NOISE PERFORMANCE
The INA118 provides very low noise in most applications.
For differential source impedances less than 1kΩ, the INA103
may provide lower noise. For source impedances greater
than 50kΩ, the INA111 FET-Input Instrumentation Ampli-
fier may provide lower noise.
Low frequency noise of the INA118 is approximately
0.28µVp-p measured from 0.1 to 10Hz (G≥100). This pro-
vides dramatically improved noise when compared to state-
of-the-art chopper-stabilized amplifiers.
OFFSET TRIMMING
The INA118 is laser trimmed for low offset voltage and
drift. Most applications require no external offset adjust-
ment. Figure 2 shows an optional circuit for trimming the
output offset voltage. The voltage applied to Ref terminal is
summed at the output. The op amp buffer provides low
impedance at the Ref terminal to preserve good common-
mode rejection.
–
V
IN
R
G
V+
IN
Microphone,
Hydrophone
etc.
INA118
47kΩ
47kΩ
Thermocouple
INA118
10kΩ
INA118
V+
INA118
Ref
V
O
100µA
1/2 REF200
Center-tap provides
bias current return.
FIGURE 3. Providing an Input Common-Mode Current Path.
INPUT COMMON-MODE RANGE
The linear input voltage range of the input circuitry of the
INA118 is from approximately 0.6V below the positive
supply voltage to 1V 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 A
1
and A
2
. Thus, the
linear common-mode input range is related to the output
voltage of the complete amplifier. This behavior also de-
pends on supply voltage—see performance curves “Input
Common-Mode Range vs Output 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 INA118 will be near 0V even
though both inputs are overloaded.
LOW VOLTAGE OPERATION
The INA118 can be operated on power supplies as low as
±1.35V.
Performance of the INA118 remains excellent with
power supplies ranging from
±1.35V
to
±18V.
Most param-
eters vary only slightly throughout this supply voltage range—
see typical performance curves. Operation at very low sup-
ply voltage requires careful attention to assure that the input
voltages remain within their linear range. Voltage swing
requirements of internal nodes limit the input common-
mode range with low power supply voltage. Typical perfor-
mance curves, “Input Common-Mode Range vs Output
Voltage” show the range of linear operation for a various
supply voltages and gains.
®
OPA177
±10mV
Adjustment Range
10kΩ
100Ω
100Ω
100µA
1/2 REF200
V–
FIGURE 2. Optional Trimming of Output Offset Voltage.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA118 is extremely high—
approximately 10
10
Ω.
However, a path must be provided for
the input bias current of both inputs. This input bias current
is approximately
±5nA.
High input impedance means that
this input bias current changes very little with varying input
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 of the INA118 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.
9
INA118
TI [ TEXAS INSTRUMENTS ]
