OFFSET TRIMMING
Microphone,
Hydrophone
etc.
The INA141 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.
INA141
47kΩ
47kΩ
Thermocouple
INA141
–
VIN
V+
INA141
Ref
VO
100µA
1/2 REF200
+
10kΩ
VIN
100Ω
100Ω
10kΩ
OPA177
±10mV
Adjustment Range
INA141
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 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 the output amplifier will be
near zero. The output of the INA141 will be near 0V even
though both inputs are overloaded.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA141 is extremely high—
approximately 1010Ω. 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.
LOW VOLTAGE OPERATION
The INA141 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 through this supply voltage range—see Typical
Performance Curves. Operation at very low supply voltage
requires careful attention to assure that the input voltages
remain within their linear range. Voltage swing require-
ments of internal nodes limit the input common-mode range
with low power supply voltage. Typical performance curves,
“Input Common-Mode Range vs Output Voltage” show the
range of linear operation for ±15V, ±5, and ±2.5V supplies.
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 of the INA141 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 thermo-
couple 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.
INPUT PROTECTION
The inputs of the INA141 are individually protected for
voltages up to ±40V. For example, a condition of –40V on
one input and +40V on the other input will not cause
damage. Internal circuitry on each input provides low series
impedance under normal signal conditions. To provide
equivalent protection, series input resistors would contribute
excessive noise. If the input is overloaded, the protection
circuitry limits the input current to a safe value of approxi-
mately 1.5 to 5mA. The typical performance curve “Input
Bias Current vs Common-Mode Input Voltage” shows this
input current limit behavior. The inputs are protected even if
the power supplies are disconnected or turned off.
INPUT COMMON-MODE RANGE
The linear input voltage range of the input circuitry of the
INA141 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 to in-
crease, however, the linear input range will be limited by the
output voltage swing of amplifiers A1 and A2. So the linear
common-mode input range is related to the output voltage of
the complete amplifier. This behavior also depends on sup-
ply voltage—see performance curves “Input Common-Mode
Range vs Output Voltage”.
®
9
INA141
TI [ TEXAS INSTRUMENTS ]
