ISL28270_0708 Datasheet, PDF (15/18 Pages) Intersil Corporation – Dual and Quad Channel Micropower, Single Supply, Rail-to-Rail Input and Output (RRIO) Instrumentation Amplifiers

ISL28270, ISL28273, ISL28470
range of input. See Input Bias Current vs Common-Mode
Input Voltage on page 8.
Input Bias Cancellation/Compensation
All three parts have an Input Bias Cancellation/Compensation
Circuit for both the input and feedback terminals (IN+, IN-, FB+
and FB-), achieving a low input bias current throughout the
input common-mode range and the operating temperature
range. While the PNP bipolar input stages are biased with an
adequate amount of biasing current for speed and increased
noise performance, the Input Bias Cancellation/Compensation
Circuit sinks most of the base current of the input transistors
leaving a small portion as input bias current, typically 500pA. In
addition, the Input Bias Cancellation/Compensation Circuit
maintains a smooth and flat behavior of input bias current over
the common mode range and over the operating temperature
range. The Input Bias Cancellation/Compensation Circuit
operates from input voltages of 10mV above the negative
supply to input voltages slightly above the positive supply. See
Input Bias Current vs Common-Mode Input Voltage in the
“Typical Performance Curves” on page 8.
Output Stage and Output Voltage Range
A Class AB common-source output stage drives the output.
The pair of complementary MOSFET devices drive the
output VOUT to within a few millivolts of the supply rails. At a
100kΩ load, the PMOS sources current and pulls the output
up to 4mV below the positive supply. The NMOS sinks
current and pulls the output down to 4mV above the negative
supply, or ground in the case of a single supply operation.
The current sinking and sourcing capability are internally
limited to 29mA. When disabled, the outputs are in a high
impedance state.
Gain Setting
VIN, the potential difference across IN+ and IN-, is replicated
(less the input offset voltage) across FB+ and FB-. The
function of the 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 in-amp gain is set by two external resistors, the
feedback resistor R_F, and the gain resistor R_G.
VIN = IN+ – IN-
VOUT
=
⎛
⎜1
⎝
+
R^-R----_G^-F--_{⎠⎟⎞}
VIN
(EQ. 1)
In Figure 49, the FB+ pin and one end of resistor R_G are
connected to GND. With this configuration, Equation 1 is
only true for a positive swing in VIN; negative input swings
will be ignored because the output will be at ground.
IN+
IN-
VCM
2.4V TO 5.5V
IN+
V+ EN
+
IN-
-
FB+ ^ISL28270
+
FB-
-
V-
EN
VOUT
R_G
R_F
FIGURE 49. GAIN IS SET BY TWO EXTERNAL RESISTORS,
R_F AND R_G
Reference Connection
Unlike a three op amp in-amp realization, 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 50 uses the FB+
pin to provide a high impedance REF terminal.
2.4V TO 5.5V
EN
IN+
IN-
2.9V to 5.5V
VCM
R₁
REF
R_{2 RG}
IN+
V+ EN
+
IN-
-
FB+ ^ISL28270
+
FB- _-
V-
R_F
VOUT
FIGURE 50. GAIN SETTING AND REFERENCE CONNECTION
.
VIN = IN+ – IN-
VOUT
=
⎛
⎜1 +
⎝
R^-R----_G^-F--_{⎠⎟⎞}
(
V
I
N)
+
⎛
⎜
⎝
1
+
R^-R----_G^-F--_{⎠⎟⎞}
(VR
E
F
)
(EQ. 2)
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 R_F and R_G. Note that any noise or unwanted
signals on the reference supply will be amplified at the
output according to Equation 2. See Figure 50.
The FB+ pin can also be connected to the other end of
resistor, R_G. See Figure 51. Keeping the basic concept that
15
FN6260.4
August 3, 2007