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Designer's Guide to the ISL55210-ABEVAL1Z Active
Balun Evaluation Board
Using the Active Balun Board to
Test Design Options
This relatively simple board is intended to provide a quick
means to test the performance of the active balun
configuration using the ISL55210. While all wideband, voltage
feedback (VFA), fully differential amplifiers (FDA) can perform
a single ended input to differential output operation, the
frequency span for acceptable input match (or low return loss)
is greatly enhanced by the >1.5GHz common mode loop
bandwidth internal to the ISL55210. The input and gain setting
elements for this board are very simple, while the output side
includes 3 possible output interfaces. As delivered, the
differential output is converted to single ended through a very
wideband transmission line transformer. This allows easy
response shape measurements with minimal transformer
rolloff effects. The two other output options include a 200Ω
load differential to single ended path for OIP3 measurements
with lighter loads and a differential 50Ω output path for direct
measurement of just the ISL55210 response using a 4 port
network analyzer.
While the board itself is completely flexible for input
impedance setting and gain by changing only 4 resistor
elements, the default circuit for the board as delivered
implements a 50Ω input match with a 16.4dB gain to the FDA
output pins. The simulation circuit for this configuration is
shown in Figure 1.
The operation of this circuit is well modeled using the Spice
model for the ISL55210 within the free iSim PE simulator
[1]
.
The board input is C2 while the output is the balun output pin.
Only +3.3V supply is required where the ISL55210 delivers
exceptional performance using only ≈34mA supply current.
The resistors here have been snapped to 1% standard values.
To get a desired input impedance matched to R
S
, and gain (Av)
from the input of R
g1
to the differential outputs, the exact
element values are given by these two simple equations
(Equations 1 and 2)
[2]
.
The two feedback resistors should be equal and set to:
Then R
g1
will be set to:
And R
g2
will be set to the sum of R
g1
and the R
S
element to
get balance in the differential feedback loop. The blocking
caps are set as necessary to pass the lowest frequency of
interest. Using this simple blocking cap approach places all the
DC operating voltages on the two input and output pins at the
default internal Vcm voltage = 1.2V for the ISL55210. This
circuit can implement a DC-coupled signal path if the supplies
are set to +2.5V and -1.2V to keep the I/O headroom
requirements satisfied but that is not supported by this board
FIGURE 1. iSim PE SIMULATION CIRCUIT FOR THE DEFAULT CONFIGURATION OF THE ACTIVE BALUN EVM
R
f
Av Av 4+()R
S
2Av 2+()
------------------------------------
=
(EQ. 1)
R
g1
R
S
1
A
v
2
------
+
----------------
=
(EQ. 2)
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Copyright Intersil Americas LLC 2013. All Rights Reserved.
1-888-INTERSIL or 1-888-468-3774
| Intersil (and design) is a trademark owned by Intersil Corporation or one of its subsidiaries.
All other trademarks mentioned are the property of their respective owners.
May 9, 2013
AN1831.1
Application Note 1831
Author: Michael Steffes
