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Analog Isolation ?

v1.5

Member
Hi
I was looking for an analog isolator IC for some kind of 0..10V (for the driver) and I found a weird one on the internet called the HCNR200. this integrated has functionality that I haven't seen before. According to me, there is a different management system. I want it to be sensitive. There should definitely be a sensitivity of about 20mV. that's why I read the entire technical document this morning. Although there are sufficient explanations in the technical document, I still have a few questions in my mind that I cannot answer.
I plan to create the circuit diagram in figure 12.b of the technical document (page 11). According to this scheme, two external opamps and 2 external resistors are required. The circuit in the figure states that it is applicable in the technical document, but no mention is made of its sensitivity. I know the sensitivity is relative to resistors and opamp. So which opamp should I choose? I have vintage opamps like LM358 in my inventory, but I don't know if they are suitable for this job? If they are not eligible I cannot spend more money. So goodbye to the HCNR200 :)
The same technical document also includes another circuit that provides precise analog isolation (Figure 17), but I don't really want to struggle that much for an isolation process, I think it's too complicated and too crowded. For this reason, I chose to establish a simpler circuit model that can provide 20mV sensitivity (Figure 12.b). The technical document states that C1 and C2 capacitors in figure 12.b are intended to increase stabilization, but it did not write at what values they should be selected. what should these values be?
And my most important question is; Do you think this integrated is suitable for this job? What are the integrations that can provide cheaper 0..10V isolation? What are the analog integrations you use? What do you think should I do?
Thank you.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
The most important info needed is: What is the voltage range that may be across the "isolation" barrier?

There are many devices to provide total isolation, but there are also simpler opamp based techniques if the voltage between the two halves of the circuit are not too extreme.

There are also ways of doing it with standard optoisolators, especially if you have two from the same batch that are likely to be well matched.

The LM358 should be fine in the example circuits, as long as you are not trying to pass particularly high frequencies.
 

v1.5

Member
The most important info needed is: What is the voltage range that may be across the "isolation" barrier?
Thank you.
For isolation, there is a mcu on one side and a motor driver on the other. the driver has 3 phases. For this reason, it should provide at least 600Vdc isolation. For this reason, I want to do this job quickly with a special integrated. 0..10v will be used in motor speed control. This motor is mounted on the vertical machining center.
There are many devices to provide total isolation, but there are also simpler opamp based techniques if the voltage between the two halves of the circuit are not too extreme.
but I wondered. I would like you to give information about the circuits that I can easily isolate for low voltages. How can I do this with Opamps? I am asking for use in my future projects.
The LM358 should be fine in the example circuits, as long as you are not trying to pass particularly high frequencies.
Yes. there will be no high frequency signal.
You really need to provide some context, and MUCH more details of exactly what you're trying to do - different requirements need different solutions,
Mr. Nigel I hope the above information is sufficient.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
For isolation, there is a mcu on one side and a motor driver on the other. the driver has 3 phases. For this reason, it should provide at least 600Vdc isolation. For this reason, I want to do this job quickly with a special integrated. 0..10v will be used in motor speed control. This motor is mounted on the vertical machining center.
Machine tool motor drives normally have isolation built in, and the control signals ground referenced?
The isolation is generally at the last stage, logic or on/off switching isolation (the PWM signals) rather than analog, in the drive signals to the power devices.

What's the make and model of the drive?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Re. the other methods:
For moderate voltage differences, a simple "differential amplifier" circuit can be used; just an opamp and two pairs of matched resistors, in essence.

That type of setup is used in many applications; it measures the difference across two points and translates it to a voltage between the opamp output and whatever is chosen as the local (input side) 0V or ground reference.

More info:

With all four resistors equal (eg. all 10K) you get a 1:1 ratio between input and output, with an input offset range of around twice the opamp common mode range. eg. With +/- 15V supplies and a rail-to-rail opamp, the inputs would need to stay within a +/- 30V range for the circuit to work.

Using higher value input resistors increases the voltage range but decreases the gain.

Many analog-input machine tool or robotics servo drives use a differential amp at the input to avoid ground noise problems in a high-current system.
 

v1.5

Member
Machine tool motor drives normally have isolation built in, and the control signals ground referenced?
The isolation is generally at the last stage, logic or on/off switching isolation (the PWM signals) rather than analog, in the drive signals to the power devices.

What's the make and model of the drive?
I developed this driver myself. I did not insert any isolation elements as I used the driver integrated in the last stage. That's why I want to protect the essential equipment, the MCU. This MCU is in constant communication with the computer. Each point in the circuit communicating with the MCU has a separate isolation. Approximately three I / O signals and one 0..10V signal.
 

v1.5

Member
Re. the other methods:
For moderate voltage differences, a simple "differential amplifier" circuit can be used; just an opamp and two pairs of matched resistors, in essence.

That type of setup is used in many applications; it measures the difference across two points and translates it to a voltage between the opamp output and whatever is chosen as the local (input side) 0V or ground reference.

More info:

With all four resistors equal (eg. all 10K) you get a 1:1 ratio between input and output, with an input offset range of around twice the opamp common mode range. eg. With +/- 15V supplies and a rail-to-rail opamp, the inputs would need to stay within a +/- 30V range for the circuit to work.

Using higher value input resistors increases the voltage range but decreases the gain.

Many analog-input machine tool or robotics servo drives use a differential amp at the input to avoid ground noise problems in a high-current system.
Which criteria depends on the absolute maximum rating of the differential amplifier you mentioned? How many volts of insulation can I provide? Is this isolation voltage limited by the opamp input voltage?
 

dougy83

Well-Known Member
With all four resistors equal (eg. all 10K) you get a 1:1 ratio between input and output, with an input offset range of around twice the opamp common mode range. eg. With +/- 15V supplies and a rail-to-rail opamp, the inputs would need to stay within a +/- 30V range for the circuit to work.

Using higher value input resistors increases the voltage range but decreases the gain.
If you feel like adding some extra resistors in the feedback path, you can boost the gain while increasing the effective common-mode range. The following circuit has 1:1 gain, but the offset voltage of the measured thing can be around ±150V. Resistor values may get a little odd though. This is probably just for fun though, as adding an extra opamp gain stage is probably more convenient.
higher gain diff amp.jpg

Which criteria depends on the absolute maximum rating of the differential amplifier you mentioned? How many volts of insulation can I provide? Is this isolation voltage limited by the opamp input voltage?
It depends on the allowable input common-mode voltage range, i.e. the voltage appearing at the opamp inputs with respect to the opamp's supply rails. The values of the resistors affect how much of the "volts of insulation" relates to the common-mode voltage applied to the opamp inputs.
 

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