Differential active splitter for ATE or RF
signal distribution
Michael Steffes - January 14, 2013
Title-3
It is sometimes required to take a single ended or differential AC source and distribute it to more
than one DUT or other element. One example would be to distribute a differential AC test signal
being generated in an ATE environment to either multiple test sites or ports of mult-channel DUT’s.
Often these resources might be limited, and distributing them differentially will retain the balanced
signal path required to retain the HD2 suppression in the original signal.
While numerous RF passive and active splitters are available, these might not match the necessary
frequency band or have limited gain settings. The simple solutions shown here will terminate a
differential or single ended source while driving multiple output paths with exceptional load
isolation. The approach can also provide fine scale gain matching with 1dB response flatness to
>200Mhz. The combination of an input balun, and very wideband Fully Differential Amplifiers
(FDA’s), can provide customizable active splitters for ATE or communications applications.
AC or RF Signal Distribution Options
Simple passive splitters are widely available. These can provide the widest bandwidth but often
require well-defined loads on each output, and they always come at the cost of some insertion loss.
The simplest 50Ω single ended splitter is shown in figure 1. If every port is terminated in 50Ω, all
three ports will see 50Ω source impedances at the cost of a 6dB insertion loss from the input of R2 to
each load (12dB loss from AC1 to each load).
Figure 1. Example 50Ω splitter as drawn for a network analyzer response measurement.
A wide range of RF active splitters are also available, particularly for the CATV applications. These
tend to be single ended with minimal gain from the input to parallel outputs. These also normally
prefer each output be terminated in the specified impedance to deliver the intended signal
distribution to all ports (ref. 1).
Extending the inverting summing design to transformer coupled FDA splitters
A generally useful circuit using FDA’s is to couple through an input balun to the two terminating
gain resistors (Rg) as shown in figure 2. In this implementation, the 2-Rg resistors feed into a
differential virtual ground due the high differential loop gain of a wideband FDA devices like the
ISL55210 (ref.2). The balun gives a zero power single to differential conversion while the FDA
provides an input termination, gain, and isolation for the input impedance to the output load. It is
normally best to leave any balun centertap unconnected in this genre of application circuit.
Figure 2. Single channel, differential FDA with Balun input.
2*Rg forms the output side termination impedance for the input balun. Assuming those are fixed at
the required impedance to achieve an input match at Vi, the overall gain can then be adjusted by
changing the Rf elements. Neglecting the transformer insertion loss, the voltage gain from the input
of the balun to the differential output pins is shown in figure 2. This topology also offers numerous
noise figure and harmonic distortion suppression advantages (ref. 3&4)
Since wideband FDA’s also provide nearly zero ohm output impedance, the load does not enter into
this input matching giving a path to exceptional load isolation if Fig. 2 is adapted to parallel FDA’s.
One direct way to provide multiple output paths is to simply fan out the differential signal at the
output of Fig. 2 to multiple loads.
Often, these loads are doubly terminated transmission lines on ATE boards where that loading will
then add up in parallel. While this can work in some applications, the heavy loading of parallel loads
will rapidly degrade the harmonic distortion of the signal delivered to the parallel loads. As an
option, consider splitting the signal on the input side with independent FDAs dedicated to each line.
A differential source
The source in Figure 2 could also be a differential source where now the balun continues to provide
DC isolation, some signal gain, and the ability to form a signal splitting into multiple FDA paths on
its outputs. Very similar to an inverting summing design with a simple op amp, parallel FDA’s can be
attached on the output side of the balun.
To continue providing an input match, the Rg resistors for each port just need to be scaled up to
where the parallel combination of each ports’ 2*Rg gets back to the necessary termination
impedance. Consider a modest step up turns ratios in the balun, figure 3 shows an example using a
1:1.41 turns ratio and a 2-port splitter configuration.
Click on figure to enlarge
Figure 3. Example 1X2 differential active splitter with 6dB gain to matched loads.
