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Voltage Difference Magnifier Schematic Circuit Diagram

Monitoring the Charge-and-Discharge Process of a 6-V Lead-Acid Battery

This particular circuit was specifically devised for overseeing the intricate charge-and-discharge phases of a 6-V lead-acid battery. These crucial processes occur within the voltage range of 6.2 V to 6.8 V. To accomplish this task, the author utilized a measuring instrument equipped with multiple measuring ranges, including 0–1 V, 0–10 V, and so forth. However, the 10 V range proved to be excessively imprecise for this measurement. A more accurate measurement outcome could be achieved by subtracting 6 V from the measured voltage, effectively narrowing the measuring range to a more precise 6 to 7 V.

Voltage Difference Magnifier Schematic Circuit Diagram

Optimal Opamp Selection for Precise Measurements

The use of a single opamp like the LF351 proved problematic due to the intertwined relationship between measured and offset voltages, rendering it unsuitable for the task at hand. In contrast, the AD620, specifically engineered by Analog Devices for this application, has proven highly effective. Within this opamp, each input signal operates through its dedicated opamp, preventing any interference between them.

Simplified Circuit Design for Accurate Offset Voltage Adjustment

The schematic for this setup is elegantly straightforward. An accurate adjustment of the offset voltage is achieved through a 10-turn potentiometer. A 5.49 kΩ resistor (with a 1% tolerance) can be included or omitted from the circuit using a jumper. Without the resistor, the gain remains at one; however, with the resistor, the difference voltage undergoes a tenfold amplification (more precisely, 9.998 times).

Versatile Power Options and Voltage Considerations

The AD620 operates with a slightly higher current draw, just over 1 mA (with a maximum idle current of 1.3 mA), making battery power a viable choice. The IC is compatible with power supply voltages ranging from ±2.3 V to ±18 V. For brief measurements, even small button cells could be utilized. However, it’s crucial to note the maximum differential voltage, capped at 25 V, especially when measuring an unknown voltage.

Optimizing DC Accuracy with AD620 Opamp Versions

The circuit demonstrates its highest DC accuracy with the opamp version illustrated. An economical alternative exists in the form of the AD620ANZ (with the ‘Z’ indicating it is lead-free). For comprehensive insights into AD620 applications, referring to Analog Devices’ evaluation board document (EVALINAMP-62RZ_82RZ_82_RMZ.pdf at [1]) alongside the datasheet is highly recommended.


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