isl55211 Intersil Corporation, isl55211 Datasheet - Page 12

• Manufacturer Part Number: isl55211
• Description: Wideband, Low Noise, Low Distortion, Fixed Gain, Differential Amplifier
• Manufacturer: Intersil Corporation

CM measurement circuit with a large signal (2V
output voltage (generated through the Vi path of Figure 31) to
measure the differential to common mode conversion - often
called the "Output Balance Error" for an FDA.
Single Supply, Input Transformer Coupled,
Design Considerations
The characterization circuit of Figure 29 shows one possible
input stage interface that offers several advantages. Where AC
coupling is adequate, the circuit of Figure 29 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
Figure 29) provide both a portion of the input termination
impedance and the gain element for the amplifier. For 50Ω
systems, these R
shown in Figure 29 to provide the full match and an external R
resistor is required. This R
gain setting using a 1:2 input turns ratio transformer.
It is also possible to adapt this circuit to other input characteristic
impedances. Figure 32 shows a 75Ω example similar to Figure 2
while Table 2 shows the necessary external R values and
resulting gains.
FIGURE 31. TEST CIRCUIT #3 COMMON MODE AC OUTPUT
V
50
i
1µF
ADT2-1T
V
CM
1:1.4
Input
MEASUREMENTS
50
G
resistors are too high with the turns ratios
1µF
V
R
R
T
CM
G
G
element goes away at the highest
12
+
+3.3V
-
500
10k
500
ISL55211
PD
P-P
Output
) differential
V
100
100
CM
G
ISL55211
1µF
in
50
T
Here, the sum of the two internal R
gain settings is too low to retain a match for the 1:2 input step up
case. There, a pair of external series resistors are added to get
the total differential input impedance up to 300Ω on the
secondary side of the transformer and the R
infinity. These two conditions are not particularly useful but
Figure 32 shows how to implement the full range of internal
conditions with the two turns ratios considered in Table 2.
Figure 32 also shows a pair of alternate input transformer types
from Pulse Engineering particularly suitable to the 75Ω case.
This input interface also simplifies the input common mode
control. The V
In most DC-coupled FDA applications, the input common mode
voltage is determined by both this output common mode and the
source signal. In a configuration like Figure 29, there is no path
for a common mode current to flow from output to input, so the
input common mode voltage equals the output. A similar effect
could be achieved with just two blocking caps on the two R
resistors. A DC-coupled, single to differential, configuration will
also have a common mode input that is moving with the input
signal. Converting to just a differential signal at the amplifier, as
in Figure 29, removes any input signal related artifacts from the
input common mode making the ISL55211 behave as a
differential only VFA amplifier. There is only a very small
differential error signal at the inputs set by the loop gain, as in a
75 INPUT
TURNS
ISL55211 INTENDED TRANSFORMER + INTERNAL GAIN SETTINGS
INPUT
XFMR
RATIO
1:1.4
1:1.4
1:1.4
Vi
TABLE 2. EXTERNAL RESISTORS FOR A 75Ω INPUT
1:2
1:2
1:2
1:1.4 -> CX2045NL
1:2 -> CX2032
FIGURE 32. 75Ω IMPEDANCE IMPLEMENTATIONS
1:n
IMPEDANCE DESIGN
INTERNAL
R
CM
G
250
125
100
250
125
100
VALUE
(Ω)
pin controls the output common mode voltage.
R
T
R
R
S
S
V
GAIN
(V/V)
2.8
5.6
6.7
6.7
O
7
4
/V
I
R
G
R
G
G
V
GAIN
16.5
16.5
(dB)
O
15
17
12
resistors at the higher two
9
ISL55211
/V
500
500
I
+
-
EXTERNAL
R
T
T
element goes to
Open
Open
214
375
600
750
VALUE
(Ω)
June 21, 2011
EXTERNAL
R
S
FN7868.0
VALUE
(Ω)
25
50
0
0
0
0
G
V
O