73M1903 Data Sheet
DS_1903_032
Appendix A – 73M1903 DAA Resistor Calculation Guide
The following procedure can be used to approximate the component values for the DAA. The optimal
values will be somewhat different due to the effects of the reactive components in the DAA (this is a DC
approximation). Simulations with the reactive components accurately modeled will yield optimal values.
The procedures for calculating the component values in the DAA are as follows. First set up R1. The
DAA should be designed to reflect 600 Ω when looking in at TIP/RING. If the transformer is 1 to 1, the
holding coil and ring detect circuit are high impedance, Cblock is a high value so in the frequency band of
interest it is negligible, the sum of R2 and R3 is much greater than R1, and the output impedance of the
drivers driving TXAP/TXAN are low then:
.
.
Rin 2 R1 RW Rohswitch 2 Rbead
RW is the sum of the winding resistance of both sides of the transformer. Measure each side of the
transformer with an Ohmmeter and sum them.
Rohswitch is the on resistance of the Off Hook Switch. Mechanical Relay switches can be ignored, but
Solid State Relays sometimes have an appreciable on resistance.
Rbead is the DC resistance of whatever series RF blocking devices may be in the design.
For Rin equal to 600 Ω:
.
600 RW Rohswitch 2 Rbead
R1
2
To maximize THL (Trans-Hybrid Loss), or to minimize the amount of transmit signal that shows up back
on the Receive pins. The RXAP/RXAN pins get their DC bias from the TXAP/TXAN pins. By capacitively
coupling the R3 resistors with the C1 caps, the DC offset can be minimized from the TXAP/TXAN to the
RXAP/RXAN because the DC offset will be divided by the ratio of the R1 resistors to the winding
resistance on the one side of the transformer.
Next make the sum of R2 + R3 much higher than 600 Ω. Make sure they are lower than the input
impedance of the RXAP/RXAN pins; otherwise they can move the frequency response of the input filter.
So let R2 + R3 = 100K.
38
Rev. 2.0
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
