Posted: Fri May 15, 2009 4:45 pm Post subject: Isolating the ADC from the signal being measured
Hi,
I need to measure four slow moving 0-5 V analog signals. I need to isolate the ADCs on the Mega8 chip from those signals. I am using a 15k ohm resistor to read the voltages (which is about the upper limit on the Mega8 ADC). In one system I am reading the voltages once per second and in another I am reading them as fast as 20 times per second.
The problem is that the 15k ohm resistance is loading the circuit and changing the signal. I need to isolate the signal from my board.
In the past, I have used an op-amp between the signal and the ADC. This seems to work OK. I am looking for other solutions that I have not thought of.
I read about opto-isolators, but have never used them.
Joined: 11 Aug 2004 Posts: 948 Location: the Netherlands
Posted: Fri May 15, 2009 9:28 pm Post subject:
Convert the measured voltage to a frequency.
Feed this frequency to an optocoupler.
Convert the frequency on the output of the optocoupler to a voltage.
For each step there is a standard IC available.
In my opinion you must always use an opamp switched as buffer between the measured signal and the input of the ADC.
Take care on the power rails if you want to measure a voltage as low as GND and high as Vcc.
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