The SYSREFREQ pins support single-ended or
differential input in AC or DC coupling mode. The SYSREFREQ pins have an internal 50Ω
termination with capacitive ground, which acts as 100Ω differential.
Figure 7-1 shows the generic SYSREFREQ input circuit recommendation to support all AC/DC,
single-ended or differential inputs. Some of the discrete components in Figure 7-1 are just placeholder for individual input signal (single-ended or differential input) and
AC or DC coupled input.
The following figures show the individual circuit diagram for each configurations:
Figure 7-2 AC-Coupled Differential
Input
Figure 7-4 DC-Coupled Differential
Input
Figure 7-3 AC-Coupled, Single-Ended
Input
Figure 7-5 DC-Coupled, Single-Ended
Input
- AC coupled differential and single-ended
input configurations required the resistor terminations (R2 and R3) to create the VCM at
each pin and resistor values must select to maintain greater than 150mV potential
difference between pin P and pin N.
- As an example, to create the VCM of
1.5V at pin P and 1.65V at pin N, with the 2.5V VCC, set R3 = 550Ω and R2 = 1kΩ
- For single-ended input
configuration, place R6 = 50Ω to avoid any reflection at complementary input pin.
- DC coupled differential and single-ended input configuration required to have the source
common-mode voltage matched with the device input common mode specifications.
- For single-ended input configuration,
keep the R1, R2, R3 and R4 resistors. This method creates the same common-mode voltage
at both pins, and the resistive dividers create 75Ω at pin P and 50Ω Thevenin's
equivalent at pin N.
- As an example, to have the
common-mode voltage of 1.35V at each pin, set the resistive divider components values
to R1 = 130Ω, R2 = 165Ω, R3 = 86.6Ω and R4 = 110Ω with the 2.5V VCC.