
8
FN6287.0
December 18, 2006
Description of Operation and Application
Information
Product Description
The ISL55020 is a full differential Current Feedback
Amplifier (CFA) featuring wide bandwidth and low power.
The device contains a pair of high impedance differential
inputs and a pair of differential outputs. It can be used in any
combination of single/differential ended input/output
configurations. A wide bandwidth unity gain, common mode
amplifier with a 100MHz -3dB bandwidth (Figure 6) is
included to provide DC offset correction or common mode
signal injection to the differential output. The ISL55020 is
internally compensated for single-ended closed loop gain
(A
VS
), differential closed gain (A
VD
) of 2, or greater.
Connected in differential gain of 5 (single ended gain of ±2.5
and driving a 200
Ω
differential load, the ISL55020 has a -
3dB bandwidth of 300MHz. Driving a 200
Ω
differential load
at gain of 10, the bandwidth is about 200MHz (Figure 3). The
ISL55020 is available with a power down feature (EN) to
reduce the power while the amplifier is disabled.
Input, Output, and Supply Voltage Range
The ISL55020 is designed to operate with dual supplies over
a range of +/-2.25V to +/-6V and can also operate with a
single supply over the range of 4.5V to 12V. For single
supply operation, the V- and GND pins must be connected
together as close to the device as possible. The amplifiers
have an input common mode voltage range from -4.3V to
3.4V when operated from ±5V supplies. The differential
mode input range (DMIR) between the two inputs is from -
2.3V to +2.3V. The input voltage range at the VCM pin is
from -3.3V to 3.7V. If the input common mode or differential
mode signal is outside the above-specified ranges, the
output signal will be distorted.
The output of the ISL55020 can swing from -3.8V to +3.8V at
100
Ω
differential load at ±5V supply. As the load resistance
becomes lower, the output swing is reduced.
Single-ended, Differential and Common Mode Gain
Settings
The ISL55020 can be used as a single/differential ended to
differential/single converter. The voltage applied at VCM pin
sets the output common mode voltage and the common
mode gain is fixed at gain is one (A
VCM
= 1).
The output differential voltage is given by the following:
Where:
R
F1
= R
F2
= R
F
The differential output gain (A
VD
) is defined by the feedback
resistors according to the following
A
VD
= 1 + 2R
F
/R
G
The single ended output voltage (V
OS
) contains a common
mode component (V
CM
) and a differential mode component
equal to one-half the differential output (V
OD
/2)., and is
given by the following:
V
OS
= V
OD
/2 + V
CM
= V
CM
+(V
IN+
- V
IN-
) x (0.5 + R
F
/R
G
) (EQ. 3)
and the single-ended gain becomes:
A
VS
= 0.5+ R
F
/R
G
FIGURE 18. BASIC APPLICATION CIRCUIT
V
OUT
+
FB+
R
G
R
F2
I
N
+
I
N
-
FB-
V
IN
+
V
IN
-
R
F1
V
OUT
-
VCM
R
IN-
R
IN+
R
L+
R
L-
+1
OUT+
OUT-
R
T-
R
T-VCM
R
T+
V+
V-
V
CM
GND-
EN
V+
V-
GND-
EN
R
S+
R
S+
V
OD
= (V
IN+
- V
IN-
) x (1 + 2R
F
/R
G
)
(EQ. 1)
(EQ. 2)
(EQ. 4)
ISL55020