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Title says it all: can an opto-isolated input be analog or are they intrinsically digital?

Why: I am looking to purchase a relay control board and am deciding between a few options.

Here is my primary option, however I need 5 analog inputs and this one states that it has opto-isolated inputs. Could those be analog?

EDIT: Page 6 of the user manual has each opto-isolated line connected to a digital and an analog input pin. Does this mean it can accept analog (voltage) inputs?

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In the context of the O.P.'s I/O box

It comes with full schematic. That could enable a little bit of hacking.
If I'm reading the schematic correctly, the opto-coupled inputs are connected to PC1/ADC1, PC2/ADC2, PC3/ADC3 on the ATMega. If U9, which is the opto-coupler IC, is removed then the ADC lines can be connected directly to the outside connectors. That would create 3 straight analog inputs.

Outside of the context of the O.P.'s relay I/O board.
More generally.

Yes, an analog input can be opto-isolated. This is usually done with specialized opto-couplers, such as LOC series.

isolation amplifier photoconductive operation

isolation amplifier photovoltaic operation

(source: IXYS application note AN-107)

These types of circuits aren't neatly as common as digital opto-couplers, though.

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    \$\begingroup\$ +1, all I can add is that for the few LED's I've looked at, the light intensity is only linear with the current over a small range. (a factor of ~10 or so.) below ~1mA there are band edge effects, and above ~10mA there start to be heating effects. \$\endgroup\$ – George Herold Nov 21 '14 at 23:09
  • \$\begingroup\$ @GeorgeHerold Note that in the above circuit there are two photodiodes coupled to a single LED so the LED ugliness (including temperature drift and aging) is mostly cancelled out by op-amp gain in the feedback loop. Using two separate optos would be much dicier. \$\endgroup\$ – Spehro Pefhany Nov 21 '14 at 23:19
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    \$\begingroup\$ @SpehroPefhany, yeah I totally noticed that! (the reason for the +1) You can do analog with a specialized opto. \$\endgroup\$ – George Herold Nov 22 '14 at 0:21
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The description of that board, and its user manual, indicate that it has only three analog inputs. The opto-isolated inputs are digital.

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  • \$\begingroup\$ I just noticed, in the user manual here: sparkfun.com/datasheets/DevTools/Arduino/KTA-223-Manual.pdf it seems that the opto-isolated lines are connected to two pins - an analog and a digital. \$\endgroup\$ – jules0075 Nov 21 '14 at 22:21
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    \$\begingroup\$ @jules0075: No - each opto input is only connected to one pin but the microcontroller can use those pins as either analog or digital inputs. As Ignacio says, you can get a degree of analog-ish input through some optoisolators, but in this board, they are intended to be used digitally. \$\endgroup\$ – Peter Bennett Nov 21 '14 at 22:29
  • \$\begingroup\$ If you look at the schematic you can see that all 4 are connected to analog inputs. The problem is that they're connected in such a way that they have very poor analog performance, so should only be used for digital input. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 21 '14 at 22:45
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To answer the larger question, optocouplers can be analog in the current domain; The "DC current transfer ratio" (CTRDC) describes the current gain/attenuation provided by the coupler.

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  • \$\begingroup\$ Thanks. Just want to clarify as this is new to me: it will not work in the voltage domain? If I'm looking to translate a battery's voltage into an input signal - that can't be done via an opto-isolator? \$\endgroup\$ – jules0075 Nov 21 '14 at 22:27
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    \$\begingroup\$ You would need to convert it into a current first, which can be done by either a resistor or a FET, and then back into a voltage on the other side, with a resistor. But it's certainly not voltage-in, voltage-out. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 21 '14 at 22:43
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You need more answers to believe, ok.

As you can see in the post from Nick Alexeev it is possible to make opto isolated analog inputs, but you have to make quite some effort to do it. To get enough precission for an AD converter even the effort shown in Nicks circuits is not enough. Be shure if that cheap box could do that, it would be written with BIG letters.

The table on page 6 in the manual shows only that the signals from the isolated inputs are connected to arduino inputs that could be used as analog or digital inputs. I'm sure they are intended as digital inputs. Eventually you can change these inputs to analog, but the (more or less) linear area would be only a smal part of the input voltage range

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