THS6212 Datasheet, PDF (32/48 Pages) Texas Instruments – Differential, Line-Driver Amplifier

THS6212
SBOS758 – MAY 2016
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Typical Applications (continued)
The two back-termination resistors (R_M = 10 Ω, each) added at each terminal of the transformer make the
impedance of the amplifier match the impedance of the line, and also provide a means of detecting the received
signal for the receiver. The value of these resistors (R_M) is a function of the line impedance and the transformer
turns ratio (n), given by Equation 7:
R_M =
Z_LINE
2n²
(7)
8.2.2.2.1 Line Driver Headroom Requirements
The first step in a transformer-coupled, twisted-pair driver design is to compute the peak-to-peak output voltage
from the target specifications. This calculation is done using Equation 8 to Equation 11:
P_L = 10 ´ log
V_RMS2
(1 mW) ´ R_L
where
• P_L = power at the load
• V_RMS = voltage at the load
• R_L = load impedance
(8)
These values produce the following:
V_RMS =
(1 mW) ´ R_L ´ 10
P_L
10
(9)
V_P
=
Crest
Factor
´
V
RMS
=
CF
´
V
RMS
where
• V_P = peak voltage at the load
• CF = crest factor
(10)
V
LPP
=
2
´
CF
´
V
RMS
where
• V_LPP = peak-to-peak voltage at the load
(11)
Consolidating Equation 8 to Equation 11 allows the required peak-to-peak voltage at the load to be expressed as
a function of the crest factor, the load impedance, and the power at the load, as given by Equation 12:
V_LPP = 2 ´ CF ´
(1 mW) ´ R_L ´ 10
P_L
10
(12)
V_LPP is usually computed for a nominal line impedance and can be taken as a fixed design target.
The next step in the design is to compute the individual amplifier output voltage and currents as a function of
peak-to-peak voltage on the line and transformer-turns ratio.
When this turns ratio changes, the minimum allowed supply voltage also changes. The peak current in the
amplifier output is given by Equation 13:
±I_P =
1
2
´
2 ´ V_LPP
n
´
1
4 R_M
where
• V_PP is as defined in Equation 12, and
• R_M is as defined in Equation 7 and Figure 85
(13)
32
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