ISL55210 Datasheet, PDF (11/18 Pages) Intersil Corporation – Wideband, Low-Power, Ultra-High Dynamic Range Differential Amplifier

ISL55210
Most of the characterization curves start with Figure 28 then get
different gains by changing the feedback resistor, R_F, use
different input transformers where then the R_G is also adjusted
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
simple matter of adjusting the two R_F resistors of Figure 29. This
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.
+3.3V
RF
10k
50
1/2 OF A 4-port
VCM
S-PARAMETER
+
PD
ISL55210
50
1/2 OF A 4-PORT
S-PARAMETER
-
50
50
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
measure the small and large signal response from the V_CM
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 (2V_P-P) differential output voltage (generated through the
V_i path of Figure 30) to measure the differential to common
mode conversion.
+3.3V
50
Vi
1:1.4
1µF
50
ADT2-1T
50
VCM INPUT
200
10k
+
PD
VCM _ISL55210
100
OUTPUT
VCM
-
100
200
1µF
50
50
50
RF
FIGURE 29. TEST CIRCUIT #2 4-PORT S-PARAMETER
MEASUREMENTS
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 (R_F) up from 200Ω to
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
FIGURE 30. TEST CIRCUIT #3 COMMON MODE AC OUTPUT
MEASUREMENTS
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
resistors (R_G in Figure 28) provide both the input termination
impedance and the gain element for the amplifier. For minimum
noise, only R_G 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.
11
FN7811.0
March 2, 2011