default. When the readout mode is enabled, OVR_SDOUT outputs the contents of the selected register serially:

  1. Set the READOUT register bit to 1. This setting puts the device in serial readout mode and disables any further writes to the internal registers except the register at address 0. Note that the READOUT bit itself is also located in register 0. The device can exit readout mode by writing READOUT = 0. Only the contents of the register at address 0 cannot be read in the register readout mode.
  2. Initiate a serial interface cycle specifying the address of the register (A7 to A0) whose content has to be read.
  3. The device serially outputs the contents (D7 to D0) of the selected register on the OVR_SDOUT pin.
  4. The external controller can latch the contents at the falling edge of SCLK.
  5. To exit the serial readout mode, the reset register bit READOUT = 0 enables writes into all registers of the device. At this point, the OVR_SDOUT pin becomes an over-range indicator pin.

8.5.2 Serial Interface

The analog-to-digital converter (ADC) has a set of internal registers that can be accessed by the serial interface formed by the SEN (serial interface enable), SCLK (serial interface clock), and SDATA (serial interface data) pins. Serial shift of bits into the device is enabled when SEN is low. Serial data SDATA are latched at every SCLK falling edge when SEN is active (low). The serial data are loaded into the register at every 16th SCLK falling edge when SEN is low. If the word length exceeds a multiple of 16 bits, the excess bits are ignored. Data can be loaded in multiples of 16-bit words within a single active SEN pulse. The first eight bits form the register address and the remaining eight bits are the register data. The interface can function with an SCLK frequency from 20 MHz down to very low speeds (of a few Hertz) and also with a non-50% SCLK duty cycle.

8.5.2.1 Register Initialization

After power-up, the internal registers must be initialized to default values. This initialization can be accomplished in one of two ways:

  1. Either through hardware reset by applying a high pulse on the RESET pin (of widths greater than 10 ns), as shown in Figure 7; or
  2. By applying a software reset. When using the serial interface, set the RESET bit (D7 in register 00h) high. This setting initializes the internal registers to default values and then self-resets the RESET bit low. In this case, the RESET pin is kept low.

8.6 Register Maps

Serial Interface Register Map summarizes the functions supported by the serial interface.

8.6.1 Serial Interface Register Map(1)

REGISTER ADDRESS DEFAULT VALUE AFTER RESET REGISTER DATA
A[7:0] (Hex) D[7:0] (Hex) D7 D6 D5 D4 D3 D2 D1 D0
00 00 0 0 0 0 0 0 RESET READOUT
01 00 LVDS SWING 0 0
03 00 0 0 0 0 0 0 HIGH PERF MODE 1
25 00 GAIN DISABLE GAIN TEST PATTERNS
26 00 0 0 0 0 0 0 LVDS CLKOUT STRENGTH LVDS DATA STRENGTH
3D 00 DATA FORMAT EN OFFSET CORR 0 0 0 0 0
3F 00 CUSTOM PATTERN HIGH D[11:4]
40 00 CUSTOM PATTERN D[3:0] 0 0 0 0
41 00 LVDS CMOS CMOS CLKOUT STRENGTH EN CLKOUT RISE CLKOUT RISE POSN EN CLKOUT FALL
42 00 CLKOUT FALL POSN 0 0 DIS LOW LATENCY STBY 0 0
43 00 0 PDN GLOBAL 0 PDN OBUF 0 0 EN LVDS SWING
4A 00 0 0 0 0 0 0 0 HIGH PERF MODE 2
BF 00 OFFSET PEDESTAL 0 0
CF 00 FREEZE OFFSET CORR 0 OFFSET CORR TIME CONSTANT 0 0
DF 00 0 0 LOW SPEED 0 0 0 0
(1) Multiple register functions can be programmed in a single write operation.

8.6.2 Register Description

For best performance, two special mode register bits must be enabled:

HI PERF MODE 1 and HI PERF MODE 2.

Table 1. Register Address 00h (Default = 00h)

7 6 5 4 3 2 1 0
0 0 0 0 0 0 RESET READOUT
Bits[7:2] Always write 0
Bit 1 RESET: Software reset applied
This bit resets all internal registers to default values and self-clears to 0 (default = 1).
Bit 0 READOUT: Serial readout
This bit sets the serial readout of the registers.
0 = Serial readout of registers disabled; the OVR_SDOUT pin functions as an over-voltage indicator.
1 = Serial readout enabled; the OVR_SDOUT pin functions as a serial data readout.

Table 2. Register Address 01h (Default = 00h)

7 6 5 4 3 2 1 0
LVDS SWING 0 0
Bits[7:2] LVDS SWING: LVDS swing programmability(1)
000000 = Default LVDS swing; ±350 mV with external 100-Ω termination
011011 = LVDS swing increases to ±410 mV
110010 = LVDS swing increases to ±465 mV
010100 = LVDS swing increases to ±570 mV
111110 = LVDS swing decreases to ±200 mV
001111 = LVDS swing decreases to ±125 mV
Bits[1:0] Always write 0
(1) The EN LVDS SWING register bits must be set to enable LVDS swing control.

