# Opto-isolation amplifier giving no output

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 !

• I would not ground your Vref . That needs to be between 4 V and Vdd inclusive. – Tony Stewart EE75 Aug 12 '20 at 16:21
• thanks, yep that did it :) – nchaud Aug 12 '20 at 17:05

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: -

$$\boxed{\dfrac{V_{REF}}{0.512}}$$

• Thank you so much, God bless, that's done it :) – nchaud Aug 12 '20 at 17:04

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.

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.