ISL28474 Intersil Corporation, ISL28474 Datasheet - Page 16

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

Description of Operation and Application
Information
Product Description
The ISL28274 and ISL28474 provide both a micropower
instrumentation amplifier (Amp A) and a low power precision
amplifier (Amp B) in the same package. The amplifiers
deliver rail-to-rail input amplification and rail-to-rail output
swing on a single 2.4V to 5V supply. They also deliver
excellent DC and AC specifications while consuming only
60µA typical supply current per amplifier. Because the
instrumentation amplifiers provide an independent pair of
feedback terminals to set the gain and to adjust the output
level, the in-amp achieve high common-mode rejection ratio
regardless of the tolerance of the gain setting resistors. The
instrumentation amplifier is internally compensated for a
minimum closed loop gain of 100 or greater. An EN pin is
used to reduce power consumption, typically 4µA for the
ISL28274 and 8µA for the ISL28474, while both amplifiers
are disabled. The user has independent control of each
amplifier via separate EN pins.
Input Protection
The input and feedback terminals have internal ESD
protection diodes to both positive and negative supply rails,
limiting the input voltage to within one diode drop beyond the
supply rails. If overdriving the inputs is necessary, the
external input current must never exceed 5mA. External
series resistor may be used as a protection to limit excessive
external voltage and current from damaging the inputs.
Input Stage and Input Voltage Range
The input terminals (IN+ and IN-) of both amplifiers “A” and
amp “B” are single differential pair P-MOSFET devices aided
by an Input Range Enhancement Circuit to increase the
headroom of operation of the common-mode input voltage.
The feedback terminals (FB+ and FB-) of amplifier “A” also
have a similar topology. As a result, the input common-mode
voltage range is rail-to-rail. These amps are able to handle
input voltages that are at or slightly beyond the supply and
ground making them well suited for single 5V or 3.3V low
voltage supply systems. There is no need then to move the
common-mode input to achieve symmetrical input voltage.
Output Stage and Output Voltage Range
A pair of complementary MOSFET devices drives the output
VOUT to within a few mV of the supply rails. At a 100kΩ
load, the PMOS sources current and pulls the output up to
4mV below the positive supply, while the NMOS sinks
current and pulls the output down to 3mV above the negative
supply, or ground in the case of a single supply operation.
The current sinking and sourcing capability of the ISL28274
are internally limited to 31mA.
Gain Setting of Instrumentation amp “A”
VIN, the potential difference across IN+ and IN-, is replicated
(less the input offset voltage) across FB+ and FB-. The goal
16
ISL28274, ISL28474
of the ISL28274 in-amp is to maintain the differential voltage
across FB+ and FB- equal to IN+ and IN-; (FB+ - FB-) =
(IN+ - IN-). Consequently, the transfer function can be
derived. The gain is set by two external resistors, the
feedback resistor RF, and the gain resistor RG.
In Figure 62, the FB+ pin and one end of resistor RG are
connected to GND. With this configuration, the above gain
equation is only true for a positive swing in VIN; negative
input swings will be ignored and the output will be at ground.
Reference Connection
Unlike a three-opamp instrumentation amplifier, a finite
series resistance seen at the REF terminal does not degrade
the high CMRR performance eliminating the need for an
additional external buffer amplifier. Figure 63 uses the FB+
pin to provide a high impedance REF terminal.
VOUT
VOUT
FIGURE 62. GAIN IS BY EXTERNAL RESISTORS R
FIGURE 63. GAIN SETTING AND REFERENCE CONNECTION
VCM
VCM
REF
2.9V to 5V
=
=
⎛
⎜
⎝
⎛
⎜
⎝
1
1
R1
R2
VIN/2
VIN/2
VIN/2
VIN/2
+
+
------- -
R
R
------- -
R
R
G
G
F
F
⎞
⎟
⎠
⎞
⎟
⎠
(
VIN
VIN
RG
RG
)
+
⎛
⎜
⎝
1
6
5
3
4
6
5
3
4
+
FB+
FB-
FB+
FB-
IN+
IN-
IN+
IN-
------- -
R
R
G
F
2.4V to 5V
2.4V to 5V
⎞
⎟
⎠
+
+
+
+
-
-
-
-
(
16
16
ISL28274
ISL28274
8
8
VREF
RF
RF
VS+
VS+
VS-
VS-
7
7
)
EN
EN
2
2
Amp “A”
December 13, 2006
Amp “A”
F
AND R
EN_BAR
EN_BAR
VOUT
VOUT
FN6345.0
G