There are several pieces to the response shape through this circuit. The source is shown as a voltage
source, but in ATE or communications systems, this is more often a complementary current source
output from a high speed DAC with shunt 50Ω output impedance on each leg. The circuit above is
equivalent for the transformer and all 10Meg elements are simulation artifacts to get DC operating
points (ref.5).
Since the source and load are shifted up to 100Ω across the 50Ω specified Coilcraft transformer, the
F-3dB passband for the PBW-2-WL used here will shift up 2X from its specified 200kHz to 400Mhz
span. The 2 - FDA paths each show 400Ω differential input impedance to the virtual ground of their
summing junctions. That combined 200Ω termination correctly matches this 1:2 ohms ratio balun
from a 100Ω source and gives the response shape to the transformer output of Figure 4 (ref. 6). This
higher source and load impedance across the balun give a lower F-3dB at 400kHz with an upper cutoff
F-3dB slightly over 800Mhz(ref. 7).
The response of Figure 4 shows a very wideband flat region for this necessarily AC coupled signal
path with 3dB gain to the balun output. Using the transformer to split the signal paths on its
secondary is a good way to handle DC common mode issues as well. A DC-coupled design could also
be delivered using this approach if all the DC I/O common mode range issues for the FDA are
carefully considered.
Figure 4. Response to the Balun output (using a 2X input signal to refer gain to blocking
cap inputs)
The ISL55210 amplifier is itself very broadband and benefits from the reflected source impedance to
lower its noise gain. From the output of the transformer to each differential FDA output, an
extremely flat 9dB gain response with 690MHz F-3dB is shown in Figure 5. The upper red curve is
from the transformer output to the differential FDA output, with the response across the transformer
the lower green curve. The 8pF capacitors on each of the inverting summing junctions act to extend
the FDA bandwidth in this application. This technique is suitable for Voltage Feedback Amplifier
(VFA) based FDA’s, but not for Current Feedback Amplifier (CFA) types.
Figure 5. FDA response for this 9dB gain setting.
The two responses of figure 5 add to 12dB midband gain where an output matching loss will take the
final gain to 6dB as shown in Figure 6. The outputs here were set up to be a differential 50Ω source
through blocking caps to a 50Ω load. This is of course completely flexible and moving to lighter loads
will improve the distortion and possibly bandwidth beyond this 400kHz to 500Mhz overall F-3dB span.
The 0.5dB flatness span is from 1MHz to 200MHz in this example design. Different bandpass
frequencies and gains can certainly be delivered selecting different baluns and FDA gains.
Figure 6. Overall response to the matched load at each of the two output ports.
Some of the unique benefits offered by this simple input splitter approach include –
1. Independent gain setting between the channels. Each channel can have different pairs of Rf
elements to either tune out transformer insertion loss, or deliver different gains to each path.
That adjustment will impact the bandwidth for that path, but will be transparent to the other
paths.
2. Complete load isolation. The load on one port could be completely different, and/or switching in
and out under ATE control, and the other paths will be ignorant of that effect.
3. Easily expandable beyond the 1X2 shown here. Staying with the 1:1.4 transformer from 100Ω
differential source, going to three paths simply means each pair of input resistors to the FDA goes
to 300Ω (with the Rf scaled accordingly), 4 port is 400Ω. There will be some limit to this, as very
high Rf elements will start to run into 2nd order bandlimiting effects. This is also why higher
turns ratios at the input should be used with caution if higher frequency applications are
intended.
4. Using an input balun allows this approach to use single ended sources and convert to differential.
Developing a good signal split at the input requires each FDA be used in a balanced differential
I/O mode to see just the differential virtual grounds on the output side of the Rg resistors with no
common mode loop activity.
5. CFA based designs can also be done if the Rf element is picked first for the intended FDA and the
remaining design worked backwards from there to the source.
Given the large range of FDAs, using different supply voltages and giving different frequency spans,
and an even larger range of wideband baluns, this basic approach can be used across many different
applications to distribute identical differential signals where needed. It is also possible to drop this
back to an even simpler approach when the source is differential.
