
ISL55211
12
FN7868.0
June 21, 2011
CM measurement circuit with a large signal (2VP-P) differential
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 (RG in
Figure
29) provide both a portion of the input termination
impedance and the gain element for the amplifier. For 50
systems, these RG resistors are too high with the turns ratios
shown in Figure 29 to provide the full match and an external RT resistor is required. This RT element goes away at the highest
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 75 example similar to Figure
2while Table
2 shows the necessary external R values and
resulting gains.
Here, the sum of the two internal RG resistors at the higher two
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 300 on the
secondary side of the transformer and the RT element goes to
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 75 case.
This input interface also simplifies the input common mode
control. The VCM pin controls the output common mode voltage.
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 RG
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
FIGURE 31. TEST CIRCUIT #3 COMMON MODE AC OUTPUT
MEASUREMENTS
50
1F
1:1.4
ISL55211
+3.3V
+
-
RG
VCM
500
100
ADT2-1T
1F
Vi
10k
PD
50
Output
VCM
50
VCM Input
1F
TABLE 2. EXTERNAL RESISTORS FOR A 75
Ω INPUT
IMPEDANCE DESIGN
ISL55211 INTENDED TRANSFORMER + INTERNAL GAIN SETTINGS
INPUT
XFMR
TURNS
RATIO
INTERNAL
RG VALUE
(
Ω)
GAIN
(V/V)
VO/VI
GAIN
(dB)
VO/VI
EXTERNAL
RT VALUE
(
Ω)
EXTERNAL
RS VALUE
(
Ω)
1:1.4
250
2.8
9
214
0
1:1.4
125
5.6
15
375
0
1:1.4
100
7
17
600
0
1:2
250
4
12
750
0
1:2
125
6.7
16.5
Open
25
1:2
100
6.7
16.5
Open
50
FIGURE 32. 75
Ω IMPEDANCE IMPLEMENTATIONS
+
-
500
RG
1:n
75 INPUT
ISL55211
Vi
VO
RT
1:1.4 -> CX2045NL
1:2 -> CX2032
RS