ISL55210
11
FN7811.2
June 6, 2013
Most of the characterization curves start with Figure
28 then get
different gains by changing the feedback resistor, RF, use
different input transformers where then the RG is also adjusted
to hold an input match, or vary the loading. For load tests below
the 200 shown in Figure
28, a simple added shunt resistor is
placed across the output pins. For loads >200, the series and
shunt load R's are adjusted to show that total load (including the
50 measurement load reflected through the 1:1 output
measurement port transformer) and provide an apparent 50
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
used only for characterization to get a doubly terminated 50
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
simple matter of adjusting the two RF resistors of Figure 29. This circuit depends on the two AC coupled source 50 of the 4 port
network analyzer and presents an AC coupled differential 100
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
simply stepping two feedback resistor values (RF) up from 200 to
1600 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 (<1 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
100 resistors (still a 200 differential load) to a center point
where the average, or common mode, output voltage may be
sensed. This is coupled through a 1F DC blocking capacitor and
measured using 50 test equipment. The common mode source
impedance for this circuit is the parallel combination of the
2 -100 elements, or 50. Figure
18 uses this circuit to
measure the small and large signal response from the VCM
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
signal (2VP-P) differential output voltage (generated through the
Vi path of Figure 30) to measure the differential to common 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
28simplifies 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
resistors (RG in Figure 28) provide both the input termination impedance and the gain element for the amplifier. For minimum
noise, only RG should be used and set to achieve the desired
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
+
-
VCM
RF
1/2 OF A 4-port
S-PARAMETER
PD
50
10k
50
1/2 OF A 4-PORT
S-PARAMETER
FIGURE 30. TEST CIRCUIT #3 COMMON MODE AC OUTPUT
MEASUREMENTS
1F
1:1.4
ISL55210
+3.3V
+
-
VCM
ADT2-1T
1F
Vi
PD
OUTPUT
VCM
VCM INPUT
100
10k
50
200