Is there a minimum current required to sense an analog input? I would like to sense 0-5 volts 250uA . Is this possible directly or what kind of interfacing is required?
Cheers,
Pracas
From the datasheet, the A/Dinput impedence is 100Mohm, so 250uA looks a little high!
The high input impedance of the input pins make them useful for voltage-sensing (as they do not draw too much current from the loads and thus make the measurement more accurate). Do you intend to measure current instead of voltage? If so, you need to add a 20K resistor from the pin to the ground to measure 250uA (which translate to 0-5V).
There is a little more to the analogue input than the impedence (which the spec sheet says is 100 M ohm)
There is a "sample & hold" (S/H) circuit in front of the analogue to digital converter so that it can 'freeze' a sample allowing it to measure varying voltages.
According to the Atmels documents, e.g. ATmega48A/48PA/88A/88PA/168A/168PA/328/328P (550 pages)
Section 23.6.1 "Analog Input Circuitry"
... When the channel is selected, the source must drive the S/H capacitor through the series resistance (combined resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately 10k[ch937] or less. If such a source is used, the sampling time will be negligible. If a source with higher impedance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely.
The impedance of the signal looks like 5V/0.00025A = 20K
The effect of this is that the input to the analog converter takes a bit longer to get to its 'true' value. If the signal changes slowly, there isn't a problem.
The Arduino is set up to sample at roughly 10K sampes/second, so assuming the signal varies at much less than 5KHz (Nyquist frequency), there shouldn't be a problem.
It is worth checking the impedance of the input signal. Depending on how the output circuit of your 5V at 250 uA works, the current supply, and hence impedance may get worse at lower voltages.
Summary if the voltage changes slowly (well under 5KHz), it shouldn't be a problem.
If it is too fast, then you have a problem anyway because the signal voltage will be too fast to sample correctly (without aliases)
HTH
GB
Do you intend to measure current instead of voltage?
No. I need to measure voltage.
Summary if the voltage changes slowly (well under 5KHz), it shouldn't be a problem.
If it is too fast, then you have a problem anyway because the signal voltage will be too fast to sample correctly (without aliases)
Thanks that makes it a bit clear. Will check on this. its a supposed output coming from another controller( a blackbox without any specs!)