Datasheet

NOISE PERFORMANCE
INTERNAL OFFSET CORRECTION
INPUT BIAS CURRENT RETURN PATH
OFFSET TRIMMING
10kW
OPA333
±10mV
AdjustmentRange
100W
100W
100 Am
1/2REF200
100 Am
1/2REF200
V+
V-
R
G
INA333
Ref
V
O
V
IN-
V
IN+
INA333
SBOS445B – JULY 2008 – REVISED OCTOBER 2008 ..................................................................................................................................................... www.ti.com
Table 1. Commonly-Used Gains and Resistor Values
DESIRED GAIN R
G
( Ω ) NEAREST 1% R
G
( Ω )
1 NC
(1)
NC
2 100k 100k
5 25k 24.9k
10 11.1k 11k
20 5.26k 5.23k
50 2.04k 2.05
100 1.01k 1k
200 502.5 499
500 200.4 200
1000 100.1 100
(1) NC denotes no connection. When using the SPICE model, the simulation will not converge unless a resistor is connected to the R
G
pins;
use a very large resistor value.
The auto-calibration technique used by the INA333
The INA333 internal op amps use an auto-calibration
results in reduced low frequency noise, typically only
technique with a time-continuous 350kHz op amp in
50nV/ √ Hz, (G = 100). The spectral noise density can
the signal path. The amplifier is zero-corrected every
be seen in detail in Figure 8 . Low frequency noise of
8 µ s using a proprietary technique. Upon power-up,
the INA333 is approximately 1 µ V
PP
measured from
the amplifier requires approximately 100 µ s to achieve
0.1Hz to 10Hz, (G = 100).
specified V
OS
accuracy. This design has no aliasing
or flicker noise.
The input impedance of the INA333 is extremely
high — approximately 100G Ω . However, a path must
Most applications require no external offset
be provided for the input bias current of both inputs.
adjustment; however, if necessary, adjustments can
This input bias current is typically ± 70pA. High input
be made by applying a voltage to the REF terminal.
impedance means that this input bias current
Figure 33 shows an optional circuit for trimming the
changes very little with varying input voltage.
output offset voltage. The voltage applied to REF
terminal is summed at the output. The op amp buffer
Input circuitry must provide a path for this input bias
provides low impedance at the REF terminal to
current for proper operation. Figure 34 illustrates
preserve good common-mode rejection.
various provisions for an input bias current path.
Without a bias current path, the inputs will float to a
potential that exceeds the common-mode range of
the INA333, and the input amplifiers will saturate. If
the differential source resistance is low, the bias
current return path can be connected to one input
(see the thermocouple example in Figure 34 ). With
higher source impedance, using two equal resistors
provides a balanced input with possible advantages
of lower input offset voltage as a result of bias current
and better high-frequency common-mode rejection.
Figure 33. Optional Trimming of Output Offset
Voltage
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