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Tweeted twitter.com/#!/StackElectronix/status/140843050288164864
added some data and updated the schematic
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Dustin
Dustin
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I've got a circuit that looks like this:

circuitcircuit http://img.skitch.com/20111127-m6jqyx9b15brese4rkipayyib6.png

My input is low voltage AC at low current and I want to measure this with an ADC at 3.3V (eventually, haven't got there yet).

Is this safe/correct? It appears to do what I want when I measure it with my oscilloscope, but it feels strange to me to be sharing an AC line with my DC circuitry (both connect to the same 0V).

I'm having trouble preventing the signal from dropping below 0V. The closest I've got was to connect a schottky with the cathode to signal and the anode to ground. I'm still seeing 300-400mV below zero on the signal line at particular instants. I'm wondering if these things are related.

Edit: Expanding a bit with more data (and updated the circuit to show opamp power).

While most measurements seem to be doing the right thing, the bigger the burst, the more likely it seems that I'd damage the device offering the reference power (in the deployment, it will be an atmega running at 3.3V). I see drops down around -80mV or so, but when I had this hooked up to my washing machine, this happened:

washer start

It leveled out pretty quickly, but my understanding of operational amplifiers is only slightly greater than my understanding of Aramaic. I've heard when you use them right, stuff like this doesn't happen. I do believe that -2.4VDC would be a Bad Thing to hand off to an ADC.

Is this the wrong way to do things?

I've got a circuit that looks like this:

circuit

My input is low voltage AC at low current and I want to measure this with an ADC at 3.3V (eventually, haven't got there yet).

Is this safe/correct? It appears to do what I want when I measure it with my oscilloscope, but it feels strange to me to be sharing an AC line with my DC circuitry (both connect to the same 0V).

I'm having trouble preventing the signal from dropping below 0V. The closest I've got was to connect a schottky with the cathode to signal and the anode to ground. I'm still seeing 300-400mV below zero on the signal line at particular instants. I'm wondering if these things are related.

I've got a circuit that looks like this:

circuit http://img.skitch.com/20111127-m6jqyx9b15brese4rkipayyib6.png

My input is low voltage AC at low current and I want to measure this with an ADC at 3.3V (eventually, haven't got there yet).

Is this safe/correct? It appears to do what I want when I measure it with my oscilloscope, but it feels strange to me to be sharing an AC line with my DC circuitry (both connect to the same 0V).

I'm having trouble preventing the signal from dropping below 0V. The closest I've got was to connect a schottky with the cathode to signal and the anode to ground. I'm still seeing 300-400mV below zero on the signal line at particular instants. I'm wondering if these things are related.

Edit: Expanding a bit with more data (and updated the circuit to show opamp power).

While most measurements seem to be doing the right thing, the bigger the burst, the more likely it seems that I'd damage the device offering the reference power (in the deployment, it will be an atmega running at 3.3V). I see drops down around -80mV or so, but when I had this hooked up to my washing machine, this happened:

washer start

It leveled out pretty quickly, but my understanding of operational amplifiers is only slightly greater than my understanding of Aramaic. I've heard when you use them right, stuff like this doesn't happen. I do believe that -2.4VDC would be a Bad Thing to hand off to an ADC.

Is this the wrong way to do things?

Source Link
Dustin
Dustin
  • 634
  • 8
  • 19

When measuring AC via an ADC, is it bad to have common ground?

I've got a circuit that looks like this:

circuit

My input is low voltage AC at low current and I want to measure this with an ADC at 3.3V (eventually, haven't got there yet).

Is this safe/correct? It appears to do what I want when I measure it with my oscilloscope, but it feels strange to me to be sharing an AC line with my DC circuitry (both connect to the same 0V).

I'm having trouble preventing the signal from dropping below 0V. The closest I've got was to connect a schottky with the cathode to signal and the anode to ground. I'm still seeing 300-400mV below zero on the signal line at particular instants. I'm wondering if these things are related.