The application circuit in Figure 7-1 shows a schematic for a battery cell voltage monitoring system. This circuit example is intended to support eight standard Lithium-Ion 4.2V batteries. The difference amplifier is used to monitor the voltage level of each battery cell within a battery pack to monitor the state of health of the batteries.

The battery-monitoring circuit functions by using the INA1H94-SP, unity-gain difference amplifier, to accurately measure the voltage from each battery cell, and level-shift the common-mode voltage to the ADC input range. The INA1H94-SP is powered with bipolar supplies of +12V (VS+) and –5V (VS–). The difference amplifier is able to accommodate the input common-mode voltage of each battery cell on the 33.6V, 8-cell battery stack.

The ADC128S12QML-SP is a radiation hardened, 12-bit, 8-channel, 50kSPS to 1MSPS successive approximation register (SAR) ADC powered with a 5V unipolar supply. The INA1H94-SP op-amp buffers the difference amplifier output, and supports driving the SAR ADC up to 500kSPS maximum sampling rate.

Figure 7-1 INA1H94-SP Battery Cell Monitor Circuit- Bipolar Supplies

Alternatively, Figure 7-2 shows the battery cell monitor circuit powered with a unipolar +12V (VS+) supply. The INA1H94-SP does not support rail-to-rail output swing, leading to the addition of a +2.5V reference. The difference amplifier output swing requires at least 1.5V headroom above the negative supply (VS–), requiring to bias the REF_A and REF_B reference input pins to a voltage above 1.5V to level-shift the output signal to meet the output linear range of the difference amplifier. The output of the INA is fed into a voltage divider using 0.05% tolerance resistors to bring the signal within the 5V full-scale range of the ADC.

Figure 7-2 INA1H94-SP Battery Cell Monitor Circuit-Unipolar Supply