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VRE304 Datasheet(PDF) 4 Page - List of Unclassifed Manufacturers |
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VRE304 Datasheet(HTML) 4 Page - List of Unclassifed Manufacturers |
4 / 41 page DISCUSSION OF PERFORMANCE THEORY OF OPERATION The following discussion refers to the schematic in figure 2 below. A FET current source is used to bias a 6.3V zener diode. The zener voltage is divided by the resistor network R1 and R2. This voltage is then applied to the noninverting input of the operational amplifier which amplifies the voltage to produce a 2.500V output. The gain is determined by the resistor networks R3 and R4: G=1 + R4/R3. The 6.3V zener diode is used because it is the most stable diode over time and temperature. The current source provides a closely regulated zener current, which determines the slope of the references’ voltage vs. temperature function. By trimming the zener current a lower drift over temperature can be achieved. But since the voltage vs. temperature function is nonlinear this compensation technique is not well suited for wide temperature ranges. Thaler Corporation has developed a nonlinear compensation network of thermistors and resistors that is used in the VRE series voltage references. This proprietary network eliminates most of the nonlinearity in the voltage vs. temperature function. By adjusting the slope, Thaler Corporation produces a very stable voltage over wide temperature ranges. VRE302 FIGURE 2 VRE302DS REV. F MAY 2001 This network is less than 2% of the overall network resistance so it has a negligible effect on long term stability. Figure 3 shows the proper connection of the VRE302 series voltage references with the optional trim resistor for initial error. The VRE302 reference has the ground terminal brought out on two pins (pin 4 and pin 7) which are connected together internally. This allows the user to achieve greater accuracy when using a socket. Voltage references have a voltage drop across their power supply ground pin due to quiescent current flowing through the contact resistance. If the contact resistance was constant with time and temperature, this voltage drop could be trimmed out. When the reference is plugged into a socket, this source of error can be as high as 20ppm. By connecting pin 4 to the power supply ground and pin 7 to a high impedance ground point in the measurement circuit, the error due to the contact resistance can be eliminated. If the unit is soldered into place, the contact resistance is sufficiently small that it does not effect performance. Pay careful attention to the circuit layout to avoid noise pickup and voltage drops in the lines. EXTERNAL CONNECTIONS FIGURE 3 8 4 6 5 + V OUT 2 + V IN VRE302 10kΩ C N 1µF OPTIONAL NOISE REDUCTION CAPACITOR OPTIONAL FINE TRIM ADJUSTMENT 3 V TEMP OUT 7 REF. GND |
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