MT91600
C9 is inserted to ensure pin 6, Vref, remains at a.c. ground. 100 nF is recommended.
Data Sheet
I
LOOP
can also be set by directly driving Vref with a low impedance voltage source. (See Figure 4). It is
recommended that a small resistor be placed in series with the Vref pin. In this case:
I
LOOP
= 1.07 * Vs
(R1 +R2)
where, Vs < 0
2kΩ
Vs
6
V
REF
C9
100nF
MT91600
Figure 4 - Direct Voltage
Calculating Component Values For AC Transmission
There are five parameters a designer should know before starting the component calculations. These five
parameters are:
1)
2)
3)
4)
5)
characteristic impedance of the line Z
o
network balance impedance Z
NB
value of the feed resistors (R1 and R2)
2 W to 4 W transmit gain
4 W to 2 W receive gain
The following example will outline a step by step procedure for calculating component values. Given:
Z
o
= 600Ω, Z
NB
= 600Ω, R1=R2= 220Ω
Gain 2 - 4 = -1dB, Gain 4 - 2 = -1dB
Step 1: Gain Setting (R12, R13, R14, R15)
Gain 2 - 4 = 20 Log [ R13 / R12]
-1 dB = 20 Log [R13 / R12]
∴
R12 = 112.2kΩ, R13 = 100kΩ.
Gain 4 - 2 = 20 Log [0.891 * [R14 / R15)]
-1 dB = 20 Log [0.891 * [R14 / R15)]
∴
R14 = 100kΩ, R15 = 100kΩ.
7
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