The DAC38RF82 (or DAC38RF89) RX [0..7]+/- differential inputs are each internally terminated to a common point via 50 Ω, as shown in Figure 27.
Common mode termination is via a 50 pF capacitor to GND. The common mode voltage and termination of the differential signal can be controlled in a number of ways to suit a variety of applications via field TERM in register SRDS_CFG2 (8.5.87), as described in Table 1.
AC coupling is recommended for JESD204B compliance.
|001||Common point set to 0.7 V. This configuration is for AC coupled systems. The transmitter has no effect on the receiver common mode, which is set to optimize the input sensitivity of the receiver. Note: this mode is not compatible with JESD204B.|
|100||Common point set to GND. This configuration is for applications that require a 0 V common mode.|
|101||Common point set to 0.25 V. This configuration is for applications that require a low common mode.|
|111||Common point floating. This configuration is for DC coupled systems in which the common mode voltage is set by the attached transmit link partner to 0 and 0.6 V. Note: this mode is not compatible with JESD204B|
Input data is sampled by the differential sensing amplifier using clocks derived from the clock recovery algorithm. The polarity of RX+ and RX- can be inverted by setting the bit of the corresponding lane in field INVPAIR in register SRDS_POL (8.5.88) to “1”. This can potentially simplify PCB layout and improve signal integrity by avoiding the need to swap over the differential signal traces.
Due to processing effects, the devices in the RX+ and RX- differential sense amplifiers will not be perfectly matched and there will be some offset in switching threshold. The DAC38RF82 (or DAC38RF89) contains circuitry to detect and correct for this offset. This feature can be enabled by setting ENOC in register SRDS_CFG1 (8.5.86) to “1”. It is anticipated that most users will enable this feature.