OPA111 Datasheet, PDF (8/12 Pages) Burr-Brown (TI) – Low Noise Precision Difet OPERATIONAL AMPLIFIER

TYPICAL PERFORMANCE CURVES (CONT)
T_A = +25°C, V_CC = ±15VDC unless otherwise noted.
INPUT OFFSET VOLTAGE CHANGE
DUE TO THERMAL SHOCK
150
75
0 25°C
85°C
–75
AM
BM
T_A = 25°C to T_A = 85°C
Air Environment
–150
–1
0
1
2
3
4
5
Time From Thermal Shock (Minutes)
APPLICATIONS INFORMATION
OFFSET VOLTAGE ADJUSTMENT
The OPA111 offset voltage is laser-trimmed and will require
no further trim for most applications. As with most amplifi-
ers, externally trimming the remaining offset can change
drift performance by about 0.3µV/°C for each 100µV of
adjusted offset. Note that the trim (Figure 1) is similar to
operational amplifiers such as 741 and AD547. The OPA111
can replace most other amplifiers by leaving the external
null circuit unconnected.
INPUT PROTECTION
Conventional monolithic FET operational amplifiers require
external current-limiting resistors to protect their inputs
against destructive currents that can flow when input FET
gate-to-substrate isolation diodes are forward-biased. Most
BIFET amplifiers can be destroyed by the loss of –V_CC.
+V_CC
7
2
3 ^OPA111
5
4
–V_CC
6
1
10kΩ to 1M trim potentiometer
(100kΩ recommended).
±10mV typical trim range.
FIGURE 1. Offset Voltage Trim.
Unlike BIFET amplifiers, The Difet OPA111 requires input
current limiting resistors only if its input voltage is greater
than 6V more negative than –V_CC. A 10kΩ series resistor
will limit input current to a safe level with up to ±15V input
levels, even if both supply voltages are lost.
2
I_IN
1
V
Maximum Safe Current
0
–1
Maximum Safe Current
–2
–15
–10
–5
0
5
Input Voltage (V)
10
15
FIGURE 2. Input Current vs Input Voltage with ±V_CC Pins
Grounded.
Static damage can cause subtle changes in amplifier input
characteristics without necessarily destroying the device. In
precision operational amplifiers (both bipolar and FET types),
this may cause a noticeable degradation of offset voltage and
drift. Static protection is recommended when handling any
precision IC operational amplifier.
GUARDING AND SHIELDING
As in any situation where high impedances are involved,
careful shielding is required to reduce “hum” pickup in input
leads. If large feedback resistors are used, they should also
be shielded along with the external input circuitry.
Leakage currents across printed circuit boards can easily
exceed the bias current of the OPA111. To avoid leakage
problems, it is recommended that the signal input lead of the
OPA111 be wired to a Teflon standoff. If the OPA111 is to
be soldered directly into a printed circuit board, utmost care
must be used in planning the board layout. A “guard” pattern
®
OPA111
8