ISL55211 Datasheet, PDF (14/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain, Differential Amplifier

ISL55211
+5V ±5%
35 55mA
24.3
3.4 4.4V
+ 2.2µF
10nF
10k
PD
RF
+
CIN _1:n
RG
RO
Vi
VCM
VO
RG
-
RO
RF
ISL55211
FIGURE 33. OPERATING FROM A SINGLE +5V SUPPLY
The ISL55211 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 -Vs and must be asserted
low to activate the shutdown feature. When not used, a 10kΩ
external resistor to +Vs 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 pull-up) or a CMOS logic line running off the same
+Vs as the amplifier. For split supply operation, the Pd pins must be
pulled to below -Vs + 0.54V to disable.
Since the ISL55211 operates as a differential inverting op amp,
there is only modest signal path isolation when disabled, as shown
in Figure 24. The inputs include 2 pairs of back to back low
capacitance diodes intended to protect any subsequent devices
from large input signals during shutdown. Those diodes limit the
maximum overdrive voltage across the input to approximately 1.0V
in each polarity. The internal 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 voltage even during shutdown. This is intended to hold the
ISL55211 outputs near the desired common mode output level
during shutdown. This improves the turn on characteristic and keeps
those output voltages in a safe range for downstream circuitry.
The very low internal power dissipation of the ISL55211, along with
the excellent thermal conductivity of the TQFN package when the
exposed metal pad is tied to a conductive plate, reduces the T_J rise
above ambient to very modest levels. Assuming a nominal 115mW
dissipation and using the 63°C/W measured thermal impedance
from Junction to ambient, gives a rise of only 0.115*63 = 7.2°C.
Operation at elevated ambient temperatures is easily supported
given this very low internal rise to junction.
The maximum internal junction temperatures would occur at
maximum supply voltage, +85°C maximum ambient operating,
and where the TQFN exposed pad is not tied to a conductive layer.
Where the TQFN must be mounted with an insulating layer to the
exposed metal plate, such as in a split supply application, device
measurements show an increased thermal impedance junction to
ambient of +120°C/W. Using this, and a maximum quiescent
internal power on 4.5V absolute maximum, which shows 45mA for
+85°C maximum operating ambient from Figure 27, we get
4.5V*45mA*+120°C/W = +24°C rise above +85°C or
approximately +109°C operating T_J maximum - still well below the
specified Absolute Maximum operating junction temperature of
+135°C.
Noise Analysis
The decompensated voltage feedback design of the ISL55211
provides very low input voltage and current noise. Based on the
ISL55210, these internal noise terms are 0.85nV/√Hz differential
voltage noise and a 5pA/√Hz current noise term on each side. Since
the ISL55211 is an internally fixed gain version, these internal noise
terms will produce only a few set of output noise values. Figure 34
shows the analysis model for just the ISL55211 with no input
transformer while Table 3 shows the resulting output and input
referred differential spot noise voltages using Equation 1.
4kTR_G
iN ^*
4kTR_F
*
R_F
500
+
R_G
e_n ^*
ISL55211
eO
4kTR_G R_G
i_N ^*
-
* ⁵⁰⁰
4kTR_F ^RF
FIGURE 34. AMPLIFIER ONLY NOISE MODEL
With equal feedback and gain resistors, the total output noise
expression becomes very simple. This is shown as Equation 1.
e₀ (e_N^∗NG)² + 2(i_NR_F)² + 2(4kTR_FNG)
(EQ. 1)
The NG term in this equation is the Noise Gain = 1 + R_F/R_G. The
last term in Equation 1 captures both the R_F and R_G resistor
noise terms. Table 3 evaluates this expression for the 3 possible
internal gains with a fixed 500Ω internal feedback. nV/√HZ
TABLE 3. OUTPUT AND INPUT SPOT NOISE FROM EQUATION 1 FOR
THE 3 GAINS OF THE ISL55211
INPUT REFERRED
R_G
GAIN NOISE GAIN
(Ω)
V/V
V/V
E_O
nV/√Hz
E_NI
nV/√Hz
250
2
3
8.19
4.09
125
4
5
10.51
2.63
100
5
6
11.60
2.32
14
FN7868.0
June 21, 2011