Eliminating the balun when the source is differential
Eliminating the balun when the source is differential
If the application already has a differential source to distribute, the parallel FDAs can also use just a
blocking cap to distribute a differential AC source signal. Figure 7 shows a 6dB gain to the matched
load example using a complementary current source DAC output model as the input signal.
Click on figure to enlarge
Figure 7. Active differential splitter with blocking cap input coupling.
All of the FDAs that might be applied to these circuits have an output common mode voltage control.
When the source is either AC coupled through capacitors, or an input transformer, that output Vcm
will also show up as the input pin common mode voltages.
As long as there is no DC path to ground in the signal paths (leave that centertap floating), that
voltage will also show up at the output side of the input capacitors or the input transformer. Figure 7
shows the default internal 1.2Vcm set up by the ISL55210 as the voltage probe at the output side of
C2.
Here, each side of the two FDA paths needs to present the same input impedance (50Ω here) to get
matching for the source. That is again accomplished as the parallel combination of the Rg resistors
into each differential virtual ground at the FDA inputs. Without the step up transformer of figure 3,
these resistors scale down but the gain in the FDA needs to increase 3dB to still achieve a net 6dB
gain to a doubly terminated output load. Using these lower resistor values, and no input
transformer, actually expands the bandwidth for the ISL55210 (even using this slightly higher gain)
as shown in figure 8.
Figure 8. Response to load from differential input voltage
Using this simpler approach, and the 10pF capacitors on the inverting nodes to ground, the F-3dB
bandwidth is 1Ghz with -.5dB span from 125kHz to 650Mhz. This is not including the self resonance
of the 0.1uF blocking caps and adding 100pF caps in parallel would be recommended for very
broadband performance.
This approach might be more expandable to >2 differential outputs than the transformer approach
as the resistor values start out lower. Since the stimulus is already a current source in this
simulation, plotting the linear Vin will give the apparent input impedance over frequency. That should
be close to 50Ω considering the shunt R’s in parallel with the signal splitter input impedance. This
will remain close to 50Ω depending on the loop gain of the FDA selected. Using a very wideband
device like the ISL55210, gives an input impedance holding a close match to 50Ω over the wide span
shown in figure 9.
Figure 9. Input impedance of the capacitive coupled differential FDA active splitter of
figure 7
Summary and conclusions
With the emergence of the FDA from several suppliers over the last 10 years, some of the legacy op
amp circuits can be adapted to new and useful applications. Here, the excellent differential input
impedance characteristic of wideband FDAs was exploited to split a single input source to multiple
paths while retaining almost exact match to the source impedance. If the original signal is single
ended, an input balun offers an easy conversion to differential for subsequent splitting and
amplification. If the source is already differential, either a balun or capacitive coupled approach to
the parallel FDAs may be used. The examples here have been AC coupled to simplify the common
mode voltage shifts through the circuit. DC coupled designs could also be done with careful
attention to common mode issues. With FDA’s available from 3.3V to 30V total supply and from
100Mhz to >5GHz bandwidths, a designer can probably find a solution for differential active
splitting in either ATE or communications requirements using the simple techniques shown here.
About the author
Michael Steffes With 27years involvement in high speed amplifier design, applications, and
marketing, Michael Steffes has introduced over 80 products spanning 5 companies while publishing
>40 contributed articles. Current focus is on high efficiency high speed ADC interfaces, DSL/PLC
line interface solutions, and online design tool development.
References
1.
2.
3.
4.
5.
For example, see the Macom active splitters for CATV.
ISL55210 Wideband, Low Power, Ultra-High Dynamic Range Differential Amplifier;
Accurately predict noise figure for transformer coupled differential amplifiers (Part 1)
Accurately predict noise figure for transformer coupled differential amplifiers (Part 2)
These circuits are simulated in Intersil’s free downloadable spice and power simulation package,
iSim PE (registration required).
6. Contact the author for an easy balun modeling technique
7. Extending the flatness frequency span for wideband baluns