ISL55210

11

FN7811.0

March 2, 2011

Most of the characterization curves start with Figure 28 then get

to hold an input match, or vary the loading. For load tests below

the 200Ω shown in Figure 28, a simple added shunt resistor is

placed across the output pins. For loads >200Ω, the series and

shunt load R's are adjusted to show that total load (including the

50Ω measurement load reflected through the 1:1 output

measurement port transformer) and provide an apparent 50Ω

differential source to that transformer. This output side

transformer is for measurement purposes only and is not

necessary for final applications circuits. There are output

interface designs that do benefit from a transformer as part of

the signal path, but the one shown at the right of Figure 28 is

used only for characterization to get a doubly terminated 50Ω

measurement path going differential to single ended.

Where just the amplifier is tested, a 4 port network analyzer is

used and the very simple test circuit of Figure 29 is

implemented. This is used to extract the differential S21 curves

and differential output impedance vs gain. Changing the gain is a

circuit depends on the two AC coupled source 50Ω of the 4 port

network analyzer and presents an AC coupled differential 100Ω

load to the amplifier as the input impedance of the remaining

two ports of the network analyzer.

Using this measurement allows the full small single bandwidth of

the ISL55210 to be exposed. Many of the other measurements are

using I/O transformers that are limiting the apparent bandwidth to

reduced level. Figure 16 shows a series of normalized differential

S21 curves for gains of 12dB to 30dB in 6dB steps. These are

1600Ω in 2X steps. The lowest gain of 12dB (4V/V) is showing a

2.2GHz small signal bandwidth. This response gets some

bandwidth extension due to phase margin <60degree effects, but

by the gain of 24dB (16V/V), the bandwidth is following a Gain

Bandwidth type characteristic showing 300MHz bandwidth or

>4GHz Gain Bandwidth Product (GBP).

The closed loop differential output impedance of Figure 17 is

simulated using Figure 29 in ADS. This shows a relatively low

output impedance (<1Ω through 100MHz) constant with signal

gain setting. Typical FDA outputs show a closed loop output

impedance that increases with signal gain setting. The ISL55210

holds a more constant response vs gain due to internal design

elements unique to this device.

Common mode output measurements are made using the circuit

Figure 30. Here, the outputs are summed together through two

100Ω resistors (still a 200Ω differential load) to a center point

where the average, or common mode, output voltage may be

sensed. This is coupled through a 1µF DC blocking capacitor and

measured using 50Ω test equipment. The common mode source

impedance for this circuit is the parallel combination of the

2Ω -100Ω elements, or 50Ω. Figure 18 uses this circuit to

control pin to the output common mode. This pin includes an

internal 50pF capacitor on the default bias network (to filter

supply noise when there is no connection to this pin) which

bandlimits the response to approximately 30MHz. This is far

lower than the actual bandwidth of the common mode loop.

Figure 19 uses this output CM measurement circuit with a large

mode conversion.

Single Supply, Input Transformer Coupled,

Design Considerations

The characterization circuit of Figure 28 shows one possible

input stage interface that offers several advantages. The

ISL55210 can also support a DC coupled differential to

differential or single ended input to differential requirement if

needed. Where AC coupling is adequate, the circuit of Figure 28

simplifies the input common mode voltage control. If the source

coming into this stage is single ended, the input transformer

provides a zero power conversion to differential. The two gain

impedance and the gain element for the amplifier. For minimum

input impedance. Since the ISL55210 is a VFA device, these

resistor values can be scaled up and down a bit more freely than

a current feedback based FDA.

FIGURE 29. TEST CIRCUIT #2 4-PORT S-PARAMETER

MEASUREMENTS

ISL55210

+3.3V

+

-

1/2 OF A 4-port

S-PARAMETER

PD

50

10k

50

50

50

1/2 OF A 4-PORT

S-PARAMETER

FIGURE 30. TEST CIRCUIT #3 COMMON MODE AC OUTPUT

MEASUREMENTS

1µF

1:1.4

ISL55210

+3.3V

+

-

ADT2-1T

1µF

PD

OUTPUT

100

100

10k

50

50

50

50

50

50

200

200