7
®
OPA177
APPLICATIONS INFORMATION
The OPA177 is unity-gain stable, making it easy to use and
free from oscillations in the widest range of circuitry. Ap-
plications with noisy or high impedance power supply lines
may require decoupling capacitors close to the device pins.
In most cases 0.1µF ceramic capacitors are adequate.
The OPA177 has very low offset voltage and drift. To
achieve highest performance, circuit layout and mechanical
conditions must be optimized. Offset voltage and drift can
be degraded by small thermoelectric potentials at the op amp
inputs. Connections of dissimilar metals will generate ther-
mal potential which can mask the ultimate performance of
the OPA177. These thermal potentials can be made to cancel
by assuring that they are equal in both input terminals.
1. Keep connections made to the two input terminals close
together.
2. Locate heat sources as far as possible from the critical
input circuitry.
3. Shield the op amp and input circuitry from air currents
such as cooling fans.
OFFSET VOLTAGE ADJUSTMENT
The OPA177 has been laser-trimmed for low offset voltage
and drift so most circuits will not require external adjust-
ment. Figure 1 shows the optional connection of an external
potentiometer to adjust offset voltage. This adjustment should
not be used to compensate for offsets created elsewhere in a
system since this can introduce excessive temperature drift.
INPUT PROTECTION
The inputs of the OPA177 are protected with 500Ω series
input resistors and diode clamps as shown in the simplified
circuit diagram. The inputs can withstand ±30V differential
inputs without damage. The protection diodes will, of course,
conduct current when the inputs are overdriven. This may
disturb the slewing behavior of unity-gain follower applica-
tions, but will not damage the op amp.
NOISE PERFORMANCE
The noise performance of the OPA177 is optimized for
circuit impedances in the range of 2kΩ to 50kΩ. Total noise
in an application is a combination of the op amp’s input
voltage noise and input bias current noise reacting with
circuit impedances. For applications with higher source
impedance, the OPA627 FET-input op amp will generally
provide lower noise. For very low impedance applications,
the OPA27 will provide lower noise.
INPUT BIAS CURRENT CANCELLATION
The input stage base current of the OPA177 is internally
compensated with an equal and opposite cancellation cur-
rent. The resulting input bias current is the difference
between the input stage base current and the cancellation
current. This residual input bias current can be positive or
negative.
When the bias current is cancelled in this manner, the input
bias current and input offset current are approximately the
same magnitude. As a result, it is not necessary to balance
the DC resistance seen at the two input terminals (Figure 2).
A resistor added to balance the input resistances may actu-
ally increase offset and noise.
FIGURE 1. Optional Offset Nulling Circuit.
OPA177
V
2
3
1
8
Trim Range is approximately ±3.0mV
V+
20kΩ
OUT
V
IN
Op Amp
(a)
R
B
OPA177
(b)
No bias current
cancellation resistor needed
Conventional op amp with
external bias current
cancellation resistor.
OPA177 with no external
bias current cancellation
resistor.
= R
2
|| R
1
R
2
R
1
R
2
R
1
FIGURE 2. Input Bias Current Cancellation.
