Optimizing High Voltage Common Mode Circuit

Here’s a design circuit for to measure the Avago Technologies ACPL-M61L’s high-voltage common mode rejection (HVCMR). Three production samples were randomly selected for the measurements. VDD1 is the transmitter supply voltage used to turn on the LED. Limiting resistor R1 is connected to the LED anode and R2 is connected to the LED cathode. R1 and R2 connected in this common-mode fashion, rather than a single resistor, enhances CMR performance. Here’s the figure of the design circuit;

Common mode noise is often seen at the system application level where there is a diff erence in the ground levels of an isolating component’s input control circuitry and output control circuitry. This is especially true when a ground line is floating (device ground connected to a common line). In the ACPL-x6xL family, the common mode rejection (CMR) specification indicates the ability to reject common mode noise. This is also known as common mode transient rejection (CMTR). CMTR s). Thedescribes the maximum tolerable rising/falling rate of a common mode voltage (given in volts per microsecond, V/ CMTR specification includes the amplitude of the common mode voltage (VCM) that can be tolerated. The common mode voltage slew rate that the optocoupler can tolerate and hold the correct output state is referred to as common mode transient immunity (CMTI).

Common mode noise can be coupled to the opto coupler output by external circuitry. Common mode noise, especially in a high electromagnetic interference (EMI) environment, can adversely affect the output state of the opto coupler through a conductive medium, primarily capacitive and inductive parasitics, Metallic printed circuit board (PCB) tracks that operate at high frequency can couple charge to the LED input pin or to the opto coupler output pin through parasitic capacitors between adjacent metal tracks. It is often difficult to identify the root cause of common mode noise or interference that is introduced by the circuit/system/application or by other forms of external factors that couple noise. When the source of common mode noise is identified, corrective measures are easy to implement by adding decoupling capacitors or filters to the system, or by adding some form of shielding.