AD8138
The power supply pins should be bypassed as close as possible
to the device to the nearby ground plane. Good high frequency
ceramic chip capacitors should be used. This bypassing should
be done with a capacitance value of 0.01 mF to 0.1 mF for each
supply. Further away, low frequency bypassing should be provided
with 10 mF tantalum capacitors from each supply to ground.
shows the differentially driven balance response. The 100 MHz
balance is 35 dB better when using the AD8138.
The well-balanced outputs of the AD8138 will provide a drive
signal to each of the transformer’s primary inputs that are of equal
amplitude and 180Њ out of phase. Thus, depending on how the
polarity of the secondary is connected, the signals that conduct
across the interwinding capacitance will either both assist the
transformer’s secondary signal equally, or both buck the secondary
signals. In either case, the parasitic effect will be symmetrical
and provide a well balanced transformer output (see Figure 5).
The signal routing should be short and direct to avoid parasitic
effects. Wherever there are complementary signals, a symmetrical
layout should be provided to the extent possible to maximize the
balance performance. When running differential signals over a
long distance, the traces on the PCB should be close together or
any differential wiring should be twisted together to minimize
the area of the loop that is formed. This will reduce the radiated
energy and make the circuit less susceptible to interference.
SIGNAL WILL BE COUPLED
ON THIS SIDE VIA C
STRAY
C
STRAY
500⍀
V
UNBAL
0.005%
BALANCED TRANSFORMER DRIVER
PRIMARY
SECONDARY V
52.3⍀
DIFF
500⍀
0.005%
Transformers are among the oldest devices used to perform a
single-ended-to-differential conversion (and vice versa). Trans-
formers also can perform the additional functions of galvanic
isolation, step-up or step-down of voltages, and impedance
transformation. For these reasons, transformers will always find
uses in certain applications.
C
STRAY
NO SIGNAL IS COUPLED
ON THIS SIDE
Figure 3. Transformer Single-Ended-to-Differential
Converter Is Inherently Imbalanced
However, when driving a transformer single-endedly and then
looking at its output, there is a fundamental imbalance due to the
parasitics inherent in the transformer. The primary (or driven) side
of the transformer has one side at dc potential (usually ground),
while the other side is driven. This can cause problems in systems
that require good balance of the transformer’s differential output
signals.
499⍀
C
STRAY
49.9⍀
499⍀
+IN
–IN
500⍀
0.005%
OUT–
V
UNBAL
If the interwinding capacitance (CSTRAY) is assumed to be uni-
formly distributed, a signal from the driving source will couple
to the secondary output terminal that is closest to the primary’s
driven side. On the other hand, no signal will be coupled to the
opposite terminal of the secondary because its nearest primary
terminal is not driven (see Figure 3). The exact amount of this
imbalance will depend on the particular parasitics of the trans-
former, but will mostly be a problem at higher frequencies.
AD8138
V
DIFF
499⍀
500⍀
0.005%
OUT+
49.9⍀
C
STRAY
499⍀
Figure 4. AD8138 Forms a Balanced Transformer Driver
The balance of a differential circuit can be measured by connecting
an equal-valued resistive voltage divider across the differential
outputs and then measuring the center point of the circuit with
respect to ground. Since the two differential outputs are supposed
to be of equal amplitude, but 180Њ opposite phase, there should
be no signal present for perfectly balanced outputs.
0
–20
V
, FOR TRANSFORMER
UNBAL
–40
–60
WITH SINGLE-ENDED DRIVE
The circuit in Figure 3 shows a Minicircuits T1-6T transformer
connected with its primary driven single-endedly and the second-
ary connected with a precision voltage divider across its terminals.
The voltage divider is made up of two 500 W, 0.005% preci-
sion resistors. The voltage VUNBAL, which is also equal to the
ac common-mode voltage, is a measure of how closely the outputs
are balanced.
–80
V
, DIFFERENTIAL DRIVE
UNBAL
–100
The plots in Figure 5 compare the transformer being driven
single-endedly by a signal generator and being driven differen-
tially using an AD8138. The top signal trace of Figure 5 shows
the balance of the single-ended configuration, while the bottom
0.3
1
10
100
500
FREQUENCY – MHz
Figure 5. Output Balance Error for Circuits of
Figures 3 and 4
REV. E
–11–
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