# Opto-isolation amplifier output is too off

(As a follow-up to an earlier question answered by the community - thanks!) So I've got the test circuit setup using the HCPL 7520 (datasheet) now and planning to use an Arduino Uno to measure the voltage at the VOUT.

I've done some tests now though and the VOUT seems to be too off. Below, I've got vRef voltage-divided to 4.41V and the VIN+ to 154mV. The rough VOUT I was expecting was VIN * GAIN = .154 * (4.41/.512) = 1.32V. But I read 3.05V at VOUT and, once adjusted (by -vRef/2 by looking at Fig.5 in the datasheet), I get 0.845 which is completely off.

(Pls ignore pin numbers in the diagram)

Another attempt was VIN+ = 104mV, expected = 0.89V. But I read 2.73V at VOUT, which adjusted becomes 0.525V. Again, not right.

I'm clearly doing something wrong - could anyone correct me or point me to the right direction?

Thanks!

• Do you get 2 volts out if you short out R2 or open circuit R1? Have you used the pin numbers in the diagram for the chip as per above? Where did the diagram come from? – Andy aka Aug 14 '20 at 10:36
• I got 2.26v for shorting out R2 and 2.16V for open circuit R1. Apologies, those pin numbers in the diagram are wrong so pls ignore pin numbers (I created the diagram with a generic IC part and didn't change pin numbers) – nchaud Aug 14 '20 at 12:04
• Maybe you need to show your actual circuit. 2 volts is about right with the resistor checks I suggested. Have you confirmed the input voltage with a multimeter and the reference voltage also (just in case of resistor value error). – Andy aka Aug 14 '20 at 12:13
• Your C1, C2 and C3 are still in backwards. If you draw your schematic conventionally with positive on top this becomes instantly apparent. Also if R1 and R2 were drawn vertically and in line with each other it becomes instantly apparent that they form a voltage divider.. – Transistor Aug 14 '20 at 12:20