RTD measurement with Wheatstone Bridge and ADC

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Tue, 01 Apr 2025 20:27:29 GMT

Using Zephyr! I solved this by editing the io-channels in devicetree. Thanks a lot!

RE: RTD measurement with Wheatstone Bridge and ADC

hmolesworth — Tue, 01 Apr 2025 16:22:15 GMT

Using zephyr or nrfx within zephyr? Maybe try using the maximum SAADC acquisition time (40uSec) to see if it is a simple settling issue when using other ADC inputs as well, not sure what value ADC_ACQ_TIME_DEFAULT is in your version. If that doesn't help and you are already reversing samples in differential mode then maybe disable the other inputs.

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Tue, 01 Apr 2025 16:04:53 GMT

Using the ratiometric and alternate differential measurements, the results are very good.

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Tue, 01 Apr 2025 16:03:41 GMT

Hi! Thanks for the advice, it has improved the measurements.

I've realized that when the rest of the ADC's are enabled in the device tree, the differential measurement is unstable again. Should I disable them in each measurement?

RE: RTD measurement with Wheatstone Bridge and ADC

hmolesworth — Mon, 31 Mar 2025 16:19:56 GMT

1/6 gain and 0.6V ref gives a SAADC full-scale range value of 3.6V which means most of the SAADC resolution is wasted. Increase the SAADC gain such that the expected full-scale voltage across 100 Ohm is near the full-scale range. x4 gain and 0.6V ref would give much better results.

Remember to reverse the differential SAADC input assignments p & n for alternate groups of readings to provide automatic calibration of SAADC offsets without having to run the SAADC calibration periodically (SAADC calibration introduces step-changes in output).

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Mon, 31 Mar 2025 14:20:07 GMT

Hi! This is the detalied circuit:

With 1/6 gain, 0.6V ref value, differential ADC and 1024 samples I got an main error of 0.33%, with a standar deviation of 1C. The PT100 is 40cm lenght untwisted. I think wheatstone Bridge, instead of just one ref resistor, generates more noise.

The measurements has improved when I used ratiometric measurement, measuring VDD to get the variation of the bridge power instead of a fixed 3V value.

Output (measured RTD with DMM 109.6C):

00>  adc = 4.67mV, vin = 2.64V, diff = 0.00V, temp = 23.67, main_temp =  24.23, desv_temp =  1.04, rtd = 109.22, error =  0.11%, mov_error =  0.31%
00>  elapsed_time = 218
00>  adc = 4.64mV, vin = 2.64V, diff = 0.00V, temp = 23.54, main_temp =  24.23, desv_temp =  1.04, rtd = 109.17, error =  0.06%, mov_error =  0.31%
00>  elapsed_time = 218
00>  adc = 5.13mV, vin = 2.64V, diff = 0.01V, temp = 26.00, main_temp =  24.30, desv_temp =  1.04, rtd = 110.12, error =  0.94%, mov_error =  0.33%
00>  elapsed_time = 218
00>  adc = 4.85mV, vin = 2.64V, diff = 0.00V, temp = 24.61, main_temp =  24.30, desv_temp =  1.04, rtd = 109.58, error =  0.44%, mov_error =  0.33%
00>  elapsed_time = 217
00>  adc = 4.80mV, vin = 2.64V, diff = 0.00V, temp = 24.35, main_temp =  24.29, desv_temp =  1.04, rtd = 109.48, error =  0.35%, mov_error =  0.33%
00>  elapsed_time = 218
00>  adc = 4.77mV, vin = 2.64V, diff = 0.00V, temp = 24.16, main_temp =  24.29, desv_temp =  1.04, rtd = 109.41, error =  0.28%, mov_error =  0.33%
00>  elapsed_time = 218
00>  adc = 4.85mV, vin = 2.64V, diff = 0.00V, temp = 24.56, main_temp =  24.27, desv_temp =  1.02, rtd = 109.56, error =  0.42%, mov_error =  0.32%
00>  elapsed_time = 218
00>  adc = 4.76mV, vin = 2.64V, diff = 0.00V, temp = 24.12, main_temp =  24.30, desv_temp =  1.03, rtd = 109.39, error =  0.27%, mov_error =  0.33%
00>  elapsed_time = 218
00>  adc = 4.95mV, vin = 2.64V, diff = 0.00V, temp = 25.09, main_temp =  24.31, desv_temp =  1.04, rtd = 109.77, error =  0.61%, mov_error =  0.34%
00>  elapsed_time = 218
00>  adc = 4.74mV, vin = 2.64V, diff = 0.00V, temp = 23.99, main_temp =  24.31, desv_temp =  1.04, rtd = 109.34, error =  0.22%, mov_error =  0.34%
00>  elapsed_time = 217
00>  adc = 4.58mV, vin = 2.64V, diff = 0.00V, temp = 23.24, main_temp =  24.28, desv_temp =  1.05, rtd = 109.05, error = -0.05%, mov_error =  0.33%
00>  elapsed_time = 218
00>  adc = 4.57mV, vin = 2.64V, diff = 0.00V, temp = 23.13, main_temp =  24.27, desv_temp =  1.06, rtd = 109.01, error = -0.08%, mov_error =  0.32%
00>  elapsed_time = 218
00>  adc = 4.57mV, vin = 2.64V, diff = 0.00V, temp = 23.14, main_temp =  24.22, desv_temp =  1.05, rtd = 109.01, error = -0.08%, mov_error =  0.31%
00>  adc = 5.19mV, vin = 2.64V, diff = 0.01V, temp = 26.34, main_temp =  24.28, desv_temp =  1.04, rtd = 110.26, error =  1.06%, mov_error =  0.33%

Thanks!

