Datasheet

APPLICATION INFORMATION
SETTING THE GAIN
A
1
A
2
A
3
6
150kW150kW
150kW150kW
7
4
3
8
1
2
V
IN-
V
IN+
R
G
V+
V-
INA333
G=1+
100kW
R
G
5
RFIFilter
50kW
50kW
RFIFilter
Load
V =G (V´ V- )
O IN+ IN-
0.1 Fm
0.1 Fm
+
-
V
O
R
G
Alsodrawninsimplifiedform:
INA333
Ref
V
O
V
IN-
V
IN+
Ref
RFIFilter
RFIFilter
INA333
www.ti.com ..................................................................................................................................................... SBOS445B – JULY 2008 – REVISED OCTOBER 2008
Figure 32 shows the basic connections required for Table 1 lists several commonly-used gains and
operation of the INA333. Good layout practice resistor values. The 100k Ω term in Equation 1 comes
mandates the use of bypass capacitors placed close from the sum of the two internal feedback resistors of
to the device pins as shown. A
1
and A
2
. These on-chip resistors are laser trimmed
to accurate absolute values. The accuracy and
The output of the INA333 is referred to the output
temperature coefficient of these resistors are included
reference (REF) terminal, which is normally
in the gain accuracy and drift specifications of the
grounded. This connection must be low-impedance to
INA333.
assure good common-mode rejection. Although 15 Ω
or less of stray resistance can be tolerated while The stability and temperature drift of the external gain
maintaining specified CMRR, small stray resistances setting resistor, R
G
, also affects gain. The contribution
of tens of ohms in series with the REF pin can cause of R
G
to gain accuracy and drift can be directly
noticeable degradation in CMRR. inferred from the gain Equation 1 . Low resistor values
required for high gain can make wiring resistance
important. Sockets add to the wiring resistance and
contribute additional gain error (possibly an unstable
Gain of the INA333 is set by a single external
gain error) in gains of approximately 100 or greater.
resistor, R
G
, connected between pins 1 and 8. The
To ensure stability, avoid parasitic capacitance of
value of R
G
is selected according to Equation 1 :
more than a few picofarads at the R
G
connections.
Careful matching of any parasitics on both R
G
pins
G = 1 + (100k Ω /R
G
) (1)
maintains optimal CMRR over frequency.
Figure 32. Basic Connections
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