ISL55211 Datasheet, PDF (10/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain, Differential Amplifier

ISL55211
Typical Performance Curves V_s+ = 3.3V, T_A ≈ +25°C, unless otherwise noted. (Continued)
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TEST CIRCUIT 1
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4
MAXIMUM DIFFERENTIAL V_P-P
OUTPUT USING DEFAULT V_CM
3
2
INTERNALLY SET V_CM
1
3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
SUPPLY VOLTAGE (V)
FIGURE 27. DEFAULT V_CM AND MAX V_OPP vs SUPPLY VOLTAGE
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44 TEST CIRCUIT 1
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T_A = +85°C
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40
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T_A = +25°C
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35
T_A = -40°C
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3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
SINGLE SUPPLY VOLTAGE (V)
FIGURE 28. SUPPLY CURRENT vs SUPPLY VOLTAGE
Applications
Basic Operation
The ISL55211 is a very wideband, voltage feedback based,
differential amplifier including an output common mode control
loop and optional power shutdown feature. Intended for very low
distortion differential signal driving, this internally fixed gain
device provides 3 possible gain settings by simply picking the
input side connections as shown in Table 1. Including internal
compensation, the ISL55211 holds a constant bandwidth over
gain settings. Most applications are intended for AC-coupled I/O
using a single 3.3V supply and an input transformer. The internal
resistor values have been scaled up slightly to require an external
termination element along with the two internal resistors where
a 50Ω differential input match is desired. This does increase the
output noise slightly but narrows up the input VSWR tolerance
and lowers the added loading of the feedback resistors
improving SFDR.
Where DC-coupled differential I/O operation is desired, the
ISL55211 can be connected directly to the source as long as the
internal input common mode range limits are observed (1.1V to
1.7V for a 3.3V single supply operation). For a DC-coupled, single
to differential requirement, consider the ISL55210. This device is
an external resistor version of the ISL55211 where the flexibility
in the external resistors will enable single to differential
operation. For a ground referenced input signal, this will require a
negative supply when using the ISL55210.
Most applications behave as a differential inverting op amp
design. There is therefore an input gain resistor on each side of
the inputs that must be driven. The 3 possible connections to the
two pairs of input pins will give a 100Ω, 125Ω, or 250Ω input
resistor on each side. Combined with the two input turns ratio's
shown in Table 1, gives a 9dB to 20dB operating gain range in
approximately 3dB steps.
The device can be powered down to < 400µA supply current
using the optional disable pin. To operate normally, this pin
should be asserted high using a simple logic gate to +V_CC or tied
high through a 10kΩ resistor to +V_CC. When disabled, the power
dissipation drops to < 1mW but, due to the inverting op amp type
architecture, the input signal will feed-forward through the
feedback and gain resistors giving limited isolation.
Application and Characterization Circuits
Test Circuit 1 of Figure 29 forms a starting point for many of the
characterization curves for the ISL55211. Since most lab sources
and measurement devices are single-ended, this circuit converts
to differential at the input through a wideband transformer and
would also be a typical application circuit coming from a
single-ended source. Assuming the source is a 50Ω impedance,
the internal R_G resistors and external R_T are set to provide both
the input termination and the gain. Since the inverting summing
nodes act as virtual ground points for AC signal analysis, the total
termination impedance across the input transformer secondary
will be (2*R_G)||R_T. Setting this equal to n²*R_S will give a
matched input impedance inside the bandwidth of the
transformer (where "n" is the turns ratio). The amplifier gain is
fixed by the selected input R_G element and the internal 500Ω
feedback resistors. While the ISL55211 is internally a Voltage
Feedback Design (VFA) to give the lowest possible noise, internal
compensation caps hold the bandwidth over gain setting
approximately constant at 1.4GHz. For wider small signal
bandwidth at lower gains, consider the ISL55210, which provides
>2.2GHz at a gain of 12dB.
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FN7868.0
June 21, 2011