This circuit was designed for monitoring the charge- and discharge process of a 6-V lead-acid battery. This process takes place between 6.2 V and 6.8 V. The author used a measuring instrument that has several measuring ranges (0–1 V, 0–10 V, etc.). The 10 V range, however, is too coarse for this measurement. A better measurement result could be obtained if 6 V were subtracted from the measured voltage. The measuring range would then be from 6 to 7 V.
A single opamp such as the LF351 suffered from the mutual dependency of the measured and offset voltages and was therefore not suitable. The AD620 from Analog Devices, however, has been specially designed for this type of application and works well. In this opamp, each of the input signals has its own opamp so that they do not interfere with each other.
The schematic is simple. The offset voltage can be set accurately with a 10-turn potentiometer. The resistor of 5.49 kΩ (1%) can be connected or removed from the circuit with a jumper; without the resistor, the gain is one, with a resistor the difference voltage will be amplified 10 times (9.998 times, to be more accurate).
The AD620 draws slightly more than 1 mA (idle current is 1.3 mA max.), so that battery power is also an option. The IC can be used with power supply voltages ranging from ±2.3 V to ±18 V. Small button cells could even be considered when doing only brief measurements. The maximum differential voltage is 25 V, something that you will have to take into account, particularly if you are going to measure an unknown voltage.
The greatest DC accuracy is obtained with the version of opamp shown in the circuit. There is also a cheaper version, the AD620ANZ (the Z stands for lead-free). For a good application note about the AD620, we can recommend the document that describes the evaluation board made by Analog Devices (EVALINAMP-62RZ_82RZ_82_RMZ.pdf at ), in addition to the datasheet, of course.