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Helpful ReplyHot!MCP6L04 used as differential amplifier doesn't work

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DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:35:42 (permalink)
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DominusT
But the negative 2M terminal according to the diagram of # 17 isn't connected to the M which is a negative power terminal of the PLC.  

Are you sure?
It may not be a zero ohm connection, but it's almost certainly biased to the PLC's negative power supply/ground.


As I understand the philosophy of a PLC is that its outputs and inputs are isolated from power supply.

For example, in diagram # 17 the 1M terminal is the ground of the logic inputs, 2M of the analog inputs and 1L for the logic outputs.
 
Logical inputs and outputs are easy to isolate, using optocouplers.
 
For analog inputs they may also be optocoupled analogue, or in some other way not so cheap.
 
An economical "isolate" option is a differential voltage input.
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stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 13:00:03 (permalink)
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Again I suggest you ground the negative side of the input in your simulation, either directly or through and impedance and see what happens at the output.
 
If you don't, you are just fooling yourself and you'll be unhappy when your customers complain.
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DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/11 12:27:42 (permalink)
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I built this circuit and we are testing it and it works quite well:


 
The one indicated in the #9 could only be used in a range of 0 to 8.25 volts.
 
With the new circuit, X4 generates a constant voltage of 0.975 V and with that it is achieved that the range is from 0 to 10V.
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simong123
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/11 17:48:49 (permalink) ☄ Helpfulby DominusT 2019/12/11 17:56:22
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DominusT
As I understand the philosophy of a PLC is that its outputs and inputs are isolated from power supply.

 
I have worked as a commissioning engineer in water/wastewater treatment for ~30 years, and have used PLC's from all the major suppliers here in the UK (Allen Bradley, Siemens, Modicon, Mitsubishi, Telemechanique, Toshiba etc.) and a fair few standalone telemetry systems, and what they all have in common is that the analogue inputs, both current and voltage, are ground referenced.
 
Which is a real pain in the posterior for current loops as the PLC/telemetry system has to be at the end of the chain. Which is awkward when you have both.
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