ISL55210 Datasheet, PDF (12/18 Pages) Intersil Corporation – Wideband, Low-Power, Ultra-High Dynamic Range Differential Amplifier

ISL55210
For instance, if a minimum noise configuration is not required,
but it is desirable to increase the feedback resistors to reduce the
added loading they present to the output stage, the R_G and R_F
resistors can be scaled up to achieve the same gain with an
additional termination resistance added across the input
transformer to adjust the termination impedance. Figure 31
shows an example using a 1:2 input turns ratio where the R_G and
R_F elements have been scaled up and a shunt termination
resistance added. This example provides a single to differential
signal gain of 20dB and input impedance of 50Ω to the source.
The 1:2 turn ratio transformer needs a 200Ω differential
secondary impedance to provide an input side 50Ω match. This is
provided here by the parallel combination of the 2Ω - 200Ω R_G
resistors and the 400Ω parallel impedance at the transformer
secondary.
+3.3V
1k
RF
50
1:2
200
+
RG
Vi ^1µF
400
VCM
ISL55210
VO
ADT4-
1WT
RG
-
200
RF
1k
FIGURE 31. SINGLE TO DIFFERENTIAL WITH REDUCED FEEDBACK
LOADING
This circuit has scaled the feedback resistor up to 1kΩ to still
achieve the amplifier gain of 5V/V which gives the overall gain of
10V/V (20dB) when the 1:2 step up at the input is considered.
The particular transformer shown is typical of 1:2 turns ratio
broadband transformers, but there a many alternates with the
similar or improved characteristics.
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 31, 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 31, removes any input signal related artifacts from the
input common mode making the ISL55210 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
normal single ended VFA application, but no common mode
signal related terms.
The examples shown are using the transformer to convert from
single to differential. However, if the source is already
differential, these same transformer input circuits can drive the
transformer differentially still providing impedance scaling if
needed and common mode rejection for both DC and AC
common mode issues. A good example would be differential
mixer outputs or SAW filter outputs. Those differential sources
could also be connected into the ISL55210 R_G resistors through
blocking caps as well eliminating the input transformer. The AC
termination impedance for the differential source will then be
the sum of the two R_G resistors when simple blocking caps are
used.
Amplifier I/O Range Limits
The ISL55210 is intended principally to give the lowest IM3
performance on the lowest power for a differential I/O
application. The amplifier will work DC coupled and over a
relatively wide supply range of 3.0V to 4.2V supplies. The outputs
have both a differential and common mode operating range
while the input pins have a common operating range. For single
supply operation, the ground pins are at ground as is the exposed
metal pad on the underside of the package. The ISL55210 can
operate split supply where then the ground pins will be a
negative supply voltage and the exposed metal pad is either
connected to this negative supply or left unconnected on an
insulating board layer.
Briefly, the I/O and V_CM limits are:
1. Maximum V_CM setting = -V_S + 2V
2. Input common mode operating range of -V_S + 1.1V or the
output V_CM + 0.5V
3. Output V_O minimum (on each side) is either -V_S + 0.3V or
output V_CM - 0.9V
4. Output V_O maximum (on each side) is +V_S - 1.5V
The output swing limits are often asymmetrical around the V_CM
voltage. The maximum single ended swings are set by these two
limits:
V_OMIN is either -V_S + 0.3V or V_CM - 0.9V whichever is less. So for
instance on a single 3.3V supply with the default V_CM voltage of
1.2V, these two limits give the same result and the output pins
can swing down to 0.3V above -V_S = 0V. If, however, the V_CM pin
is raised to 1.5V, then the minimum output voltage will become
1.5V - 0.9V = 0.6V.
V_OMAX is set by a headroom limit to the positive supply to be:
V_OMAX = +V_S - 1.5V. Again, on a 3.3V single supply and the
default 1.2V V_CM setting, this mean the maximum referenced to
ground output pin voltages can be 3.3V - 1.5V = +1.8V or 0.6V
above the default V_CM voltage.
Using these default conditions, and the maximum positive
excursion of 0.6V above the 1.2V output V_CM setting, the
maximum differential V_P-P swing will be 4X this 0.6V single
ended limit or 2.4V_P-P. Where +V_S is increased the limit then
becomes the 0.9V below V_CM, but then the absolute maximum
differential V_P-P is then 4X 0.9V to 3.6V_P-P. So, for instance, to
get this maximum output swing, increase the supply voltage until
+V_S - 1.5V > V_CM + 0.9V. If we assume a V_CM voltage of 1.3V for
instance, then 1.3V + 0.9V + 1.5V = 3.7V will give an unclipped
12
FN7811.0
March 2, 2011