XR620A Datasheet, PDF (11/15 Pages) Exar Corporation – Instrumentation Amplifier

XR620A
Application Information
Basic Information
The XR620A is a monolithic instrumentation amplifier based
on the classic three op amp solution, refer to the Functional
Block Diagram shown in Figure 1. The XR620A produces a
single-ended output referred to the REF pin potential.
-IN
+
–
R_G
–
OUT
+
1% R_G (Ω)
49.9k
12.4k
5.49k
2.61k
1.00k
499
249
100
49.9
Caclulated
Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
0.1% R_G (Ω)
49.3k
12.4k
5.49k
2.61k
1.01k
499
249
98.8
49.3
Calculated
Gain
2.002
4.984
9.998
19.93
49.91
100.0
199.4
501.0
1,003.0
Table 1: Recommended R_G Values
–
+IN
+
REF
Figure 1: Functional Block Diagram
Follow these guidelines for improved performance:
■■ To maintain gain accuracy, use 0.1% to 1% resistors
■■ To minimize gain error, avoid high parasitic resistance in
series with R_G
■■ To minimize gain drift, use low TC resistors (<10ppm/°C)
The internal resistors are trimmed which allows the gain to be
accurately adjusted with one external resistor R_G.
G = ^49.4k
R_G
+ 1;
R_G
=
49.4k
G-1
R_G also determines the transconductance of the preamp
stage. As R_G is reduced for larger gains, the transconductance
increases to that of the input transistors. Producing the
following advantages:
■■ _{Open-loop gain increases as the gain is increased, reducing}
gain related errors
■■ Gain-bandwidth increases as the gain is increased,
optimizing frequency response
■■ Reduced input voltage noise which is determined by the
collector current and base resistance of the input devices
Common Mode Rejection
The XR620A offers high CMRR. To achieve optimal CMRR
performance:
■■ Connect the reference terminal (pin 5) to a low impedance
source
■■ Minimize capacitive and resistive differences between the
inputs
In many applications, shielded cables are used to minimize
noise. Properly drive the shield for best CMRR performance
over frequency. Figures 1 and 2 show active data guards
that are configured to improve AC common-mode rejections.
the capacitances of input cable shields are “bootstrapped”
to minimize the capacitance mismatch between the inputs.
Gain Selection
The impedance between pins 1 and 8, R_G, sets the gain
of the XR620A. Table 1 shows the required standard table
values of R_G for various calculated gains. For G = 1, R_G = ∞.
+ Input
+V _S
_
_
R_G / 2
100
XR620A
CLCxxx
+
R_G / 2
+
REF
- Input
-V _S
Figure 2: Common-mode Shield Driver
Output
© 2013-2015 Exar Corporation
11 / 15
exar.com/XR620A
Rev 1B