So I'm measuring a varying voltage ( based apon what band the radio is on ) from a ham radio ( Yaesu ) and it seems that the supplied current is fairly low.
The input resistance of the analog pin is as expected-- high, but when the power is off to the Aduino Nano the input resistance changes ... to my dismay.
I dont want to buffer the voltage from the radio because of possible non-unity gain issues -- but also I don't want to build more circuitry. -- go figure --
I'm planning on having more than one Nano measuring this voltage so that other equipment can be automatically controlled by other Audiono Nano's and the voltage ( information) needs to be unadulterated as much as possible.
Are all Auduino offerings going to behave in this manner?
Yaesu is still mum about the specifics of the band data voltage limitations -- almost a month has passed from the time I asked for more information from them.
Hi,
I assume you want the Nano to change things like aerials and matching units that connect to the Yaesu for each band.
A simple fix would be a relay with its coil connected to the Nano 5V supply, use the NO contacts of the relay so when you power up the Nano the output signal from the radio is then connected to the Nano input.
The atmega328 on the Nano has input protection to prevent voltages that exceed Vcc from damaging the chip, basically a diode that clamps the input voltage to Vcc + < voltage drop across diode > . When you remove power from the Nano, any voltage on the input pin will flow through the diode to Vcc, and if the voltage/current capacity is sufficient can actually provide enough power for the chip to run. You can place a fairly high value resistor between the radio and input pin to limit the current flow, 1MΩ would probably be sufficient and is insignificant compared to the input impedance of the analog input.
1M could impact voltage readings if analogRead() is attempted at high rates; the ADC's sample and hold capacitor will charge more slowly to the input level with 1M in series with the input, the datasheet recommends a 10K source impedance.
I will take a SWAG, try a series resistor in the 10K range. Higher if conversion speed is not important and you can also add a cap maybe 100 nF on the radio side of the resistor. The higher the resistance the more the cap will help as you have an unknown and changing input impedance on the analog input to contend with.
Pretty much every chip period is going to behave in that manner, since it's a consequence of the ESD protection diodes. You need to physically disconnect the Nano when power is removed from it. TomGeorge's relay idea is good for that.
I'd say make it 10k. 1M might lead to unwanted low-pass filtering whereas 10k definitely
won't and is plenty for protection purposes. Or compromize on 100k if you want. 1M will
also risk picking up interference too.
If you have power available you can use an analog switch. The 4000 series allows them to operate upto about 18V. They are basically a transmission gate. You can use it as a 4 channel mux if you want. If my memory is correct we used some in the late 70's that did not have input protection diodes and were fine with voltages up to about 10V without VCC, they simply stayed off. I do not remember the part number for them. I looked at the 4016 and get different stories on input protection, you may want to try one but no promises.
