Interpreting 10-bit ADC output

Question

I am having difficulty understanding the output from my ADC. I am using a 10-bit MCP3008 with a photoresistor and CT current sensor connected into the analog inputs. I am reading the signals with this module. The module has a 'read_adc' function which 'Reads the current value of the specified ADC channel. The values can range from 0 to 1023 (10-bits).'

So far in the photoresistor channel, I am seeing values range from 100-200 (when the lights are low) to near 1000 when the lights are very bright. What exactly do these values mean? Are they a value of voltage being passed through the analog input port? Are they some sort of voltage measurement as a proportion of the reference voltage?

Same goes for the current sensor. The values remain steady at around 6 (which is the same for other grounded analog inputs), but then jumps to around 60-80 when a load the sensor is monitoring turns on, put then plummets back down to 6 after a few samples. Are the 60 and 80 values again reading voltage ?

Sorry if this is a simple question about ADCs. Thanks for the help!

-Tuomas

Answer 1

In general, if your ADC is 10-bit with a 0-1023 return range, this value indicates a voltage that is proportional to the V_REF (reference voltage) input on your ADC. If your V_REF was 5 V, then a value of 1000 would indicate that the ADC is reading

octave:1> 5*1000/1023
ans =  4.8876

4.9 volts. What this voltage means then depends on your sensor type.

An LDR is usually constructed as part of a voltage divider circuit. Assuming the LDR goes between +5 V and the ADC pin, and ground goes to the the ADC pin via a 10K resistor (R), then the resistance measured by the LDR is calculated by R*V_IN/V_OUT+R which in this case gives

octave:2> 10000*5/4.88-10000
ans =  245.90

where V_IN is the reference 5 V voltage and V_OUT is the measured voltage. The datasheet for the LDR may give a calibration from resistance to Lux, a measurement of light intensity, like this datasheet does for the GL5528. Reading off the graph gives the relationship lux = exp((ln(R/1000) - 4.125)/-0.6704) which in this case gives

octave:3> exp((log(245.9/1000) - 4.125)/-0.6704)
ans =  3810.7

around 4000 lux. Note there will be pretty big error bars on this though and shade makes a huge impact!

The datasheet for the current meter should give a similar relationship between voltage and current.