OPA128 Datasheet, PDF (7/11 Pages) Burr-Brown (TI) – Difet Electrometer-Grade OPERATIONAL AMPLIFIER

APPLICATIONS INFORMATION
OFFSET VOLTAGE ADJUSTMENT
The OPA128 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 effort. Note that the trim (Figure 1) is similar to
operational amplifiers such as HA-5180 and AD515. The
OPA128 can replace many other amplifiers by leaving the
external null circuit unconnected.
+V_CC
The amplifier case should be connected to any input shield or
guard via pin 8. This insures that the amplifier itself is fully
surrounded by guard potential, minimizing both leakage and
noise pickup (see Figure 2).
Non-Inverting
Buffer
8
2
8
2
OPA128 ^{6 Out}
OPA128 ^{6 Out}
In
3
3
In
7
2
OPA128
3
5
4
(1)
–V_CC
6
1
±10mV Typical
Trim Range
NOTE: (1) 10kΩ to 1MΩ
Trim Potentiometer
(100kΩ Recommended)
FIGURE 1. Offset Voltage Trim.
INPUT PROTECTION
Conventional monolithic FET operational amplifiers’ inputs
must be protected against destructive currents that can flow
when input FET gate-to-substrate isolation diodes are for-
ward-biased. Most BIFET^® amplifiers can be destroyed by
the loss of –V_CC.
Because of its dielectric isolation, no special protection is
needed on the OPA128. Of course, the differential and
common-mode voltage limits should be observed.
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 preci-
sion 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 OPA128. To avoid leakage problems, it is
recommended that the signal input lead of the OPA128 be
wired to a Teflon standoff. If the input is to be soldered
directly into a printed circuit board, utmost care must be used
in planning the board layout. A “guard” pattern should
completely surround the high impedance input leads and
should be connected to a low impedance point which is at the
signal input potential.
Inverting
TO-99 Bottom View
In
2
3
4
5
6
OPA128 ^{6 Out}
7
2
3
8
1
8
BOARD LAYOUT
FOR INPUT GUARDING
Guard top and bottom of board.
Alternate: use Teflon^® standoff
for sensitive input pins.
Teflon^® E.I. Du Pont de Nemours & Co.
FIGURE 2. Connection of Input Guard.
Triboelectric charge (static electricity generated by friction)
can be a troublesome noise source from cables connected to
the input of an electrometer amplifier. Special low-noise cable
will minimize this effect but the optimum solution is to mount
the signal source directly at the electrometer input with short,
rigid, wiring to preclude microphonic noise generation.
TESTING
Accurately testing the OPA128 is extremely difficult due to its
high level of performance. Ordinary test equipment may not
be able to resolve the amplifier’s extremely low bias current.
Inaccurate bias current measurements can be due to:
1. Test socket leakage
2. Unclean package
3. Humidity or dew point condensation
4. Circuit contamination from fingerprints or anti-static
treatment chemicals
5. Test ambient temperature
6. Load power dissipation
BIFET^® National Semiconductor Corp.
®
7
OPA128