Opto-isolation amplifier giving no output

Question

First of all, thanks to the community for having such helpful answers here - don't post very often but here's one I could really do some help with.
I want to measure a voltage range of 0-100V DC in an isolated manner and using an Arduino Uno. I've got a HCPL 7520 (datasheet) and created an initial test circuit with just 5V input but I'm getting no output. I've got a voltage divider to bring the 5V down to the +/- 200mV range and chosen resistors to bring the current to ~10mA (which is less than the 20mA max).
But whenever I measure voltage between VOut and Vdd2_Gnd, I get 0V. I'm no expert so any advice would be much, much appreciated !

Andy aka · Accepted Answer

You've got $V_{REF}$ tied to 0 volts - the recommended value is between 4 volts and $V_{DD2}$: -

The whole premise for this device amplifying is that it has a gain determined by: -
$$boxed{dfrac{V_{REF}}{0.512}}$$

TimWescott · Answer

From the datasheet:

Gain is defined as the slope of the best-fit line of the output voltage vs. the differential input voltage (VIN+ - VIN-) over the specified input range.
Gain is derived from VREF/512 mV; e.g. VREF = 5.0, gain will be 9.77 V/V

And when VREF = 0, gain will be 0.  For starters, hook VREF to VDD2; if you need more precision than your supply on that side provides, hook VREF to a better voltage reference.

Cristobol Polychronopolis · Answer

That part provides an output between VDD2_GND and VREF, which the spec states should be between 4V and VDD2.  You're scaling the output to zero by connecting it to ground.  Try connecting it to VDD2.

Opto-isolation amplifier giving no output

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