Electrical – Isolated Voltage Measurement

In my application I need to measure a voltage that is in the area of 50V, but my microcontroller has to be isolated from my power circuit. I came across the ACPL-C87A-000E IC. I followed the typical application circuit found in page 12 of the datasheet, but at first, I am not using an op amp. The circuit I am now using, converts the 50V to around 1.9V via a voltage divider, which is sent into the IC. This is my circuit:

^{simulate this circuit – Schematic created using CircuitLab}

By looking at figure 13 of the datasheet, page 9 (Vin vs Vout+,Vout-), I thought that I could send the 1.9V in the IC and by measuring just the Vout-, I could get a value that is proportional to the voltage measured (to be exact, in figure 13, Vout- and Vin seem to be inversely proportional). I left the Vout+ pin unconnected. Also, let me point out that all the voltage measurements are taken from digital multimeters.

However, there are some problems with the above-mentioned idea.

1. Firstly, immediately after connecting the voltage divider's output voltage to the Vin of the IC, there seems to be a drop in that voltage (goes from 1.9V down to ~1.88V). The IC's Vout- value however seems to be correct for the given Vin, around the one expected from figure 13.
2. Furthermore, the voltage divider's output voltage continues to change slowly, going from 1.84 to 1.89, as long as it is connected to the Vin of the IC. Obviously, the Vout- changes accordingly.
3. After leaving the circuit on for some time, the voltage divider's output suddenly dropped significantly, to around 0.3 Volts, which I think may be the result of the IC going bad for some reason.

Another option I am considering for voltage measurement is the LTC6992-1, as suggested here. However, I don't know how to implement the last part, of measuring the PWM's duty cycle and converting it back to an analog voltage that can be read from my microcontroller. I am using a Beaglebone Black.

I would be really grateful for any other suggestions, perhaps simpler solutions, for measuring an isolated voltage with my microcontroller.