ISL55211_14 Datasheet, PDF (12/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain Differential Amplifier

ISL55211
CM measurement circuit with a large signal (2V_P-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.
+3.3V
10k
RG
+
Vi
ADT2-1T
1:1.4
1µF
1µF
50
VCM
PD
500
100
Output
VCM
1µF
50
-
RG
500
100
VCM Input
ISL55211
50
FIGURE 31. TEST CIRCUIT #3 COMMON MODE AC OUTPUT
MEASUREMENTS
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_G in
Figure 29) provide both a portion of the input termination
impedance and the gain element for the amplifier. For 50Ω
systems, these R_G resistors are too high with the turns ratios
shown in Figure 29 to provide the full match and an external R_T
resistor is required. This R_T 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 75Ω example similar to Figure 2
while Table 2 shows the necessary external R values and
resulting gains.
ISL55211
500
R_S
R_G
+
Vi
75 INPUT
R_T
V_O
1:n
R_S
1:1.4 -> CX2045NL
1:2 -> CX2032
-
R_G
500
FIGURE 32. 75Ω IMPEDANCE IMPLEMENTATIONS
Here, the sum of the two internal R_G 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 300Ω on the
secondary side of the transformer and the R_T 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 75Ω case.
TABLE 2. EXTERNAL RESISTORS FOR A 75Ω INPUT
IMPEDANCE DESIGN
ISL55211 INTENDED TRANSFORMER + INTERNAL GAIN SETTINGS
INPUT
XFMR
TURNS
RATIO
1:1.4
INTERNAL
R_G VALUE
(Ω)
250
GAIN
(V/V)
V_O/V_I
2.8
GAIN
(dB)
V_O/V_I
9
EXTERNAL EXTERNAL
R_T VALUE R_S VALUE
(Ω)
(Ω)
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
This input interface also simplifies the input common mode
control. The V_CM 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 R_G
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
12
FN7868.0
June 21, 2011