ISL55210_14 Datasheet, PDF (13/18 Pages) Intersil Corporation – Wideband, Low-Power, Ultra-High Dynamic Range Differential Amplifier

ISL55210
3.6V_P-P output capability. The V_P-P reported in Figure 26 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. This
coincides with the default output V_CM from Figure 26. Operating
at +4.1V single supply in a Figure 28 type configuration will give
the maximum linear available output swing of 3.6V_P-P.
The differential inputs of the ISL55210 also have operating
range limits relative to the supply voltages. Operating in an AC
coupled circuit like Figure 28 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 GND
connection (or -V_S 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
V_CM above the -V_S supply is 2V so there is more room to move
the output V_CM up than down from the default value.
When operated as a DC coupled single to differential amplifier,
the input common mode voltage will move with the input signal
and will be different than the output common mode voltage
when the external resistors are set for gain. When the input
common mode can be different than the output, the additional
constraint that must be observed is that the input common mode
voltage cannot be > output V_CM +0.5V. This would only occur if
the single source was coming from a higher voltage than the
output V_CM setting.
Power Supply, Shutdown, and Thermal
Considerations
The ISL55210 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 27 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
referenced from the negative supply giving the exceptional 90dB
low frequency PSRR shown in Figure 25.
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 32. Here, a simple IR drop from the +5V supply
will bring the operating supply voltage for the ISL55210 into its
allowed range. Figure 32 shows example calculations for the
voltage range at the ISL55210 +V_S 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 27 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 SFDR performance.
C_{in 1:n}
V_i
+5V ±5%
35 55mA
RF
24.3
3.4 4.4V
+ 2.2µF
10nF
10k
RG
VCM
+
PD
ISL55210
RO
VO
RO
RG
-
RF
FIGURE 32. OPERATING FROM A SINGLE +5V SUPPLY
The ISL55210 includes a power shutdown feature that can be
used to reduce system power dissipation when signal path
operation is not required. This pin (PD) is referenced to the
ground pins and must be asserted low to activate the shutdown
feature. When not used, a 10kΩ external resistor to +V_S should
be used to assert a high level at this pin. Digital control on this
pin can be either an open collector output (using that 10kΩ
pullup) or a CMOS logic line running off the same +V_S as the
amplifier. For split supply operation, the PD pins must be pulled
to below -V_S + 0.54V to disable.
Since the ISL55210 operates as a differential inverting op amp,
there is only modest signal path isolation when disabled as
shown in Figure 23. For small input signals, Figure 23 shows
about 5dB to 6dB isolation while for large signals, back to back
protection diodes across the inputs compress the signal to show
actually an improved isolation. This is intended to protect any
subsequent devices from large input signals during shutdown.
Those diodes limit the maximum overdrive voltage across the
input to approximately 0.5V in each polarity. The R_G resistors of
Test Circuit #1 limit the current into those diodes under this
condition.
The supply current in shutdown does not reduce to zero as
internal circuitry is still active to hold the output common mode
voltage at the V_CM control input voltage even during shutdown (or
the default value). This is intended to hold the ISL55210 output
near the desired common mode output level during shutdown.
This improves turn on characteristic and keeps the output
voltages in a safe range for downstream circuitry.
DISABLED OPERATION WARNING IN DC COUPLED DESIGNS
When disabled, the output stage provides a nominal DC voltage
at the Vcm control pin input or the default internal 1.2V value.
Being very low power, any external circuit condition that can
cause the output pins to source or sink DC current can move the
ISL55210 internal operating points into regions from which it
may not recover when the device is enabled. If the external
circuitry can force >20µA into the output pins or pull > 1.5mA out
of the output pins correct operation is not guaranteed. For
designs that might force current into the output stage during
disable, adding a resistor to ground on the outputs might provide
13
FN7811.2
June 6, 2013