Setting up the LTC2485
The physical set-up of the ADC provided a few more challenges than the temperature and humidity sensors I handled previously. Using the Diolan to make the I2C to USB connection to my PC.
The SCL & SDR connections to this board were a bit precarious and I eventually wound up latching onto those from the bottom of the board with an extender, and to Vin & Gnd from the top side. Each time I setup, I used i2cdetect to find the ADC before instantiating it. Using the Diolan, you need to use the -r option (use receive byte) of i2cdetect because the Diolan does not support SMBus quick write commands. (man i2cdetect) Oftentimes, it would require a bit of wiggling and repositioning to get this just right.
Powering the external vref
After setting up the board I realized the onboard vref had no power. I didn't know why, I just knew that when I tested it with a voltmeter it was 0. So, that led me off to try using an external vref. I knew I just needed enough volts to be greater than what I would use for the differential inputs. Here's my test setup with 2 D batteries clamped together and connected as externel vref with old headphone wires. Those 2 wires (bread ties) coming off the front of the plate are attached to the differential inputs and I'd touch other batteries to create data.
Using the on board vref
I kept revisiting that failing on board vref and eventually spotted the problem. The onboard reference voltage needed power from the DC590 controller, an optional part of this evaluation kit. The DC590 would have been attached via a ribbon cable and as you can see in the diagram below, J1 - 1 would have powered the onboard vref.
I can make that connection using the ribbon cable that was included in the kit and those bread ties by sharing the Vin from the Diolan. Looking at this photo now, I'm not sure why I used the ribbon cable. I guess I wanted to make a big loop away from the board. It didn't get me 5 volts, maybe 4.6, but it was enough to test with an onboard vref.
Feeding the Differential Inputs
I just used a few batteries at different voltage levels to do the testing.
The one piece of 'real' equipment I did have was a voltmeter which I used to sanity check everything.
Labels: IIO Driver, LTC2485, Setup