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SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005
OFFSET TRIMMING
Microphone,
Hydrophone
etc.
The INA128/INA129 is laser trimmed for low offset voltage
and offset voltage drift. Most applications require no
external offset adjustment. Figure 2 shows an optional
circuit for trimming the output offset voltage. The voltage
applied to Ref terminal is summed with the output. The op
amp buffer provides low impedance at the Ref terminal to
preserve good common-mode rejection.
INA128
Ω
47k
Ω
47k
−
VIN
Thermocouple
INA128
V+
VO
RG
INA128
Ref
100µA
1/2 REF200
+
VIN
Ω
10k
Ω
100
Ω
100
Ω
10k
OPA177
10mV
Adjustment Range
INA128
100µA
1/2 REF200
Center−tap provides
bias current return.
V−
Figure 3. Providing an Input Common-Mode
Current Path
Figure 2. Optional Trimming of Output Offset
Voltage
INPUT COMMON-MODE RANGE
INPUT BIAS CURRENT RETURN PATH
The linear input voltage range of the input circuitry of the
INA128/INA129 is from approximately 1.4V below the
positive supply voltage to 1.7V above the negative
supply. As a differential input voltage causes the output
voltage increase, however, the linear input range will be
The input impedance of the INA128/INA129 is
extremely high—approximately 10 Ω. However, a path
must be provided for the input bias current of both
inputs. This input bias current is approximately 2nA.
High input impedance means that this input bias current
changes very little with varying input voltage.
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limited by the output voltage swing of amplifiers A and
1
A . So the linear common-mode input range is related
2
to the output voltage of the complete amplifier. This
behavior also depends on supply voltage—see
performance curves, Input Common-Mode Range vs
Output Voltage.
Input circuitry must provide a path for this input bias
current for proper operation. Figure 3 shows various
provisions for an input bias current path. Without a bias
current path, the inputs will float to a potential which
exceeds the common-mode range, and the input
amplifiers will saturate.
Input-overload can produce an output voltage that
appears normal. For example, if an input overload
condition drives both input amplifiers to their positive
output swing limit, the difference voltage measured by
If the differential source resistance is low, the bias
current return path can be connected to one input (see
the thermocouple example in Figure 3). With higher
source impedance, using two equal resistors provides
a balanced input with possible advantages of lower
input offset voltage due to bias current and better
high-frequency common-mode rejection.
the output amplifier will be near zero. The output of A
3
will be near 0V even though both inputs are overloaded.
LOW VOLTAGE OPERATION
The INA128/INA129 can be operated on power supplies
as low as 2.25V. Performance remains excellent with
power supplies ranging from 2.25V to 18V. Most
parameters vary only slightly throughout this supply
voltage range—see typical performance curves.
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