125kHz RFID Access-Control Reader Design: EM4100 / EM4200 Tags and the EM4095 Reader AFE
A 125kHz access-control system is built from read-only tags (EM4100/EM4200), a reader analog front-end (EM4095) and an MCU that decodes. This guide explains how the system works, the difference between EM4100 and EM4200, what the EM4095 does, and when to step up to the read/write EM4305.
How a 125kHz access-control system works
125kHz is low-frequency (LF) passive RFID: the reader uses an analog front-end such as the EM4095 to generate a 125kHz carrier, drive the antenna’s LC resonant circuit and radiate a magnetic field that powers the card. The card (EM4100/EM4200) rectifies the field for power, derives its clock from the field, and uses load modulation (switching an internal modulation transistor) to send its stored ID back — the reader senses the tiny amplitude change on the carrier, performs AM demodulation, and passes the recovered bitstream to the MCU, which does the Manchester/Bi-phase decoding, checks the header and parity, and recovers the card ID. Read range depends on antenna size and Q, how accurately the LC is tuned, field strength, and the card’s own resonant capacitor and coil.
EM4100 vs EM4200: read-only tags
The EM4100 is a 125kHz (rated 100–150kHz) read-only tag IC holding a 64-bit laser-programmed unique code, supporting Manchester, Bi-phase or PSK encoding at 64/32/16 carrier periods per bit; it integrates a 74pF resonant capacitor, a rectifier bridge and a voltage limiter on-chip, needing only an external coil. The EM4200 is EM Micro’s successor designed to “directly replace the EM4100/4102 and EM4005/4105,” fully protocol-compatible (existing EM4100 readers read it directly), but with a 128-bit laser ROM (64/96/128-bit options) and a larger on-chip resonant capacitor (75/210/250pF options); EM Micro states it gives better read range than the EM4100. Both are read-only and cannot be rewritten, so the ID can be cloned — something to weigh where security matters.
The EM4095 reader AFE: what it does and does not do
The EM4095 is the reader’s analog front-end (AFE), not a tag. It uses an on-chip PLL that locks to the antenna’s resonant frequency to generate a 100–150kHz carrier (no external crystal needed), drives the antenna directly with bridge drivers, and AM-demodulates the tag’s load modulation, outputting the recovered bitstream on the DEMOD_OUT pin. It does not decode Manchester or recognize the card ID — decoding and ID matching are done by an external MCU. Key pins: DEMOD_OUT (the demodulated bitstream to the MCU), MOD (modulation input; tied to VSS for a continuous field when reading read-only cards, used for 100% AM when writing EM4305-class tags), SHD (sleep/start, must be high at power-up to initialize) and RDY/CLK (signals PLL lock and provides a carrier-synchronous clock). The EM4095 reads both read-only (EM4100/EM4200) and read/write (EM4305-class) tags.
Read-only or read/write? When you need the EM4305
If the card only needs a fixed ID and lowest cost, use the read-only EM4100, or the EM4200 for better range and a longer code. If you need to rewrite data in the field, password protection, or to use the card as a stored-value wallet, choose the read/write EM4205/EM4305: 512-bit EEPROM (16 words × 32 bits), 32-bit password read/write protection, per-word write-lock, a factory 32-bit unique UID, ISO 11784/11785 compliance, 10-year data retention and 100,000 write cycles. The EM4305 differs from the EM4205 in having enlarged bump pads for direct coil/antenna connection. Where security matters, do not rely on a read-only card alone (the ID is easily cloned) — use the EM4305 with its password and write protection. A read/write card still uses the EM4095 reader AFE; the only difference is using the MOD pin to apply 100% AM and send commands to the tag.
Products mentioned
EM4100 125kHz Read-Only RFID Tag IC
The EM4100 is EM Microelectronics' classic 125kHz read-only LF RFID tag IC with a 64-bit laser-programmed unique ID (not field-rewritable), supporting Manchester/Biphase/PSK coding and RF/64, RF/32, RF/16 data rates. Fully passive, with an on-chip 74pF resonance capacitor, rectifier, and voltage limiter — ubiquitous in access cards, key fobs, and asset ID.
EM4200 125kHz Read-Only RFID Tag IC (EM4100-Compatible)
The EM4200 is a 125kHz read-only LF RFID transponder from EM Microelectronics with 128-bit ROM, protocol-compatible with the EM4100/4102 (and EM4005/4105) as a drop-in replacement with better read range. Supports Manchester/Biphase/PSK modulation and RF/32, RF/64 data rates. Fully passive — common in access cards, animal ID, and asset tracking.
EM4095 125kHz RFID Reader Front-End IC
The EM4095 is an EM Microelectronics 125kHz RFID reader/base-station analog front-end that directly drives the antenna and handles AM modulation/demodulation. An integrated PLL self-adapts to antenna resonance (no external quartz), connecting to an MCU via a simple interface (MOD / DEMOD_OUT / RDY_CLK), with a 4.1~5.5V supply in an SO16 package — common in access control, animal ID, and immobilizer applications.
EM4305 125kHz Read/Write RFID Transponder
EM4305 is a 125kHz low-frequency RFID read/write transponder IC powered by the inductive coil. Supports ASK/FSK/PSK modulation with 512-bit EEPROM — widely used in access control cards, employee ID cards, animal identification, and industrial tracking.
Need these components or design help?
JLink Technology provides parts, datasheets, and engineering support.
Request a Quote