Datasheet

INA126, INA2126
8
SBOS062A
www.ti.com
INPUT BIAS CURRENT RETURN
The input impedance of the INA126/2126 is extremely
high—approximately 10
9
Ω. However, a path must be pro-
vided for the input bias current of both inputs. This input
bias current is typically –10nA (current flows out of the
input terminals). 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 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
advantages of lower input offset voltage due to bias current
and better high-frequency common-mode rejection.
FIGURE 3. Providing an Input Common-Mode Current Path.
INPUT COMMON-MODE RANGE
The input common-mode range of the INA126/2126 is
shown in the typical characteristic curves. The common-
mode range is limited on the negative side by the output
voltage swing of A
2
, an internal circuit node that cannot be
measured on an external pin. The output voltage of A
2
can
be expressed as:
V
O2
= 1.25 V
IN
– (V
IN
– V
IN
) (10kΩ/R
G
) (2)
(Voltages referred to Ref terminal, pin 5)
–+–
The internal op amp A
2
is identical to A
1
and its output
swing is limited to typically 0.7V from the supply rails.
When the input common-mode range is exceeded (A
2
’s
output is saturated), A
1
can still be in linear operation and
respond to changes in the non-inverting input voltage. The
output voltage, however, will be invalid.
LOW VOLTAGE OPERATION
The INA126/2126 can be operated on power supplies as low
as ±1.35V. Performance remains excellent with power sup-
plies ranging from ±1.35V to ±18V. Most parameters vary
only slightly throughout this supply voltage range—see
typical characteristic curves. Operation at very low supply
voltage requires careful attention to ensure that the common-
mode voltage remains within its linear range. See “Input
Common-Mode Voltage Range.”
The INA126/2126 can be operated from a single power
supply with careful attention to input common-mode range,
output voltage swing of both op amps and the voltage
applied to the Ref terminal. Figure 4 shows a bridge ampli-
fier circuit operated from a single +5V power supply. The
bridge provides an input common-mode voltage near 2.5V,
with a relatively small differential voltage.
INPUT PROTECTION
The inputs are protected with internal diodes connected to
the power supply rails. These diodes will clamp the applied
signal to prevent it from exceeding the power supplies by
more than approximately 0.7V. If the signal source voltage
can exceed the power supplies, the source current should be
limited to less than 10mA. This can generally be done with
a series resistor. Some signal sources are inherently current-
limited and do not require limiting resistors.
CHANNEL CROSSTALK—DUAL VERSION
The two channels of the INA2126 are completely indepen-
dent, including all bias circuitry. At DC and low frequency
there is virtually no signal coupling between channels.
Crosstalk increases with frequency and is dependent on
circuit gain, source impedance and signal characteristics.
As source impedance increases, careful circuit layout will
help achieve lowest channel crosstalk. Most crosstalk is
produced by capacitive coupling of signals from one channel
to the input section of the other channel. To minimize
coupling, separate the input traces as far as practical from
any signals associated with the opposite channel. A grounded
guard trace surrounding the inputs helps reduce stray cou-
pling between channels. Carefully balance the stray capaci-
tance of each input to ground, and run the differential inputs
of each channel parallel to each other, or directly adjacent on
top and bottom side of a circuit board. Stray coupling then
tends to produce a common-mode signal that is rejected by
the IA’s input.
47kΩ47kΩ
10kΩ
Microphone,
Hydrophone
etc.
Thermocouple
Center-tap provides
bias current return.
INA126
INA126
INA126