AD8138
BALANCED TRANSFORMER DRIVER
SIGNAL WILL BE COUPLED
ON THIS SIDE VIA C
STRAY
Transformers are among the oldest devices that have been used
to perform a single-ended-to-differential conversion (and vice
versa). Transformers also can perform the additional functions
of galvanic isolation, step-up or step-down of voltages and im-
pedance transformation. For these reasons, transformers will
always find uses in certain applications.
C
STRAY
500⍀
V
UNBAL
0.005%
PRIMARY
SECONDARY V
52.3⍀
DIFF
500⍀
0.005%
C
STRAY
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 (usu-
ally ground), while the other side is driven. This can cause prob-
lems in systems that require good balance of the transformer’s
differential output signals.
NO SIGNAL IS COUPLED
ON THIS SIDE
Figure 37. Transformer Single-Ended-to-Differential Con-
verter Is Inherently Imbalanced
499⍀
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 37.) The exact amount of
this imbalance will depend on the particular parasitics of the
transformer, but will mostly be a problem at higher frequencies.
C
STRAY
49.9⍀
499⍀
+IN
–IN
500⍀
0.005%
OUT–
V
UNBAL
AD8138
V
DIFF
499⍀
500⍀
0.005%
OUT+
49.9⍀
C
STRAY
499⍀
The balance of a differential circuit can be measured by con-
necting an equal-valued resistive voltage divider across the dif-
ferential outputs and then measuring the center point of the
circuit with respect ground. Since the two differential outputs
are supposed to be of equal amplitude, but 180 degrees opposite
phase, there should be no signal present for perfectly balanced
outputs.
Figure 38. AD8138 Forms a Balanced Transformer Driver
0
The circuit in Figure 37 shows a Minicircuits T1-6T trans-
former 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.
–20
V
, FOR TRANSFORMER
UNBAL
–40
–60
WITH SINGLE-ENDED DRIVE
The plots in Figure 39 show a comparison between the case
where the transformer is driven single-endedly by a signal gen-
erator and driven differentially using an AD8138. The top signal
trace of Figure 39 shows the balance of the single-ended con-
figuration, while the bottom shows the differentially driven
balance response. The 100 MHz balance is 35 dB better when
using the AD8138.
–80
V
, DIFFERENTIAL DRIVE
UNBAL
–100
0.3
1
10
100
500
FREQUENCY – MHz
Figure 39. Output Balance Error for Circuits of Figures 37
and 38
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 180 degrees 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 39.)
REV. A
–11–
ADI [ ADI ]
