ISL28474 Intersil Corporation, ISL28474 Datasheet - Page 17

• Manufacturer Part Number: ISL28474
• Description: (ISL28274 / ISL28474) Rail-to-Rail Input-Output Instrumentation Amplifier and Precision Operational Amplifier
• Manufacturer: Intersil Corporation

The FB+ pin is used as a REF terminal to center or to adjust
the output. Because the FB+ pin is a high impedance input,
an economical resistor divider can be used to set the voltage
at the REF terminal without degrading or affecting the CMRR
performance. Any voltage applied to the REF terminal will
shift VOUT by VREF times the closed loop gain, which is set
by resistors RF and RG as shown in Figure 63.
The FB+ pin can also be connected to the other end of resistor,
RG. See Figure 64. Keeping the basic concept that the in-amps
maintain constant differential voltage across the input terminals
and feedback terminals (IN+ - IN- = FB+ - FB-), the transfer
function of Figure 64 can be derived.
A finite resistance Rs in series with the VREF source, adds
an output offset of VIN*(RS/RG). As the series resistance Rs
approaches zero, the gain equation is simplified to the above
equation for Figure 64. VOUT is simply shifted by an amount
VREF.
External Resistor Mismatches
Because of the independent pair of feedback terminals
provided by the ISL28274, the CMRR is not degraded by
any resistor mismatches. Hence, unlike a three opamp and
especially a two opamp in-amp, the ISL28274 reduce the
cost of external components by allowing the use of 1% or
more tolerance resistors without sacrificing CMRR
performance. The ISL28274 CMRR will be 100dB
regardless of the tolerance of the resistors used.
Disable/Power-Down
The ISL28274 Amplifiers “A” and “B” can be powered down
reducing the supply current to typically 4µA. When disabled,
the output is in a high impedance state. The active low EN
bar pin has an internal pull down and hence can be left
floating and the in-amp and Opamp enabled by default.
When the EN bar is connected to an external logic, the
VOUT
FIGURE 64. REFERENCE CONNECTION WITH AN AVAILABLE
VCM
VREF
=
⎛
⎜
⎝
1
VIN/2
VIN/2
+
VREF
------- -
R
R
G
F
⎞
⎟
⎠
(
VIN
RG
)
+
(
VREF
6
5
3
4
FB+
FB-
17
IN+
IN-
2.4V to 5V
)
+
+
-
-
16
ISL28274
8
RF
VS+
VS-
7
EN
2
Amp “A”
ISL28274, ISL28474
EN_BAR
VOUT
amplifiers will power down when EN bar is pulled above 2V,
and will power on when EN bar is pulled below 0.8V.
Using Only the Instrumentation Amplifier
If the application only requires the instrumentation amp, the
user must configure the unused Opamp to prevent it from
oscillating. The unused Opamp 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 in-amp. The proper way to prevent this oscillation is to
short the output to the negative input and ground the positive
input (as shown in Figure 65).
Proper Layout Maximizes Performance
To achieve the maximum performance of the high input
impedance and low offset voltage, care should be taken in
the circuit board layout. The PC board surface must remain
clean and free of moisture to avoid leakage currents
between adjacent traces. Surface coating of the circuit board
will reduce surface moisture and provide a humidity barrier,
reducing parasitic resistance on the board. When input
leakage current is a concern, the use of guard rings around
the amplifier inputs will further reduce leakage currents.
Figure 66 shows a guard ring example for a unity gain
amplifier that uses the low impedance amplifier output at the
same voltage as the high impedance input to eliminate
surface leakage. The guard ring does not need to be a
specific width, but it should form a continuous loop around
both inputs. For further reduction of leakage currents,
components can be mounted to the PC board using Teflon
standoff insulators.
Current Limiting
The ISL28274 has no internal current-limiting circuitry. If the
output is shorted, it is possible to exceed the Absolute
Maximum Rating for output current or power dissipation,
potentially resulting in the destruction of the device.
FIGURE 65. PREVENTING OSCILLATIONS IN UNUSED
FIGURE 66. GUARD RING EXAMPLE FOR UNITY GAIN
HIGH IMPEDANCE INPUT
IN
CHANNELS
AMPLIFIER
+
-
V+
1/2 ISL28274
1/4 ISL28474
December 13, 2006
FN6345.0