Datasheet

INA326, INA327
11
SBOS222D
www.ti.com
Applications sensitive to the spectral characteristics of high-
frequency noise may require consideration of the spurious
frequencies generated by internal clocking circuitry. “Spurs”
occur at approximately 90kHz and its harmonics (see typical
characteristic “Input-Referred Ripple Spectrum”) which may
be reduced by additional filtering below 1kHz.
Insufficient filtering at pin 5 can cause nonlinearity with large
output voltage swings (very near the supply rails). Noise
must be sufficiently filtered at pin 5 so that noise peaks do not
“hit the rail” and change the average value of the signal.
Figure 3 shows guidelines for filter cutoff frequency.
HIGH-FREQUENCY NOISE
C
2
and C
O
form filters to reduce internally generated auto-
correction circuitry noise. Filter frequencies can be chosen to
optimize the trade-off between noise and frequency re-
sponse of the application, as shown in Figure 3. The cutoff
frequencies of the filters are generally set to the same
frequency. Figure 3 shows the typical output noise for four
gains as a function of the –3dB cutoff frequency of each filter
response. Small signals may exhibit the addition of internally
generated auto-correction circuitry noise at the output. This
noise, combined with broadband noise, becomes most evi-
dent in higher gains with filters of wider bandwidth.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA326 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 ±0.2nA. 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 4 shows provision for an input
bias current path in a thermocouple application. Without a
bias current path, the inputs will float to an undefined poten-
tial and the output voltage may not be valid.
INPUT COMMON-MODE RANGE
Common instrumentation amplifiers do not respond linearly with
common-mode signals near the power-supply rails, even if “rail-
to-rail” op amps are used. The INA326 uses a unique topology
to achieve true rail-to-rail input behavior (see Figure 5, “Inside
the INA326”). The linear input voltage range of each input
terminal extends to 20mV below the negative rail, and 100mV
above the positive rail.
INPUT PROTECTION
The inputs of the INA326 are protected with internal diodes
connected to the power-supply rails. These diodes will clamp
the applied signal to prevent it from damaging the input
circuitry. If the input signal voltage can exceed the power
supplies by more than 0.5V, the input signal current should
be limited to less than 10mA to protect the internal clamp
diodes. This can generally be done with a series input
resistor. Some signal sources are inherently current-limited
and do not require limiting resistors.
FILTERING
Filtering can be adjusted through selection of R
2
C
2
and
R
O
C
O
for the desired trade-off of noise and bandwidth.
Adjustment of these components will result in more or less
ripple due to auto-correction circuitry noise and will also
affect broadband noise. Filtering limits slew rate, settling
time, and output overload recovery time.
It is generally desirable to keep the resistance of R
O
relatively
low to avoid DC gain error created by the subsequent stage
loading. This may result in relatively high values for C
O
to
produce the desired filter response. The impedance of R
O
C
O
can be scaled higher to produce smaller capacitor values if
the load impedance is very high.
Certain capacitor types greater than 0.1µF may have dielec-
tric absorption effects that can significantly increase settling
time in high-accuracy applications (settling to 0.01%). Polypro-
pylene, polystyrene, and polycarbonate types are generally
good. Certain “high-K” ceramic types may produce slow
settling “tails.” Settling time to 0.1% is not generally affected
by high-K ceramic capacitors. Electrolytic types are not
recommended for C
2
and C
O
.
INA326
Thermocouple
5
FIGURE 4. Providing Input Bias Current Return Path.