I work with hydraulic equipment, and I built a small wireless transmitter that allows me to use industrial 4...20 mA pressure transducers for diagnostics - I have used it extensively and quite successfully with wika A-10 sensors in the field for some time now. It uses a Silicon Labs BLE121LR Bluetooth module based on CC2541 SOC. My current design makes use of the built in 12 bit ADC in differential mode (internal 1.24 Vref) with negative input tied to the GND and positive tied to the 56 ohm current sensing resistor, which gives the span of 0.224 to 1.12 V (left part of the schematics). I have experienced no problems so far regarding noise, stability, etc. With this simple setup the ADC outputs 380 at 4 mA and 1890 at 20 mA, which provides a reasonable resolution of +-1500 steps.
I am trying to "upgrade" it by doubling the size of the current sensing resistor and using an external 1.25 vref, that's tied to the negative ADC input (the right part of the schematics), which allows (I think) spreading the doubled voltage across the positive and negative spans of the ADC due to the "shifted zero". I've tested this setup without a precision vref - just a voltage divider and the internal 1.24V Vref - and I did see that the amount of steps across measured span roughly doubled. My question/doubt is (before I start ordering components and dive into re-designing the pcb, etc...) -
In principle - is this a correct way to "make use" of the negative span of the differential ADC, or can I run into some issues by doing it this way? Seems like a very cheap way to double resolution (in situation when I onlly need to measure positive voltage in relation to common GND), but I have very little experience with this type of ADCs, and I want to use as few components as possible.
Some notes - GND is common to everything. Sensor is powered by a booster, not showed in the schematics. Pins 3 and 4 of the BLE121LR are the positive and negative inputs of the module's differential ADC, pin 7 would be the external Vref. The module transmits the ADC reading via Blueooth to the Android App that was created for this purpose.
I am trying to "upgrade" it by doubling the size of the current sensing resistor and using an external 1.25 vref, that's tied to the negative ADC input (the right part of the schematics), which allows (I think) spreading the doubled voltage across the positive and negative spans of the ADC due to the "shifted zero". I've tested this setup without a precision vref - just a voltage divider and the internal 1.24V Vref - and I did see that the amount of steps across measured span roughly doubled. My question/doubt is (before I start ordering components and dive into re-designing the pcb, etc...) -
In principle - is this a correct way to "make use" of the negative span of the differential ADC, or can I run into some issues by doing it this way? Seems like a very cheap way to double resolution (in situation when I onlly need to measure positive voltage in relation to common GND), but I have very little experience with this type of ADCs, and I want to use as few components as possible.
Some notes - GND is common to everything. Sensor is powered by a booster, not showed in the schematics. Pins 3 and 4 of the BLE121LR are the positive and negative inputs of the module's differential ADC, pin 7 would be the external Vref. The module transmits the ADC reading via Blueooth to the Android App that was created for this purpose.
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