A2K Absolute Optical Encoder: Kit Version

A2K Features

  • Installs onto shafts up to 10mm diameter
  • 12-bit resolution, and resolution field programmable from 2 to 4,096 codes per revolution (3,600 factory default)
  • Full 360 degrees range, 7 msec update time
  • Low power drain of 18.5 mA max., and 2.5 mA in sleep mode
  • Field programmable parameters such as setting zero position point (free demo software provided)
  • EEPROM stores downloadable parameters
  • 9,600 baud default data rate adjustable up to 115K baud
  • 12-bit analog voltage output option (0 to +3.599 volts factory default setting. Field programmable up to 0 to +4.095 volts)
  • Multi-turn mode (note: power must be maintained to prevent reset to zero)
  • -25 to 70 degrees C. operating temperature

See more info below

Configure the A2K

A2K Product Description

Restriction of Hazardous Substances

The A2K optical encoder is a 12-bit absolute rotary kit style encoder that reports a shaft angle within a single 360-degree rotation of a shaft. The kit style A2K allows the encoder to be easily assembled onto an existing shaft and bearing assembly. Unlike incremental, sometimes called relative, style encoders, the A2K provides true (absolute) shaft position eliminating the need for a home or zero cycle after a supply voltage power cycle. The A2K communicates over a RS 485 style serial bus utilizing US Digital's SEI (Serial Encoder Interface), which allows for simple, quick and convenient networking of multiple SEI devices on a single network. PLCs, motion controllers, and computers can also reside on the SEI bus by using US Digital's SEI to USB interface device. For complete information about the SEI bus, please refer to the SEI Communications Protocol webpage.

The A2K is also available with an optional analog output. The analog output option provides a maximum voltage range of 0 to 4.095 volts with 12-bit resolution. The output voltage can be scaled by simple SEI commands to provide user-defined voltage ranges. From the factory, the analog output voltage is set to 0 to 3.599 VDC range. Please note that with the A2K analog output option, only one device may reside on an SEI bus.

Product Specifications

View here or download the specifications


Operating Temperature -25 to 70 C
Vibration (5Hz to 2kHz) 20 G
Electrostatic Discharge, IEC 61000-4-2  ± 4 kV


Parameter Value Units
Max. Shaft Axial Play ±0.010 in.
Max. Shaft Runout 0.010 T.I.R. in.
Max. Acceleration 100000 rad/sec²
Weight 2.5 oz.
Codewheel Moment of Inertia 8.9 x 10^-5 oz-in-s²
Max. Shaft Length 0.60  to .80 from mounting surface in.
Technical Bulletin TB1001 - Shaft and Bore Tolerances   Download


  • Specifications apply over entire operating temperature range.
  • Typical values are specified at Vcc = 12V and 25C.
Parameter Min. Typ. Max. Units
Supply Voltage 7.5 12 16 V
Supply Current @ 12V supply


Analog Output Impedance   51   Ohms
Zero Scale Analog Voltage 0 2 12 mV
Full Scale Analog Voltage 4.066 4.095 4.124 V
Output Noise (Analog version)   10   mV rms
Differential Nonlinearity (Analog version) -1.0   1.0 LSB
Integral Nonlinearity (Analog version) -1.0   1.0 LSB
Absolute Accuracy (SEI interface version)   0.18 0.25 Degrees
Angle tracking speed
  Single-turn mode
  Multi-turn mode
 Position Update Rate (1)      7 msec.

(1) The internal microcontroller takes a snapshot of the disk every 7 msec. and stores the position in memory. It responds immediately to a "report position request" by sending the most recently computed position.


 Parameter  Default value  Volatile?
 SEI address  2  Non-volatile
 Resolution  3600  Non-volatile
 Origin offset  0  Non-volatile
 Baud rate  9600  Volatile
 Mode  0 (1)

(1) Mode is always restored from non-volatile EEPROM on power-up. However, there are separate SEI commands for setting the RAM copy only, or both the RAM copy and the non-volatile EEPROM copy. For an explanation of the Mode bits see SEI Absolute Encoder Communications Protocol.


The analog version of the A2K has a 12-bit DAC on the output which feeds to 2 lines that are otherwise used for the BUSY handshaking pair. This DAC has a full range of 0 to 4.095 Volts which is 1 mV per bit. The absolute position value which the internal microcontroller sends to that DAC is the same as the digital value that it sends to the host over SEI. Since the resolution (which represents the number of codes per revolution) is field programmable, the range of the DAC will also follow that setup. The default resolution is 3600 codes per revolution which yields 1 count per a tenth of a degree. This makes the DAC output equal to 1 mV per tenth of a degree or 0 to 3.599 Volts. When the DAC needs to have the full range to 4.095 Volts, the single turn resolution should be set to 4096. This is easily done with the available software which runs on a PC.  Cable CA-MD6A-SS-MD6-6FT and the SEI-USB are needed to interface the A2K analog version to a USB port or RS-232 serial COM port.

Please Note: The BUSY handshaking lines are replaced by the analog output option. This means that only one device will be able to be connected to the SEI bus when using the analog output option.


1 Ground
2 Busy+
3 Busy-
4 Power
5 DataL
6 DataH


Each A2K is shipped with all encoder components, centering tool, hex wrench, and two #4-40 x 1" cover screws. The following part numbers are provided if spares are needed.

1. Centering Tool

Part #: CTOOL - (Shaft Diameter)
Description: This reusable tool provides a simple method for accurately centering the A2K base onto the shaft.

Instructions: When mounting encoder base, slide centering tool down shaft until it slips into centering hole of encoder base. Tighten mounting screws, then remove centering tool.

2. Hex Tool

Part #: HEXD-050
Description: Hex driver, 0.050" flat-to-flat for #3-48 or #4-48 set screws.

Part #: HEXW-050
Description:  Hex wrench, .050" flat-to-flat for #3-48 or #4-48 set screws.

3. Spacer Tool

Part #: SPACER-A2


Title Date Description Download
Hub Set Screw Production Change - PCN 5367 7/20/2015 As part of our ongoing continuous improvement efforts, US Digital is implementing a change related to the production of our 5/16" (.313), 8mm (.315), 3/8" (.375), and 10mm (.394) HUBDISK assemblies. We are adding a low strength thread locker to the set screw during our assembly process to assist in securing the set screw in the hub during transportation. This will aid in the set screw retention of these specific hub sizes; ensuring the retention is sufficient and avoid the potential of them backing out of the hub assembly during transit. This change does not affect form, fit or function. Download
A2/HD25A Product Lines Detector Upgrade 8/18/2014 As part of our ongoing continuous improvement efforts, US Digital is updating our A2 and HD25A product lines design by utilizing surface mount devices for the detector and LED. Previously these were through hole devices. This change is transparent with the exception of minor cosmetic differences for the A2 kit style encoder, and Spacer Tool used in kit assembly process. Download


Parameter Value Units
Hub Set Screw 2-3 in-lbs
Cover Screw 2-4 in-lbs
Base Mounting Screw (#2-56) 2-3 in-lbs
Base Mounting Screw (#4-40) 4-6 in-lbs
Adapter Plate Mounting Screw (#2-56) 2-3 in-lbs
Adapter Plate Mounting Screw (#4-40) 4-6 in-lbs

Additional Information

Product Notes

  • Cables and connectors are not included and must be ordered separately.
  • US Digital® warrants its products against defects in materials and workmanship for two years. See complete warranty for details.


User Guides

Assembly Instructions



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