View Full Version : Differential ADC and power supply options

11-25-2012, 06:46 PM
Hi everyone,

I considered the many good observations here carefully re: the power supply on the Teensy 3.0 and the considerations for the ADC one should make. Here are some questions of mine...

As a starting point, I want to incorporate a external power supply to produce more stable voltage as an input into the Teensy. To that end, I propose to cut the Vin-VUSB jumper and then run regulated 3.3V power into the Vdd supply directly. I presume that I would not have to worry about back feeding or fighting with the on-chip regulator as long as the jumper is cut and Vin has been left floating, right?

To produce a stable voltage, I was thinking of using a linear regulator like the MCP1825 (http://ww1.microchip.com/downloads/en/DeviceDoc/22056b.pdf) in a SOT223-3 package. Produces allegedly a nice 0.5% regulated power supply. For the ADC AREF, I was thinking of using a series reference. Multiple caps (100uF + 1uF for 5V, 10uF + 0.1uF for 3.3V and 1uF + 0.1uF for 2.5V) smooth the input and output on each stage.

One of the components I want to interface with is a LTSR-6NP (http://www.lem.com/hq/en/component/option,com_catalog/task,displaymodel/id, hall-effect current sensor from LEM which has two input/outputs, the internal reference voltage (2.5V) (can be input or output) and the current signal (2.5VDC +/-2V) (output only) itself. I am running the output through a voltage divider (2x 4.7k @ 0.1% ea) while supplying 2.5V from the precision reference for the REF input. For the voltage-measurement circuit, I am using a transformer to isolate the line voltage, then a series of step-down resistors, terminators, etc. feeding a Analog 8137 ADC driver. The Vocm is derived from the 2.5 AREF voltage via precision resistors (again, 4.7kOhm, 0.1%) Does this seem reasonable?

From what I have learned, A11 and A10 are one of the differential inputs, while A12 and A13 are the other one. Thus, I am planning on attaching a SMD pin header to the underside of the Teensy. Does the above sound reasonable as far as generating a stable ADC is concerned? Can I expect 13 bits or so of ADC happiness? I only really need 12 bits but am happy to get the extra, if possible, since the marginal cost is so low.

Many thanks for any insights, Constantin