Following my previous topic, I'm looking for some input on an element of my data acquisition design. My 32 analogue inputs will be interfaced with 2 16-channel, 16-bit ADS1158 ADCs. The analogue inputs can come in two flavours (depending on how the end user sets up the device):
- Powered signal line, which is pulled up to 5V via a 1kOhm resistor prior to connecting to my device.
- An unpowered signal line, which my device will need to pull up to 5V via a 1kOhm resistor.
The following pseudo-diagrams illustrate the two potential input configurations (blue represents my device):
Unpowered signal line:
Powered signal line:
Note: proper grounding would obviously be taken into account (not shown in the diagrams).
I posed this question some time back on another forum, and it was recommended I use a P-channel MOSFET to act as a switch. When the MOSFET is turned on, the line up to 5V via the 1kOhm resistor is enabled, when the MOSFET is off the line up to 5V via the 1kOhm resistor is open circuited.
Is this the optimum approach? I've got plenty of spare GPIO on my SoM, so can spare 32 dedicated lines to turn on/off each input MOSFET as required. It looks like 2-channel P-channel MOSFETs are the best bang for buck, so I'd have 16 MOSFETs controlling the 32 analogue inputs.
It's worth noting that I have 3V3 GPIOs, so the solution needs to facilitate switching on/off the 5V MOSFET with a 3V3 input.
Any confirmation, alternate methods, suggested MOSFETs (or otherwise) would be greatly appreciated.