RX Analog Front End

Versal ACAP GTM Transceivers Architecture Manual (AM017)

Document ID
AM017
Release Date
2022-04-27
Revision
1.0 English

The RX analog front end (AFE) is a high-speed input differential buffer with configurable RX termination voltage, on-chip coupling capacitors, and calibrated termination resistor. When the on-chip coupling capacitor is used, a level shifter circuit is also enabled to maintain DC balance when receiving unbalanced patterns.

Figure 1. RX Analog Front End

AC Coupling Mode

This is the typical and default transceiver operation mode. In this use mode, the external AC coupling capacitor must be present and should follow data sheet recommendations. The device's internal capacitor is bypassed. The termination voltage is set to VCOM_VREF provided by CTLE.

Table 1. AC Coupling Mode Settings
Use Mode External AC Coupling Termination Voltage Setting
AC Coupling Yes VCOM_VREF (typically 590 mV)
  • ACCAP_EN = 1'b0
  • LSHIFT_EN = 1'b0
  • RTERM_VCOM_MODE = 2'b11
Figure 2. AC Coupling

DC Coupling Mode with GTM_AVTT Termination

This mode enables an internal AC capacitor circuit that decouples the analog front end from the equalization block, allowing greater flexibility on the common mode input voltage. The TX or board circuit should ensure that the RX input common mode voltage is set to between 0.6V ~ 1.0V.

Table 2. DC Coupling Mode with GTM_AVTT Termination
Use Mode External AC Coupling Termination Voltage Setting
DC Coupling No GTM_AVTT
  • ACCAP_EN = 1'b1
  • LSHIFT_EN = 1'b1
  • RTERM_VCOM_MODE = 2'b00
Figure 3. DC Coupling with GTM_AVTT Termination

DC Coupling Mode with GND Termination

This mode enables an internal AC capacitor circuit that decouples the analog front end from the equalization block, allowing greater flexibility on the common mode input voltage. The TX or board circuit should ensure that the RX input common mode voltage is set to between 0.2V ~ 0.6V.

Table 3. DC Coupling Mode with GND Termination
Use Mode External AC Coupling Termination Voltage Setting
DC Coupling No GND
  • ACCAP_EN = 1'b1
  • LSHIFT_EN = 1'b1
  • RTERM_VCOM_MODE = 2'b01
Figure 4. DC Coupling with GND Termination

DC Coupling Mode with FLOAT Termination

This mode enables an internal AC capacitor circuit that decouples the analog front end from the equalization block, allowing greater flexibility on the common mode input voltage. The TX or board circuit should ensure that the RX input common mode voltage is set to between 0.2V ~ 1.0V.

Table 4. DC Coupling Mode with FLOAT Termination
Use Mode External AC Coupling Termination Voltage Setting
DC Coupling No FLOAT
  • ACCAP_EN = 1'b1
  • LSHIFT_EN = 1'b1
  • RTERM_VCOM_MODE = 2'b10
Figure 5. DC Coupling with FLOAT Termination