Table 3. Register Address 03h (Default = 00h)

7 6 5 4 3 2 1 0
0 0 0 0 0 0 HI PERF MODE 1
Bits[7:2] Always write 0
Bits[1:0] HI PERF MODE 1: High-performance mode 1
00 = Default performance after reset
01 = Do not use
10 = Do not use
11 = For best performance across sampling clock and input signal frequencies, set the HIGH PERF MODE 1 bits

Table 4. Register Address 25h (Default = 00h)

7 6 5 4 3 2 1 0
GAIN DISABLE GAIN TEST PATTERNS
Bits[7:4] GAIN: Gain programmability
These bits set the gain programmability in 0.5-dB steps.
0000 = 0-dB gain (default after reset)
0001 = 0.5-dB gain
0010 = 1.0-dB gain
0011 = 1.5-dB gain
0100 = 2.0-dB gain
0101 = 2.5-dB gain
0110 = 3.0-dB gain		 0111 = 3.5-dB gain
1000 = 4.0-dB gain
1001 = 4.5-dB gain
1010 = 5.0-dB gain
1011 = 5.5-dB gain
1100 = 6.0-dB gain
Bit 3 DISABLE GAIN: Gain setting
This bit sets the gain.
0 = Gain enabled; gain is set by the GAIN bits only if low-latency mode is disabled
1 = Gain disabled
Bits[2:0] TEST PATTERNS: Data capture
These bits verify data capture.
000 = Normal operation
001 = Outputs all 0s
010 = Outputs all 1s
011 = Outputs toggle pattern
Output data D[11:0] is an alternating sequence of 010101010101 and 101010101010.
100 = Outputs digital ramp
Output data increments by one LSB (12-bit) every fourth clock cycle from code 0 to code 4095
101 = Output custom pattern (use registers 3Fh and 40h for setting the custom pattern)
110 = Unused
111 = Unused

Table 5. Register Address 26h (Default = 00h)

7 6 5 4 3 2 1 0
0 0 0 0 0 0 LVDS CLKOUT STRENGTH LVDS DATA STRENGTH
Bits[7:2] Always write 0
Bit 1 LVDS CLKOUT STRENGTH: LVDS output clock buffer strength
This bit determines the external termination to be used with the LVDS output clock buffer.
0 = 100-Ω external termination (default strength)
1 = 50-Ω external termination (2×strength)
Bit 0 LVDS DATA STRENGTH: LVDS data buffer strength
This bit determines the external termination to be used with all of the LVDS data buffers.
0 = 100-Ω external termination (default strength)
1 = 50-Ω external termination (2×strength)

Table 6. Register Address 3Dh (Default = 00h)

7 6 5 4 3 2 1 0
DATA FORMAT EN OFFSET CORR 0 0 0 0 0
Bits[7:6] DATA FORMAT: Data format selection
These bits selects the data format.
00 = The DFS pin controls data format selection
10 = Twos complement
11 = Offset binary
Bit 5 ENABLE OFFSET CORR: Offset correction setting
This bit sets the offset correction.
0 = Offset correction disabled
1 = Offset correction enabled
Bits[4:0] Always write 0

Table 7. Register Address 3Fh (Default = 00h)

7 6 5 4 3 2 1 0
CUSTOM PATTERN D11 CUSTOM PATTERN D10 CUSTOM PATTERN D9 CUSTOM PATTERN D8 CUSTOM PATTERN D7 CUSTOM PATTERN D6 CUSTOM PATTERN D5 CUSTOM PATTERN D4
Bits[7:0] CUSTOM PATTERN
These bits set the custom pattern.

Table 8. Register Address 40h (Default = 00h)

7 6 5 4 3 2 1 0
CUSTOM PATTERN D3 CUSTOM PATTERN D2 CUSTOM PATTERN D1 CUSTOM PATTERN D0 0 0 0 0
Bits[7:2] CUSTOM PATTERN
These bits set the custom pattern.
Bits[3:0] Always write 0

Table 9. Register Address 41h (Default = 00h)

7 6 5 4 3 2 1 0
LVDS CMOS CMOS CLKOUT STRENGTH EN CLKOUT RISE CLKOUT RISE POSN EN CLKOUT FALL
Bits[7:6] LVDS CMOS: Interface selection
These bits select the interface.
00 = The DFS pin controls the selection of either LVDS or CMOS interface
10 = The DFS pin controls the selection of either LVDS or CMOS interface
01 = DDR LVDS interface
11 = Parallel CMOS interface
Bits[5:4] CMOS CLKOUT STRENGTH
Controls strength of CMOS output clock only.
00 = Maximum strength (recommended and used for specified timings)
01 = Medium strength
10 = Low strength
11 = Very low strength
Bit 3 ENABLE CLKOUT RISE
0 = Disables control of output clock rising edge
1 = Enables control of output clock rising edge
Bits[2:1] CLKOUT RISE POSN: CLKOUT rise control
Controls position of output clock rising edge
LVDS interface:
00 = Default position (timings are specified in this condition)
01 = Setup reduces by 500 ps, hold increases by 500 ps
10 = Data transition is aligned with rising edge
11 = Setup reduces by 200 ps, hold increases by 200 ps
CMOS interface:
00 = Default position (timings are specified in this condition)
01 = Setup reduces by 100 ps, hold increases by 100 ps
10 = Setup reduces by 200 ps, hold increases by 200 ps
11 = Setup reduces by 1.5 ns, hold increases by 1.5 ns
Bit 0 ENABLE CLKOUT FALL
0 = Disables control of output clock falling edge
1 = Enables control of output clock falling edge

