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Current to Voltage op-amp

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VirusX2

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Hi all, i am trying to measure AC mains using the DL-PT202D, witch doesn't have a useful datasheet.
From the figures with op-amp below i can understand that that is a current to voltage converter. Using lm741 op-amp i can't make it work.. on the output of the op-amp i am taking only noise.. Has someone used it to drive me..?
the topology with the shunt resistor works perfectly

Thanks

T2UB.PXjtXXXXXXXXX_!!392231134.jpg

20140102091715011501.png
 

KlausST

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Hi,

for your OPAMP you need a dual power supply
and try a 2500 Ohms feedback resistor.

Klaus
 

VirusX2

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Hi,

for your OPAMP you need a dual power supply
and try a 2500 Ohms feedback resistor.

Klaus

I am using dual power supply, +- 15V. For feedback i am using 1500 ohm resistor
 

d123

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Have you had to use the OpAmp offset null pins. The TI uA741 datasheet says something about using them to offset input voltage caused by unavoidable internal transistor mismatches. Maybe that causes the noisy output.

This TI application note goes into detail: "Nulling Input Offset Voltage of Operational Amplifiers"

https://www.ti.com/lit/an/sloa045/sloa045.pdf

And on page 11 of the 741 the datasheet it explains how to balance the offset with a 10K trimpot or two.

If the 741 is good for that function, great, and maybe a more application specific OpAmp would help with the objective you have, as the 741 is a general purpose device, not that it should matter.

I admit I have no idea, but maybe, or maybe not, the OpAmp is oscillating, and I remember that you can add cap + r + cap from output to input on some configuration(s), but it looks like you've done that already, unless it's a different type of OpAmp configuration.

Will that circuit transmit mains noise and 50/60Hz ripple to your measurement set-up? Does it matter or not? If it matters and it fits (safely) with the circuit function, maybe a little decoupling cap after the transformer secondary output could help, or not - you know your circuit operation parameters.
 

Audioguru

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A lousy old 741 opamp has a noisy output because its design is 47 years old and it was not designed for low noise like some newer audio opamps. Use an OPA134 opamp instead.
 

VirusX2

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I can't figure it out, finally i achived this with the shunt topology, and drive the output to an non-inverting opa-mp with proper gain (gain=4). Works perfectly!
 

d123

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Good, I'm glad for you, sometimes it's just nice when things work after putting in the effort! Great.
 

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