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DominusT
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# MCP6L04 used as differential amplifier doesn't work

Hi.

I need to take a sample from a 10 V voltage source for which I perform the following circuit: Figure 1

In theory the circuit reduces the voltage from 10 volts to 2.72 V and when simulated with mindi I get the following at the output of amplifier X3: (Figure 2)

The output voltage is approximately equal to 2.4 V.

Now I have performed the physical circuit on a PCB to test the theory and it doesn't work, the voltage on the 3M resistor is equal to 2.8mV (average voltage according to an oscilloscope) and obviously the voltage in output (X3) is very small.

Now if I test the positive terminals of X1 and X2, 2.7 Volts is applied, the system works very well so I suspect that the problem is the input impedances of X1 and X2 in conjunction with the resistances that reduce the voltage of 10V.

My question is:

How can I solve the problem or is there a better circuit for what I want?
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ric
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/07 14:07:26 (permalink)
5 (1)
If your simulator says that circuit will work, it is not a very good simulator.
Those op-amps do feature "rail to rail" input range, but you can't go more than 0.3V above the supply rail.
You are supplying the op-amps with 3.3V, and trying to take the X1 and X2 input pins way outside the supply rails.
As the + inputs are both floating, it's indeterminate which one will go too high or low, but it's impossible for both to be within your supply range.
The circuit might just work if you remove X1 and X2 and go straight to X3, but then you've lost the high input impedance you're obviously trying to achieve.

post edited by ric - 2019/12/07 14:08:29

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Bob White
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/07 22:15:32 (permalink)
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Why not use the usual op-amp based instrumentation amplifier?

ric
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/07 23:15:42 (permalink)
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That circuit still requires the inputs to be within the supply rails of the op-amps.

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hudejun
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 07:28:07 (permalink)
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Simple resistors and OP-Amp filter.

DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 09:22:02 (permalink)
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robertvwhite
Why not use the usual op-amp based instrumentation amplifier?

DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 09:22:43 (permalink)
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robertvwhite
Why not use the usual op-amp based instrumentation amplifier?

Thanks for the comments, I'm in the circuit design stage, so I'm testing several options.

stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:04:19 (permalink)
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What is the real source of the 10V?

If it's not  referenced to ground in some manner, no circuit will work.
DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:08:05 (permalink)
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I am testing with the following circuit, it seems to work quite well, it would be a possible option.

stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:35:05 (permalink)
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You still haven't said what your real source of 10V is?  If it's referenced to ground in any way, you results could change.
DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:46:41 (permalink)
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stumichaels
You still haven't said what your real source of 10V is?  If it's referenced to ground in any way, you results could change.

It is a pressure sensor whose output voltage is 0 to 10V.

Obviously, connecting it to the negative ground terminal of the circuit would be more effective to design a circuit to condition the signal.

But my intention is to avoid a damage by the technical personnel that install the equipment by an error, or due to a defect in the cable and a short circuit exists. For example, the sensor cable that reaches the ground of our circuit for some reason (technician error, defective cable, etc.) makes contact with another electrical terminal an over voltage permanently damage our PCB.

Now of course, if you have a better idea, any comment or idea is welcome.
stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:50:08 (permalink)
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How is that pressure sensor powered?  Is it totally isolated from the amplifier power?  How so?
DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 11:54:43 (permalink)
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stumichaels
How is that pressure sensor powered?  Is it totally isolated from the amplifier power?  How so?

I would not know how to tell you exactly, since it is a device that we develop for the user to program. The requirement is that this has an analog input of 0 to 10V with an impedance of at least 500k or more. Those who are going to buy the equipment have told us that they use pressure and other sensors that generate voltages between 0 to 10V and 0 to 5V.
DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:03:19 (permalink)
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stumichaels
How is that pressure sensor powered?  Is it totally isolated from the amplifier power?  How so?

stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:06:01 (permalink)
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If the pressure sensor is referenced to the amplifier ground in some manner, it may affect the output voltage by changing the common mode voltage.  Try grounding the negative terminal of your voltage source in your simulation and see what happens to the output.
stumichaels
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:08:05 (permalink)
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Your schematic in panel #14 shows the negative output of the pressure sensor grounded.

DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:13:41 (permalink)
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stumichaels
Your schematic in panel #14 shows the negative output of the pressure sensor grounded.

.

ric
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:20:58 (permalink)
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You diagram in post#17 also shows both analog inputs with the negative side connected to a single terminal, which is almost certainly analog ground.
This all makes perfect sense, and is a vital detail missing from your original question.

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DominusT
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:25:33 (permalink)
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ric
You diagram in post#17 also shows both analog inputs with the negative side connected to a single terminal, which is almost certainly analog ground.
This all makes perfect sense, and is a vital detail missing from your original question.

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.

Or am I in error?
ric
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Re: MCP6L04 used as differential amplifier doesn't work 2019/12/08 12:29:15 (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.

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