REV. E

AD8138

–11–

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.

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.

BALANCED TRANSFORMER DRIVER

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.

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.

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.

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 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-

ac common-mode voltage, is a measure of how closely the outputs

are balanced.

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

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 180 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).

PRIMARY

NO SIGNAL IS COUPLED

ON THIS SIDE

SIGNAL WILL BE COUPLED

52.3

500

0.005%

500

0.005%

Figure 3. Transformer Single-Ended-to-Differential

Converter Is Inherently Imbalanced

500

0.005%

500

0.005%

AD8138

+IN

–IN

499

OUT+

OUT–

499

499

499

49.9

49.9

Figure 4. AD8138 Forms a Balanced Transformer Driver

FREQUENCY – MHz

0

0.3

500

110

100

–20

–40

–60

–80

–100

WITH SINGLE-ENDED DRIVE

Figure 5. Output Balance Error for Circuits of

Figures 3 and 4