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TC911BCOA Arkusz danych(PDF) 4 Page - Microchip Technology |
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TC911BCOA Arkusz danych(HTML) 4 Page - Microchip Technology |
4 / 7 page 4 TC911A TC911/B © 2001 Microchip Technology Inc. DS21481A Monolithic Auto-Zeroed Operational Amplifers Pin Compatibility The CMOS TC911 is pin compatible with the industry standard ICL7650 chopper-stabilized amplifier. The ICL7650 must use external 0.1 µF capacitors connected at pins 1 and 8. With the TC911, external offset voltage error cancel- ing capacitors are not required. On the TC911 pins 1, 8 and 5 are not connected internally. The ICL7650 uses pin 5 as an optional output clamp connection. External chopper capacitors and clamp connections are not necessary with the TC911. External circuits connected to pins 1, 8 and 5 will have no effect. The TC911 can be quickly evaluated in existing ICL7650 designs. Since external capacitors are not required, system part count, assembly time, and total sys- tem cost are reduced. Reliability is increased and PC board layout eased by having the error storage capacitors inte- grated on the TC911 chip. The TC911 pinout matches many existing op-amps: 741, LM101, LM108, OP05–OP08, OP-20, OP-21, ICL7650 and ICL7652. In many applications operating from +5V supplies the TC911 offers superior electrical performance and can be a functional pin-compatible replacement. Offset voltage correction potentiometers, compensation capaci- tors, and chopper-stabilization capacitors can be removed when retrofitting existing equipment designs. Thermocouple Errors Heating one joint of a loop made from two different metallic wires causes current flow. This is known as the Seebeck effect. By breaking the loop, an open circuit voltage Figure 1. Unwanted Thermocouple Errors Eliminated by Reducing Thermal Gradients and Balancing Junctions J3 J4 J5 J2 J1 J6 PACKAGE PIN J = J J = J J = J 3 2 1 4 5 6 NO TEMPERATURE DIFFERENTIAL AND SAME METALLIC CONNECTION + – – + + – + – + – + – V2 V1 V5 V6 V4 V3 J4 J3 J2 J1 J5 J6 V = 0 T V = V + V + V – V – V – V = 0 T1 2 3 4 5 6 (Seebeck voltage) can be measured. Junction tempera- ture and metal type determine the magnitude. Typical values are 0.1 µV/°C to 10µV/°C. Thermal-induced voltages can be many times larger than the TC911 offset voltage drift. Unless unwanted thermocouple potentials can be con- trolled, system performance will be less than optimum. Unwanted thermocouple junctions are created when leads are soldered or sockets/connectors are used. Low thermo-electric coefficient solder can reduce errors. A 60% Sn/36% Pb solder has 1/10 the thermal voltage of common 64% Sn/36% Pb solder at a copper junction. The number and type of dissimilar metallic junctions in the input circuit loop should be balanced. If the junctions are kept at the same temperature, their summation will add to zero-canceling errors (Figure 1). Shielding precision analog circuits from air currents — especially those caused by power dissipating components and fans — will minimize temperature gradients and ther- mocouple-induced errors. Avoiding Latch-Up Junction-isolated CMOS circuits inherently contain a parasitic p-n-p-n transistor circuit. Voltages exceeding the supplies by 0.3V should not be applied to the device pins. Larger voltages can turn the p-n-p-n device on, causing excessive device power supply current and excessive power dissipation. TC911 power supplies should be established at the same time or before input signals are applied. If this is not possible input current should be limited to 0.1mA to avoid triggering the p-n-p-n structure. Overload Recovery The TC911 recovers quickly from the output satura- tion. Typical recovery time from positive output saturation is 20msec. Negative output saturation recovery time is typi- cally 5msec. |
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