Other Parts Discussed in Thread: ADS131M08, , ADS114S08, ADS124S08
We are working on a project that will use 4-20mA inputs. I was planning to use ADS114S08BIPBS for this but trying to figure out what the best approach is to managing these single ended inputs. I have a question about which general topology is recommended and also some specific questions about the part that I could not determine from the datasheet.
Regarding the topologies, I was considering these four options. Could someone comment on which would be best?
1. 125 ohm shunt resistor, PGA bypassed, single ended mode, unipolar supply = 3.3V. AINN = GND.
2. 125 ohm shunt resistor, PGA gain=1, pseudo-differential mode, unipolar supply = 3.3V, AINN = 2.5 (direct from voltage reference).
3. 156.25 ohm shunt resistor, PGA gain=1, pseudo-differential mode, unipolar supply = 3.3V, AINN = 0.25V ( voltage divider from VREFOUT).
4. 156.25 ohm shunt resistor, PGA gain=2, pseudo-differential mode, unipolar supply = 3.3V (does this need to be bipolar?), AINN = 1.375 (voltage divider from VREFOUT).
I created a table to analyze the effective resolution of each approach. In reality we will add some margin to the range (ie 3-21mA) and use "real" resistor values, but these are ideal numbers.
Approach | AINN | Shunt R | PGA Gain | 4mA | 20mA | 4-20mA ADC Range |
4-20 Resolution (Bits) |
4-20 Underfill Factor |
||||
Input Voltage | ADC Voltage | ADC Counts | Input Voltage | ADC Voltage | ADC Counts | |||||||
1 | 0 | 125 | 1 | 0.5 | 0.5 | 6553 | 2.5 | 2.5 | 32768 | 26215 | 14.68 | 2.50 |
2 | 2.5 | 125 | 1 | 0.5 | -2 | -26215 | 2.5 | 0 | 0 | 26215 | 14.68 | 2.50 |
3 | 0.625 | 156.25 | 1 | 0.625 | 0 | 0 | 3.125 | 2.5 | 32768 | 32768 | 15.00 | 2.00 |
4 | 1.875 | 156.25 | 2 | 0.625 | -2.5 | -32768 | 3.125 | 2.5 | 32768 | 65536 | 16.00 | 1.00 |
Here are my specific questions about the ADC
· When the PGA is in bypass mode, does that instruct the ADC to operate in single-ended mode? Does the output now generate 65535 counts when the input is = +VREF or still just 32,767? Or is this just a differential input with one side = GND?
· I didn’t see how to tell the ADC to use pseudo-differential mode instead of differential mode. Does the output still scale from -VREF to VREF in pseudo differential mode? Do I lose half the range?
· If I use this differential ADC in single ended or pseudo differential mode, then the common mode input to the ADC varies as the input scales. How does this impact the performance of the ADC? Does it create nonlinearities?
· I am assuming that in order to calculate the accuracy of the ADCs, I will need to multiply the various sources of error by the underfill factor. In other words, a 0.2% error from the reference will result in about 0.6% if my underfill factor is 3X. Right?
· If I run this in single ended mode, with PGA bypassed, I have no buffer between the shunt resistor and the ADC. I will have to have an antialias filter there that will add source impedance. I think this is not ideal. How are other customers using this for current loops?
· There’s a note on page 70 of the datasheet that says the range for pseudo differential inputs is VAINN to VAINN + VIN. Does this mean negative voltages are not allowed? I would think that the arrangement shown on figure 100 is not correct. Can I instead connect 2.5V to AINP instead of AINN and achieve essentially the same results or is it best to have the AINP input always be the “input”.