1 Introduction

Isolated amplifiers are essential building blocks in systems that require galvanic isolation for electrical isolation of two parts of a circuit. Good examples of systems that require galvanic isolation are high-voltage DC/DC converters, motor drives, hybrid electric vehicle (HEV) and electric vehicles (EV) traction inverters. Most commonly, isolated amplifiers sense current or voltage and transfer this information to the controller across the galvanic isolation barrier. Isolated amplifiers are similar to delta-sigma modulators in function; however, the output of the isolated amplifier is analog, whereas the output of the modulator is a digital signal. Many systems and engineers benefit from the analog output, as the system implementation and testing are generally better understood and do not require assistance from a microcontroller (MCU).

Typically, the output of the isolated amplifier interfaces with a SAR ADC.

The SAR ADC is the most common type present in MCUs. The resolution ranges from 10-b for low-cost to 16-b for high-end microcontrollers. A critical aspect of this implementation is that the SAR ADC's sample-and-hold (S/H) circuit momentarily disrupts the analog signal chain during data acquisition. MCUs have multiple analog inputs but only one, two, or three ADC blocks. For this reason, analog inputs share the ADC through the analog multiplexer. The multiplexed system makes things more difficult because the C_SH capacitor is typically not reset and therefore remembers the state of the previous channel. Figure 1-1 shows a simplified diagram as an example.

Analog amplifiers have two possible output types.

Differential output (Figure 1-2) is a preferred choice for systems where the physical distance between the isolated amplifier and the ADC is long or passes through connectors. The information carried to the controller is the voltage difference between two complementary outputs, not the absolute value with regard to the common ground. For this reason, this output effectively suppresses common-mode noise that can intrude the circuit between the ADC and the isolated amplifier. The drawback is that many ADCs cannot work directly with the differential signaling. In this case there is a differential-to-single-ended conversion in close proximity to the ADC. The difference amplifier converts the signal for the ADC but introduces additional measurement errors and increases the system complexity.

Single-ended output (Figure 1-3) interfaces directly with the ADC and does not require a difference amplifier. Typically, this device has a reference voltage input (REFIN) to add an offset to the output (OUT) or set the gain. This is easier to implement but this configuration cannot reject the common-mode noise. For this reason, this type of output is a preferred design for situations where the distance between the ADC and the isolated amplifier is relatively short (<10cm), or performance degradation due to the common mode noise is acceptable.

For both types, the engineer must understand very well how the sampling process works to avoid AC and DC signal chain performance loss.