AD8138
BALANCED TRANSFORMER DRIVER
Transformers are among the oldest devices used to perform a
single-ended-to-differential conversion (and vice versa). Trans-
formers can also perform the additional functions of galvanic
isolation, step-up or step-down of voltages, and impedance
transformation. For these reasons, transformers always find
uses in certain applications.
The well-balanced outputs of the AD8138 provide a drive signal
to each of the transformer’s primary inputs that are of equal
amplitude and 180° out of phase. Therefore, depending on how
the polarity of the secondary is connected, the signals that
conduct across the interwinding capacitance either both assist
the transformer’s secondary signal equally, or both buck the
secondary signals. In either case, the parasitic effect is
symmetrical and provides a well-balanced transformer output
(see Figure 45).
However, when driving the transformer in a single-ended
manner, there is an imbalance at the output 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.
SIGNAL IS COUPLED
ON THIS SIDE VIA C
STRAY
C
STRAY
500Ω
0.005%
V
UNBAL
PRIMARY
SECONDARY V
52.3Ω
DIFF
500Ω
0.005%
If the interwinding capacitance (CSTRAY) is assumed to be
uniformly distributed, a signal from the driving source couples
to the secondary output terminal that is closest to the primary’s
driven side. On the other hand, no signal is coupled to the
opposite terminal of the secondary because its nearest primary
terminal is not driven (see Figure 43). The exact amount of this
imbalance depends on the particular parasitics of the trans-
former, but is mostly a problem at higher frequencies.
C
STRAY
NO SIGNAL IS COUPLED
ON THIS SIDE
Figure 43. Transformer Single-Ended-to-Differential Converter Is Inherently
Imbalanced
499Ω
C
STRAY
49.9Ω
OUT–
The balance of a differential circuit can be measured by
499Ω
+IN
–IN
500Ω
V
0.005%
UNBAL
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 180° opposite phase,
there should be no signal present for perfectly balanced outputs.
AD8138
OUT+
V
DIFF
499Ω
500Ω
0.005%
49.9Ω
C
STRAY
499Ω
Figure 44. AD8138 Forms a Balanced Transformer Driver
The circuit in Figure 43 shows a Mini-Circuits® T1-6T
transformer connected with its primary driven single-endedly
and the secondary connected with a precision voltage divider
across its terminals. The voltage divider is made up of two
500 Ω, 0.005% precision resistors. The voltage VUNBAL, which is
also equal to the ac common-mode voltage, is a measure of how
closely the outputs are balanced.
0
–20
V
, FOR TRANSFORMER
UNBAL
WITH SINGLE-ENDED DRIVE
–40
Figure 45 compares the transformer being driven single-
endedly by a signal generator and being driven differentially
using an AD8138. The top signal trace of Figure 45 shows the
balance of the single-ended configuration, while the bottom
shows the differentially driven balance response. The 100 MHz
balance is 35 dB better when using the AD8138.
–60
–80
V
, DIFFERENTIAL DRIVE
100 500
UNBAL
–100
0.3
1
10
FREQUENCY (MHz)
Figure 45. Output Balance Error for Circuits of Figure 43 and Figure 44
Rev. F | Page 20 of 24
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