TIDUEZ8 May 2021
As stated, the isolation voltage is monitored with an AMC3330 across the isolation barrier. The AMC3330 is really convenient for this application as it offers an integrated DC/DC converter and high CMTI bipolar input (VINP – VINN) in the range from –1 V to 1 V.
The output of the AMC3330 is a fully-differential analog signal comprised of the OUTP and OUTN pins centered around a common-mode voltage of 1.44 V that can be fed directly to a stand-alone analog-to-digital converter (ADC).
The MSP430 and C2000 family of processors have embedded single-ended input ADCs. The addition of a differential to single-ended amplifier output stage, illustrated in Figure 2-10, allows the full output range of the AMC3330 to be served to the 3.3-V range single-ended embedded ADC. The signal range is amplified, and the common-mode voltage is set to half of the ADC range with the REF2033, dual-output Vref and Vref / 2 voltage reference.
The TLV6001 operational amplifier for cost-sensitive systems is used for this purpose. Further details on the TI isolated amplifier family differential output conversion to singled ended are found in the Interfacing a Differential-Output (Isolated) Amplifier to a Single-Ended Input ADC application brief.
At ideal conditions, when the isolation barrier is unbroken and one of the resistive branches is switched in, the current flowing through the isolation barrier is 0 V and Vref / 2 should be present at the input of the ADC. The opposite occurs when the isolation barrier is fully broken, as the full DC bus voltage is seen at the isolation voltage. The voltage drop at the input of the AMC3330 is –1 V for the negative side switching or +1 V for the positive side switching. This translates into the 0 V and 3.3 V at the single-ended 0-V to 3.3-V range ADC.
Hence, a gain of 0.825 is set with the ratio of R1/R2 and R4/R3. Common-mode voltage of 1.65 V is set with the Vref / 2 output of the REF2033 voltage reference.