Table 10. Register Address 42h (Default = 00h)

7 6 5 4 3 2 1 0
CLKOUT FALL CTRL 0 0 DIS LOW LATENCY STBY 0 0
Bits[7:6] CLKOUT FALL CTRL
Controls position of output clock falling edge
LVDS interface:
00 = Default position (timings are specified in this condition)
01 = Setup reduces by 400 ps, hold increases by 400 ps
10 = Data transition is aligned with rising edge
11 = Setup reduces by 200 ps, hold increases by 200 ps
CMOS interface:
00 = Default position (timings are specified in this condition)
01 = Falling edge is advanced by 100 ps
10 = Falling edge is advanced by 200 ps
11 = Falling edge is advanced by 1.5 ns
Bits[5:4] Always write 0
Bit 3 DIS LOW LATENCY: Disable low latency
This bit disables low-latency mode.
0 = Low-latency mode is enabled. Digital functions such as gain, test patterns, and offset correction are disabled.
1 = Low-latency mode is disabled. This setting enables the digital functions. See the Digital Functions and Low-Latency Mode section.
Bit 2 STBY: Standby mode
This bit sets the standby mode.
0 = Normal operation
1 = Only the ADC and output buffers are powered down; internal reference is active; wake-up time from standby is fast
Bits[1:0] Always write 0

Table 11. Register Address 43h (Default = 00h)

7 6 5 4 3 2 1 0
0 PDN GLOBAL 0 PDN OBUF 0 0 EN LVDS SWING
Bit 0 Always write 0
Bit 6 PDN GLOBAL: Power-down
This bit sets the state of operation.
0 = Normal operation
1 = Total power down; the ADC, internal references, and output buffers are powered down; slow wake-up time.
Bit 5 Always write 0
Bit 4 PDN OBUF: Power-down output buffer
This bit set the output data and clock pins.
0 = Output data and clock pins enabled
1 = Output data and clock pins powered down and put in high-impedance state
Bits[3:2] Always write 0
Bits[1:0] EN LVDS SWING: LVDS swing control
00 = LVDS swing control using LVDS SWING register bits is disabled
01 = Do not use
10 = Do not use
11 = LVDS swing control using LVDS SWING register bits is enabled

Table 12. Register Address 4Ah (Default = 00h)

7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 HI PERF MODE 2
Bits[7:1] Always write 0
Bit[0] HI PERF MODE 2: High-performance mode 2
This bit is recommended for high input signal frequencies greater than 230 MHz.
0 = Default performance after reset
1 = For best performance with high-frequency input signals, set the HIGH PERF MODE 2 bit

Table 13. Register Address BFh (Default = 00h)

7 6 5 4 3 2 1 0
OFFSET PEDESTAL 0 0 0 0
Bits[7:4] OFFSET PEDESTAL
These bits set the offset pedestal.
When the offset correction is enabled, the final converged value after the offset is corrected is the ADC mid-code value. A pedestal can be added to the final converged value by programming these bits.
OFFSET PEDESTAL VALUE PEDESTAL
0111
0110
0101
—
000000
—
1111
1110
—
1000		 7 LSB
6 LSB
5 LSB
—
0 LSB
—
–1 LSB
–2 LSB
—
–8 LSB
Bits[3:0] Always write 0

Table 14. Register Address CFh (Default = 00h)

7 6 5 4 3 2 1 0
FREEZE OFFSET CORR BYPASS OFFSET CORR OFFSET CORR TIME CONSTANT 0 0
Bit 7 FREEZE OFFSET CORR
This bit sets the freeze offset correction.
0 = Estimation of offset correction is not frozen (bit EN OFFSET CORR must be set)
1 = Estimation of offset correction is frozen (bit EN OFFSET CORR must be set). When frozen, the last estimated value is used for offset correction every clock cycle. See Offset Correction.
Bit 6 Always write 0
Bits[5:2] OFFSET CORR TIME CONSTANT
These bits set the offset correction time constant for the correction loop time constant in number of clock cycles.
VALUE TIME CONSTANT (Number of Clock Cycles)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011		 1 M
2 M
4 M
8 M
16 M
32 M
64 M
128 M
256 M
512 M
1 G
2 G
Bits[1:0] Always write 0

Table 15. Register Address DFh (Default = 00h)

7 6 5 4 3 2 1 0
0 0 LOW SPEED 0 0 0 0
Bits[7:1] Always write 0
Bit 0 LOW SPEED: Low-speed mode
00, 01, 10 = Low-speed mode disabled (default state after reset); this setting is recommended for sampling rates greater than 80 MSPS.
11 = Low-speed mode enabled; this setting is recommended for sampling rates less than or equal to 80 MSPS.