DV voltage from one device to another

[brownt]

Hi,

is there an issue with using a 3.3v DC output voltage from one device, to drive the 3.3v DC input of another device. And joining the grounds connections together.
Current is a few milliamps. The voltage input goes through an inline 10k ohm resistance inside the input device with rail to ground schottky transient suppression, before entering an analog to digital convertor microcontroller pin. 

[pcprogrammer]

That is basically how connections between devices work. Ground to ground and signal output to signal input. Only thing to make sure is that the output specifications meet the input specifications. So a 3.3V output can be connected to a input capable of handling 3.3V or more.

[Terry Bites]

Its ok, but there will be a voltage drop across the 10k; 10V/mA. 
Iz DV a typo of DC?

[brownt]

ok, thanks. There is a possibility that the output device will be 5v and the input device 3.3 volts.

So, aside from not having full voltage swing, if I clamped the output using a 3.3 volt zener diode, would that then get around that issue?

[ledtester]

Just to make sure we're all on the same page, is this the situation?



And are the resistor and diodes external to the microcontroller?

There is a possibility that the output device will be 5v and the input device 3.3 volts.

So, aside from not having full voltage swing, if I clamped the output using a 3.3 volt zener diode, would that then get around that issue?

Note that ADC inputs are generally high impedance so for a 3.3V input you shouldn't expect really any current. And for a 5V input the current through the 10k resistor will be limited to 170 uA so there is no need for the zener.

Also, MCU pins have their own protection diodes so you probably don't need the external Schottky diodes as the 10k + the MCU diodes will do the job.

For more discussion and ideas on this situation do a search on "how to protect ADC inputs".

[brownt]

yes that's it. The schottkys and resister are internal to the input device. At least they are in this one specific device that I have, as attached. But I don't know what's in all available units.

The inline resistor, seems reasonable to be in all devices, but it may not be. And so I'd like to clamp it to 3.3v just in case and live with the lesser range that the A/D will produce.

[pcprogrammer]

In the schematic you attached the input has a low pass filter made up with the 10K resistor and the 100nF capacitor. This has a cutoff frequency of ~160Hz. And the two diodes to ground and vdd protect it from out of range voltages.

For other circuits instead of a zener a TVS will provide better protection.

[RJSV]

Uh, my understanding of the 10k input resistor is that the input impedance of the AtoD is already very high (like >100 k ohms) so not to view things as 'voltage divider' on the input.
Better see if the text on best practice for protecting AtoD input mentions placing a zener...instead of just 'throwing one in' just for good luck...might even mess up your readings, ok.

[brownt]

thanks. being that it is a DC voltage is the 160 Hz cutoff relative?

TVS, ok. I don't know much about them. Does it matter that this could involve a constant 5v on a pin designed for 3.3v, rather than a transient?

[brownt]

The A/D readings are not at all critical, but yes OK, I will look into it further. thanks

[RJSV]

In that one specific case (attached schematic), if you DID place 5 volt device output driving the AtoD input, the 10 k resistor will limit current, as then the voltage is clamped, by diode (to 3.3 v), but continuous current won't damage, as the resistor power dissipation will be very low, while clamping.