User manual TRUST WB-1400T

[. . . ] Terminals are also provided for use in applications requiring input offset voltage nulling. The use of PMOS field effect transistors in the input stage results in common mode input voltage capability down to 0. 5V below the negative supply terminal, an important attribute for single supply applications. The output stage uses bipolar transistors and includes built-in protection against damage from load terminal short circuiting to either supply rail or to ground. The CA3140 Series has the same 8-lead pinout used for the "741" and other industry standard op amps. [. . . ] The 1, 000, 000/1 adjustment range is accomplished by a single variable potentiometer or by an auxiliary sweeping signal. The CA3140 functions as a noninverting readout amplifier of the triangular signal developed across the integrating capacitor network connected to the output of the CA3080A current source. Buffered triangular output signals are then applied to a second CA3080 functioning as a high speed hysteresis switch. Output from the switch is returned directly back to the input of the CA3080A current source, thereby, completing the positive feedback loop The triangular output level is determined by the four 1N914 level limiting diodes of the second CA3080 and the resistor divider network connected to Terminal No. These diodes establish the input trip level to this switching stage and, therefore, indirectly determine the amplitude of the output triangle. Compensation for propagation delays around the entire loop is provided by one adjustment on the input of the CA3080. This adjustment, which provides for a constant generator amplitude output, is most easily made while the generator is sweeping. High frequency ramp linearity is adjusted by the single 7pF to 60pF capacitor in the output of the CA3080A. It must be emphasized that only the CA3080A is characterized for maximum output linearity in the current generator function. Sine Wave Shaper The circuit shown in Figure 12 uses a CA3140 as a voltage follower in combination with diodes from the CA3019 Array to convert the triangular signal from the function generator to a sine-wave output signal having typically less than 2% THD. The basic zero crossing slope is established by the 10k potentiometer connected between Terminals 2 and 6 of the CA3140 and the 9. 1k resistor and 10k potentiometer from Terminal 2 to ground. Positive feedback via D5 and D6 establishes the zero slope at the maximum and minimum levels of the sine wave. The slew rate required of this amplifier is 28V/µs (18VP-P x x 0. 5MHz). +15V SIGNAL LEVEL ADJUSTMENT 2. 5k 200 3 2 REFERENCE VOLTAGE INPUT VOLTAGE ADJUSTMENT 3 + 7 6 REGULATED OUTPUT CA3140 2 - 4 FIGURE 15. BASIC SINGLE SUPPLY VOLTAGE REGULATOR SHOWING VOLTAGE FOLLOWER CONFIGURATION Essentially, the regulators, shown in Figures 16 and 17, are connected as non inverting power operational amplifiers with a gain of 3. 2. An 8V reference input yields a maximum output voltage slightly greater than 25V. As a voltage follower, when the reference input goes to 0V the output will be 0V. Because the offset voltage is also multiplied by the 3. 2 gain factor, a potentiometer is needed to null the offset voltage. Series pass transistors with high ICBO levels will also prevent the output voltage from reaching zero because there is a finite voltage drop (VCESAT) across the output of the CA3140 (see Figure 2). This saturation voltage level may indeed set the lowest voltage obtainable. The high impedance presented by Terminal 8 is advantageous in effecting current limiting. Thus, only a small signal transistor is required for the current-limit sensing amplifier. Resistive decoupling is provided for this transistor to minimize damage to it or the CA3140 in the event of unusual input or output transients on the supply rail. Figures 16 and 17, show circuits in which a D2201 high speed diode is used for the current sensor. [. . . ] This load current is increased by the multiplication factor R2/R1, when the load current is monitored by the power supply meter M. Thus, if the load current is 100nA, with values shown, the load current presented to the supply will be 100µA; a much easier current to measure in many systems. R1 10k +15V IL x R2 R1 3 M 2 POWER SUPPLY 0. 1µF 7 + CA3140 6 0. 1µF R2 10M IL RL 5 100k 1 4 Top Trace: Output; 50mV/Div. , 200ns/Div. BASIC CURRENT AMPLIFIER FOR LOW CURRENT MEASUREMENT SYSTEMS Note that the input and output voltages are transferred at the same potential and only the output current is multiplied by the scale factor. The dotted components show a method of decoupling the circuit from the effects of high output load capacitance and the potential oscillation in this situation. [. . . ]