ISL55211_14 Datasheet, PDF (13/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain Differential Amplifier

ISL55211
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 ISL55211 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 ISL55211 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
limits while the input pins internal to the ISL55211 have a
common mode voltage operating range. For single supply
operation, the -Vs pins are at ground as is the exposed metal pad
on the underside of the package. The ISL55211 can operate split
supply where then -Vs 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 as follows:
1. Maximum V_CM setting = -Vs +2V
2. Input common mode operating range (internal summing
junction pints of the ISL55211) of -Vs + 1.1V or to output V_CM
+ 0.5V
3. Output V_O minimum (on each side) is either -Vs + 0.3V or
output V_CM - 0.9V
4. Output V_O maximum (on each side) is +Vs - 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_O(MIN) is either -Vs + 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 -Vs (= 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_O(MAX) is set by a headroom limit to the positive supply to be
-V_O(MAX) = +Vs - 1.5V. Again, on a 3.3V single supply and the
default 1.2V V_CM setting, this means 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 +Vs is increased, the limit
then becomes the 0.9V below V_CM, but then the absolute
maximum differential V_P-P is then 4 x 0.9V to 3.6V_P-P. So for
instance, to get this maximum output swing, increase the supply
voltage until +Vs - 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 3.6V_P-P output capability. The V_P-P reported in
Figure 27 is an asymmetrically clipped maximum swing. Going
10% above this 3.7V target to 4.1V will be within the
recommended operating range and give some tolerancing
headroom that would also suggest the V_CM voltage be moved up
to approximately 1.5V, which coincides with the default output
V_CM from Figure 27. Operating at +4.1V single supply in a
Figure 29 type configuration will give the maximum linear
differential output swing of 3.6V_P-P.
The differential inputs internal to the ISL55211 also have
operating range limits relative to the supply voltages. Operating
in an AC-coupled circuit like Figure 29 will produce an input
common mode voltage equal to the outputs. The inputs can
operate with full linearity with this V_CM voltage down to 1.1V
above the -Vs supply. On the default 1.2V output V_CM on +3.3V
supplies this gives a 100mV guardband on the input V_CM
voltages. Overriding the default V_CM by applying a control voltage
to the V_CM pin should be done with care in going towards the
negative supply due to this limit. On the + side, the maximum
input V_CM above the -Vs supply is 2V so there is more room to
move the output V_CM up than down from the default value.
Power Supply, Shutdown, and Thermal
Considerations
The ISL55211 is intended for single supply operation from 3.0V
to 4.2V with an absolute maximum setting of 4.5V. The 3.3V
supply current is trimmed to be nominally 35mA at +25°C
ambient. Figure 28 shows the supply current for nominal +25°C
and -40°C to +85°C operation over the specified maximum
supply range. The input stage is biased from an internal voltage
reference from the negative supply giving the exceptional 90dB
low frequency PSRR shown in Figure 26.
Since the input stage bias is from a re-regulated internal supply,
a simple approach to single +5V operation can be supported as
shown in Figure 33. Here, a simple IR drop from the +5V supply
will bring the operating supply voltage for the ISL55211 into its
allowed range. Figure 33 shows example calculations for the
voltage range at the ISL55211 +Vs pin assuming a ±5%
tolerance on the +5V supply and a 35mA to 55mA range on the
total supply current. Considering the 34mA to 44mA quiescent
current range from Figure 28 over the -40°C to +85°C ambient,
and the 3.4V to 4.4V supply voltage range assumed here, this is
designing for a 1mA to 11mA average load current, which should
be adequate for most intended application loads. Good supply
decoupling at the device pins is required for this simple solution
to still provide exceptional HD performance.
13
FN7868.0
June 21, 2011