AD8138ARZ-R7 datasheet(20/24 Pages) AD

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AD8138

Rev. F | Page 20 of 24

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.

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.

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.

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 180° opposite phase,

there should be no signal present for perfectly balanced outputs.

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.

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.

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).

PRIMARY

CSTRAY

52.3Ω

SECONDARY VDIFF

500Ω

0.005%

500Ω

0.005%

VUNBAL

SIGNALIS COUPLED

ON THIS SIDE VIA CSTRAY

NO SIGNAL IS COUPLED

ON THIS SIDE

Figure 43. Transformer Single-Ended-to-Differential Converter Is Inherently

Imbalanced

VDIFF

VUNBAL

AD8138

+IN

–IN

OUT+

OUT–

499Ω

49.9Ω

500Ω

0.005%

500Ω

0.005%

CSTRAY

Figure 44. AD8138 Forms a Balanced Transformer Driver

FREQUENCY (MHz)

–20

–40

–60

–80

–100

0.3

100

500

VUNBAL, DIFFERENTIAL DRIVE

VUNBAL, FOR TRANSFORMER

WITH SINGLE-ENDED DRIVE

Figure 45. Output Balance Error for Circuits of Figure 43 and Figure 44

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