Datasheet OPA128 (Texas Instruments) - 7

APPLICATIONS INFORMATION
The amplifier case should be connected to any input shield or guard via pin 8. This insures that the amplifier itself is fully
OFFSET VOLTAGE ADJUSTMENT
surrounded by guard potential, minimizing both leakage and The OPA128 offset voltage is laser-trimmed and will require noise pickup (see Figure 2). no further trim for most applications. As with most amplifi- ers, externally trimming the remaining offset can change Non-Inverting Buffer 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 8 8 2 2 external null circuit unconnected. 6 Out 6 Out
OPA128 OPA128
3 3 In In +VCC 7 Inverting TO-99 Bottom View 2 6
OPA128
In 4 5 6 3 1 2 3 7 6 Out 5 ±10mV Typical
OPA128
2 4 3 Trim Range 8 8 (1) 1 NOTE: (1) 10kΩ to 1MΩ Trim Potentiometer –VCC (100kΩ Recommended) BOARD LAYOUT FOR INPUT GUARDING Guard top and bottom of board. FIGURE 1. Offset Voltage Trim. Alternate: use Teflon® standoff for sensitive input pins. Teflon® E.I. Du Pont de Nemours & Co.
INPUT PROTECTION
Conventional monolithic FET operational amplifiers’ inputs FIGURE 2. Connection of Input Guard. must be protected against destructive currents that can flow when input FET gate-to-substrate isolation diodes are for- Triboelectric charge (static electricity generated by friction) ward-biased. Most BIFET® amplifiers can be destroyed by can be a troublesome noise source from cables connected to the loss of –V . CC the input of an electrometer amplifier. Special low-noise cable will minimize this effect but the optimum solution is to mount Because of its dielectric isolation, no special protection is the signal source directly at the electrometer input with short, needed on the OPA128. Of course, the differential and rigid, wiring to preclude microphonic noise generation. common-mode voltage limits should be observed. Static damage can cause subtle changes in amplifier input characteristics without necessarily destroying the device. In
TESTING
precision operational amplifiers (both bipolar and FET types), Accurately testing the OPA128 is extremely difficult due to its this may cause a noticeable degradation of offset voltage and high level of performance. Ordinary test equipment may not drift. be able to resolve the amplifier’s extremely low bias current. Static protection is recommended when handling any preci- Inaccurate bias current measurements can be due to: sion IC operational amplifier. 1. Test socket leakage 2. Unclean package
GUARDING AND SHIELDING
3. Humidity or dew point condensation As in any situation where high impedances are involved, careful shielding is required to reduce “hum” pickup in input 4. Circuit contamination from fingerprints or anti-static leads. If large feedback resistors are used, they should also be treatment chemicals shielded along with the external input circuitry. Leakage 5. Test ambient temperature currents across printed circuit boards can easily exceed the 6. Load power dissipation bias current of the OPA128. To avoid leakage problems, it is recommended that the signal input lead of the OPA128 be BIFET® National Semiconductor Corp. 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. ® 7
OPA128