ISL28270_0708 Datasheet, PDF (16/18 Pages) Intersil Corporation – Dual and Quad Channel Micropower, Single Supply, Rail-to-Rail Input and Output (RRIO) Instrumentation Amplifiers

ISL28270, ISL28273, ISL28470
the in-amp maintains constant differential voltage across the
input terminals and feedback terminals (FB- - FB+) =
(IN+ - IN-), the transfer function of Figure 51 can be derived
from Equation 3. Note that the VREF gain term is eliminated,
and susceptibility to external noise is reduced.
2.4V TO 5.5V
EN
IN+
IN-
VCM
R_S
VREF
R_G
IN+
V+ EN
+
IN-
-
FB+ ^ISL28270
+
FB-
- _V-
R_F
VOUT
FIGURE 51. REFERENCE CONNECTION WITH AN AVAILABLE
VREF
VIN = IN+ – IN-
VOUT =
1
+
-^R----^S-----+-----^R-----^F-
R_G
+ VREF
(EQ. 3)
VOUT
=
⎛
⎜
⎝
1
+
R^-R----_G^-F--_{⎠⎟⎞}
(V
I
N
)
+
(V
R
E
F
)
(EQ. 4)
A finite resistance R_S in series with the VREF source, adds
an output offset of VIN*(R_S/R_G). As the series resistance R_S
approaches zero, Equation 3 is simplified to Equation 4 for
Figure 51. VOUT is simply shifted by an amount VREF.
External Resistor Mismatches
Because of the independent pair of feedback terminals
provided by the in-amps, the CMRR is not degraded by any
resistor mismatches. Hence, unlike a three op amp and
especially a two op amp in-amp realization, the ISL28270,
ISL28273 and ISL28470 reduce the cost of external
components by allowing the use of 1% or more tolerance
resistors without sacrificing CMRR performance. The CMRR
will be typically 110dB regardless of the tolerance of the
resistors used. Instead, a resistor mismatch results in a
higher deviation from the theoretical gain - gain error.
Gain Error and Accuracy
The gain error indicated in the “Electrical Specifications”
table on page 4 is the inherent gain error alone. The gain
error specification listed does not include the gain error
contributed by the resistors. There is an additional gain error
due to the tolerance of the resistors used. The resulting
non-ideal transfer function effectively becomes:
VOUT
=
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
× [1 ± (E_RG + E_RF + E_G)] × VIN
(EQ. 5)
Where:
E_RG = Tolerance of RG
E_RF = Tolerance of RF
E_G = Gain Error of the ISL28270
The term [1 - (E_RG +E_RF +E_G)] is the deviation from the
theoretical gain. Thus, (E_RG +E_RF +E_G) is the total gain
error. For example, if 1% resistors are used, the total gain
error would be:
TotalGainError = ±(E_RG + E_RF + E_G(typical))
TotalGainError = ±(0.01 + 0.01 + 0.005)= ±2.5%
(EQ. 6)
Disable/Power-Down
The ISL28270, ISL28273 and ISL28470 have an
enable/disable pin for each channel. They can be powered
down to reduce the supply current to typically 4µA when all
channels are off. When disabled, the corresponding output is
in a high impedance state. The active low EN pin has an
internal pull-down and hence can be left floating and the
in-amp enabled by default. When the EN is connected to an
external logic, the in-amp will shutdown when the EN pin is
pulled above 2V, and will power up when EN is pulled below
0.8V.
Unused Channels
The ISL28270, ISL28273 and ISL28470 are dual and quad
channel op amps. If the application only requires one
channel when using the ISL28270, ISL28273 or less than 4
channels when using the ISL28470, the user must configure
the unused channel(s) to prevent them from oscillating. The
unused channel(s) will oscillate if the input and output pins
are floating. This will result in higher than expected supply
currents and possible noise injection into the channel being
used. The proper way to prevent this oscillation is to short
the IN+ and IN- terminals to ground and short the FB+, FB-
and the output terminals to ground as shown in Figure 52.
IN+
+
IN-
-
FB+ ₊
FB- _-
1/2 ISL28270, ISL28273
1/4 ISL28470
FIGURE 52. PREVENTING OSCILLATIONS IN UNUSED
CHANNELS
16
FN6260.4
August 3, 2007