Dual Isolated Input to Microcontroller

[jamesh77]

OK jamesh,
One good outcome is that, as a result of your post, I have learned a lot about isolation amps, which will, no doubt, be useful in the future.
spec

I've learnt a lot about opto-isolators in general! and have a good few solutions

 

[jamesh77]

All understood, thanks again Mike!

 

[spec]

I thought about using a "digital opto-coupler", but didn't because I was too lazy to go looking for one for which I can get a Spice model. ( A perverse impact of bountyhunter 's argument).

To faithfully get the PWM signal from the isolated Arduino to the off-line PIC, the rise-time and fall-times of the PWM waveform must be matched, and the voltage swing must go from 0V to 5V.

My initial attempt used a simple resistive pull-up to +5V on the PC817, but the turn-off delay for full 5V swing was significantly longer than the turn-on time, distorting the width of the PWM pulse.

My solution was to come up with the Kluge consisting of R3 to R7 and U2. I am relying on U2's rail-to-rail output to create a (nearly) 0V to (nearly) 5V swing. By biasing the PC817 the way I did, it is now acting as a current switch, where its output voltage doesn't have to change very much (~200mV), and that (and U2) makes the turn-on and turn-off delays symmetrical, preserving the PWM information as it is optically transmitted.

The TLP2361 is an open-collector NPN output with a 6.3mA sink rating. Using a resistor pull-up to 5V, it would have a Vce(sat) of ~0.4V when low, and would pull all the way to 5V when high, creating a different type of distortion to the PWM signal...

The TLP2200 has a full TTL-totem pole output, but doesn't pull any closer to ground than the TLP2361, and only pulls up to ~3.5V when high. If they where CMOS instead of TTL, they could work.

To use either one, it would require a "level restoring" circuit like U2. Does anybody have a CMOS "digital opto-coupler" suggestion that might work directly without U2?

Hy Mike,

I think these Analogue Devices digital isolators would be worth considering.

The rise and fall times are so fast that you would not have to worry about matching the input and output rise and fall times. The timing skew is so low, at 13nS, that I don't think you will have to worry about timing errors either. The maximum data rate is 150M baud. The outputs are CMOS and swing rail to rail, under light current that is. I used similar chips when they came out and they behaved quite well. They are also low power and dirt cheap.

spec

Single channel, ADuM110N
https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM110N.pdf

Dual channel, ADuM120N
https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM120N_121N.pdf

 

[ronsimpson]

The last time I had to design an analog isolation amp:
AMC1100
Input +/-0.250V
output +/- 2.5V
It does a ADC conversion. Then pipes the digital across the isolation. Then back to analog DAC.

 

[spec]

The last time I had to design an analog isolation amp:
AMC1100
Input +/-0.250V
output +/- 2.5V
It does a ADC conversion. Then pipes the digital across the isolation. Then back to analog DAC.

Interesting, I had a look at the AMC1100 and its brother the AMC1200. How did you get on with it. I couldn't get a definitive figure for the input current from the data sheet.

Avago have a large family of similar chips, all of which are primarily intended for current monitoring of motors, mains supplies etc, with input voltage ranges of either +-100mV or +- 200mV. Some are quite cheap, around £3 UK. The input offsets amounted to between 0.5% and about 2.5% which, coupled to other errors, seemed a bit high compared to a half decent opamp.

The precision isolation amps with higher input/output voltage ranges and lower input offsets from Analog Devices, for example, were heap big expensive: £25 UK upwards, so I dismissed them on cost grounds.

Here is the little list of isolation amplifiers that I examined. The HCPL-7900 seemed to be the best of the low cost versions with an input offset voltage of 0.5% of max input voltage.

spec

 

[ronsimpson]

I tested one and then made the PCB. Got it back and lost funding. So the parts are in a box and noting is happening.

I sent the signal into the (+) input and put a pot on the (-) to do offset.
Ahead of the isolator I put a TLC6910 variable gain amp.
Because the isolator really does not like to see noise at the "switching frequency" I added a low pass filter with gain of 1 or 10.

So what is it? A isolated scope probe with a band with of 100khz. Gain knob and offset knob.

input stage

Gain

isolation:

There are two power supplies to make +/-2.5 volts for the isolator.