Datasheet

AD9277
Rev. 0 | Page 38 of 48
Table 13. Flexible Output Test Modes
Output Test Mode
Bit Sequence Pattern Name Digital Output Word 1 Digital Output Word 2
Subject to Data
Format Select
0000 Off (default) N/A N/A N/A
0001 Midscale short 10 0000 0000 0000 Same Yes
0010 +Full-scale short 11 1111 1111 1111 Same Yes
0011 −Full-scale short 00 0000 0000 0000 Same Yes
0100 Checkerboard 10 1010 1010 1010 01 0101 0101 0101 No
0101 PN sequence long N/A N/A Yes
0110 PN sequence short N/A N/A Yes
0111 One-/zero-word toggle 11 1111 1111 1111 00 0000 0000 0000 No
1000 User input Register 0x19 and Register 0x1A Register 0x1B and Register 0x1C No
1001 1-/0-bit toggle 10 1010 1010 1010 N/A No
1010 1× sync 00 0000 0011 1111 N/A No
1011 One bit high 10 0000 0000 0000 N/A No
1100 Mixed bit frequency 10 1010 0011 0011 N/A No
The PN sequence long pattern produces a pseudorandom bit
sequence that repeats itself every 2
23
− 1 bits, or 8,388,607 bits.
A description of the PN sequence long and how it is generated
can be found in Section 5.6 of the ITU-T O.150 (05/96) standard.
The only differences are that the starting value is a specific value
instead of all 1s and that the AD9277 inverts the bit stream with
relation to the ITU-T standard (see Table 14 for the initial values).
Table 14. PN Sequence
Sequence
Initial
Value
First Three Output Samples
(MSB First)
PN Sequence Short 0x0DF 0x37E4, 0x3533, 0x0063
PN Sequence Long 0x29B80A 0x191F, 0x35C2, 0x2359
See the Memory Map section for information on how to change
these additional digital output timing features through the SPI.
SDIO Pin
This pin is required to operate the SPI. It has an internal 30 kΩ
pull-down resistor that pulls this pin low and is only 1.8 V
tolerant. If applications require that this pin be driven from a
3.3 V logic level, insert a 1 kΩ resistor in series with this pin to
limit the current.
SCLK Pin
This pin is required to operate the SPI port interface. It has an
internal 30 kΩ pull-down resistor that pulls this pin low and is
both 1.8 V and 3.3 V tolerant.
CSB Pin
This pin is required to operate the SPI port interface. It has an
internal 70 kΩ pull-up resistor that pulls this pin high and is
both 1.8 V and 3.3 V tolerant.
RBIAS Pin
To set the internal core bias current of the ADC, place a resistor
nominally equal to 10.0 kΩ to ground at the RBIAS pin. Using a
resistor other than the recommended 10.0 kΩ resistor for RBIAS
degrades the performance of the device. Therefore, it is imperative
that at least a 1% tolerance on this resistor be used to achieve
consistent performance.
Voltage Reference
A stable and accurate 0.5 V voltage reference is built into the
AD9277. This is gained up internally by a factor of 2, setting
VREF to 1.0 V, which results in a full-scale differential input span
of 2.0 V p-p for the ADC. VREF is set internally by default, but
the VREF pin can be driven externally with a 1.0 V reference to
achieve more accuracy. However, the AD9277 does not support
ADC full-scale ranges below 2.0 V p-p.
When applying the decoupling capacitors to the VREF pin,
use ceramic, low ESR capacitors. These capacitors should be
close to the reference pin and on the same layer of the PCB as
the AD9277. The VREF pin should have both a 0.1 µF capacitor
and a 1 µF capacitor connected in parallel to the analog ground.
These capacitor values are recommended for the ADC to
properly settle and acquire the next valid sample.
The reference settings can be selected using the SPI. The settings
allow two options: using the internal reference or using an
external reference. The internal reference option is the default
setting and has a resulting differential span of 2 V p-p.
Table 15. SPI-Selectable Reference Settings
SPI-Selected Mode
Resulting
VREF (V)
Resulting Differential
Span (V p-p)
External Reference N/A 2 × external reference
Internal Reference (Default) 1.0 2.0