Datasheet

DS_1903_032 73M1903 Data Sheet
Rev. 2.1 41
R3
100 K
1
Rwtot 600
1200
where
Rwtot RW Rohswitch 2 Rbead
.
R2 100 K R3
Use 1% resistors for R1, R2, and R3.
To select the value for C1, make the zero at around 10 Hz.
1
2 π
.
100
.
K C1
.
10
C1
1
2 π
.
100
.
K 10
.
C1 0.15 uF
The blocking cap Cblock should also have the same frequency response, but due to the low impedance,
its value will be much higher, usually requiring a polarized cap. A blocking cap may also be needed on
the modem side of the transformer if the DC offset current of the transmit pins will exceed the current
rating for a dry transformer circuit.
Cblock
1
2 π
.
600
.
10
.
Cblock 27 uF
If you are using a Wet transformer design, as in the following figure:
The only difference is that the blocking capacitor, Cblock, it is removed. All other equations still hold true.
Trans-Hybrid Loss (THL)
Trans-Hybrid Loss is by definition the loss of transmit signal from Tip/Ring to the receive inputs on the
modem IC. This definition is only valid when driving a specific phone line impedance. In reality, phone
line impedances are never perfect, so this definition isn’t of much help. Instead, as an alternate definition
that helps in analysis for this modem design, THL is the loss from the transmit pins to the receive pins.