The circuit shown in Figure 1 is a robust battery monitoring front end designed for environments where transients may occur, such as industrial or process automation environments. This circuit uses an ADG5408 4-channel CMOS multiplexer followed by an AD8226 instrumentation amplifier to accurately monitor the voltage of each battery with low power consumption and low cost, without the need for additional external transient protection circuits.

Transient overvoltage conditions can cause latch-up in traditional CMOS switches. Through junction isolation technology, the N and P wells of PMOS and NMOS transistors form parasitic silicon-controlled rectifier (SCR) circuits. An overvoltage condition triggers this SCR, causing the current to be significantly amplified, causing latch-up. Latch-up is an undesirable high-current condition that persists until power is turned off and can cause device failure.

Latch-up can occur if one of the input or output pin voltages exceeds the supply rail by more than one diode drop, or if the power supply is improperly sequenced. If a fault occurs on the channel and the signal exceeds the maximum rating, the fault can trigger a latch-up condition in typical CMOS devices.

During circuit power-up, it is also possible for a voltage at the input to occur before the CMOS switch is powered up, especially if multiple power supplies are used to power the circuit. This condition may exceed the maximum ratings of the device and trigger a latch-up condition.

The two multiplexers and instrumentation amplifiers (IA) used in this design have robust inputs. The ADG5408 is a latch-up proof, high voltage 8:1 multiplexer. The trench isolation technology used to manufacture the ADG5408 prevents latch-up conditions and reduces external protection shorts. Anti-latch-up does not guarantee overvoltage protection, it only means that the switch will enter a high-current SCR mode. The ADG5408 also has an 8 kV human body model electrostatic discharge (ESD) rating (ANSI/ESDA/JEDEC JS-001-2010).

The AD8226 is a low-cost, low-power instrumentation amplifier with a robust input that can handle input voltages up to 40 V from opposite supply rails while limiting the output to the supply rail. For example, with a ±18 V supply, the AD8226 positive or negative input has a damage-free swing of ±22 V. All inputs of the AD8226 are ESD protected via internal diodes.

A battery monitoring system (BMS) requires applying individual voltages across each cell within the battery pack to evaluate the battery's state of charge (SOC) and state of operation (SOH). Multiplexing the battery pack pins via two multiplexers, as shown in Figure 1, allows the voltage across each cell to be evaluated.

One multiplexer for the positive pin and another for the negative pin. This differential multiplexing allows a single instrumentation amplifier to be used for up to eight channels. This way the amplifier does not require the common mode voltage of each cell for the BMS to use.

The ADG5408 has low on-resistance per channel, typically 13.5 Ω and 22 Ω maximum over temperature. With a maximum input offset current of 2 nA, the maximum error voltage across the channel resistor is 44 nV.

Figure 2 shows the results of a typical CMOS switch (using epitaxial layers) compared to the ADG5408 when subjected to latch-up testing. During the test, the stress current is applied to the pin for 1 ms. This operation is called triggering, and the current on the pin is measured after triggering. This particular test is performed with the switch open, drain (D) set to VDD and source (S) set to VSS, as shown in Figure 3. The source voltage is then driven beyond VSS until the desired trigger current is reached. If latch-up does not occur, the pin current returns to the pre-trigger value. After latch-up occurs, the pin continues to draw current without being driven by the trigger voltage. It can only be stopped by turning off the device.

As can be seen in Figure 2, this typical CMOS switch reaches latch-up current at −290 mA, whereas the ADG5408 does not latch-up unless the test ends at −510 mA.