RE: RTD measurement with Wheatstone Bridge and ADC

hmolesworth — Fri, 28 Mar 2025 23:29:31 GMT

The diagram is not clear how the 3-wire RTD is connected, can you post more details? There should be 3 separate wires going to the RTD and it's best to use differential SAADC measurements. Where is the RTD power coming from? Are the remote wires twisted or shielded?

Typical measured values: 14-bit value of 905.0 Ohm 2-Wire test resistor using 128 samples is 904.9 Ohms with -0.01% error

// Internal 0.6v Ref 14-bit S0S1 2-Wire RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Pos-Neg ADC
// VDD mV Iref uA  RdsOn  V1 mV V2 mV V3 mV    Vrtd    Vref   Rrtd  Error
// ====== ======= ======  ===== ===== =====  ======  ====== ====== ======
//   2974    1008     41   2932   911    -3  3108.5  6883.3  903.8 -0.13%
//   2975    1007     41   2933   912    -2  3108.3  6882.6  903.8 -0.13%
//   2974    1007     37   2936   913    -2  3107.9  6882.4  903.8 -0.14%
//   2976    1007     42   2933   914    -2  3108.5  6882.0  904.0 -0.11%
//   2973    1007     38   2934   911    -2  3108.3  6881.3  904.0 -0.11%
//   2977    1007     42   2934   911    -1  3110.3  6882.9  904.4 -0.07%
//   2978    1008     40   2937   913    -1  3110.1  6884.8  904.1 -0.10%
//   2977    1008     40   2936   913    -1  3109.8  6885.3  903.9 -0.12%
//
// Internal 0.6v Ref 14-bit S0S1 2-Wire RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Neg-Pos ADC
// VDD mV Iref uA  RdsOn  V1 mV V2 mV V3 mV    Vrtd    Vref   Rrtd  Error
// ====== ======= ======  ===== ===== =====  ======  ====== ====== ======
//   2977    1010     41   2935   912    -3  3120.1  6896.4  905.5  0.05%
//   2976    1009     40   2935   912    -2  3119.5  6893.8  905.7  0.07%
//   2973    1009     39   2933   912    -2  3119.4  6892.3  905.8  0.09%
//   2976    1010     40   2935   911     0  3121.9  6896.3  906.0  0.11%
//   2975    1010     39   2935   913    -2  3120.9  6896.3  905.7  0.08%
//   2975    1009     40   2934   911    -1  3120.0  6891.4  906.1  0.12%
//   2976    1009     41   2934   912     0  3118.6  6892.3  905.6  0.07%
//   2975    1009     38   2936   913    -1  3118.3  6891.4  905.6  0.07%
//
// Measured 14-bit value of 905.0 Ohm 2-Wire test resistor using 128 samples is 904.9 Ohms with -0.01% error

Sample circuit; the RTD is presumably remote:


data-mode="c_cpp">// RTD Measurement - 2- and 3-wire Unidirectional current + ----------------------------------------------------------------------+ |  nRF52832/nRF52833/nRF52840                                           | | |    VDD               VDD                 VDD                          | |   --#--             --#--               --#--                         | |     |                 |                   |                           | |   __|__               |   Excitation      |   Option                  | |    / \                +-|   On            +-|   13k                   | |   /-+-\                 |<- H0   H1         |<- Pullup                | |     |                 +-|   160R 35R      +-|   (not used)            | |                   |                           | -----#------#----------#-------------------#------------ In            | |     |      |          |   Excitation      |   Option                  | |   __|__  -----        +-|   Off           +-|   13k                   | |    / \   -----          |<- L0   L1         |<- Pulldown              | |   /-+-\    |3pF       +-|   160R 35R      +-|   (not used)            | |     |      |          |                   |                           | |   =====  =====      =====               =====                         | |    ===    ===        ===                 ===                          | |     =      =          =                   =                           | | |    VDD                                         VDD                    | |   --#--                                       --#--                   | |   __|__                    Auto-adjust:         |   160k              | |    / \                     Bias inputs at VDD/2 +-| Bias Hi/Lo/Both   | |   /-+-\                     VDD1_2                |<-                 | |     |   PIN_VOLTAGE_1                           +-|                   | |   AIN4          |----'T'-Mux-P---|        |            P SAADC  | -----#------#-------#--+ +----#-------+ +--#-----#------#------#-->    | |     |      |       |  | |    |       | |  |     |      |      |       | |   __|__  -----     |  ----   +-|    ----- |     +-|   +++     |       | |    / \   -----     |   |       |<--+  |   |       |<- | |   -----     | |   /-+-\    | 3pF   |   |     +-|   |  |   |     +-|   | |   -----     | |       |   +---- |--------#   |     |     +++     | 2.5pF | |   =====  =====     |         |     |  |   |     |      |1M0   |       | |    ===    ===      |       =====   |  |   |   =====  =====  =====     | |     =      =       |        ===    |  |   |    ===    ===    ===      | |         =    Select  |     =      =      =       | |                      |                           | |                      |    VDD                    | |  |----'T'-Mux-N---|  |   --#--                   | +--+ +----#-------+ +----+   |   160k              | | |   +-| Bias Hi/Lo/Both   | PIN_VOLTAGE_2                     | |     |<-                 | AIN5          |----'T'-Mux-----|  | |   +-|                   | -----#------#-------#--+ +----#-------+ +--# |   |            N SAADC  | +--+ +----#-------+ +----#---#------#------#-->    | | |   |      |      |       | | |   +-|   +++     |       | Note V3 can be negative          | |     |<- | |   -----     | PIN_VOLTAGE_3                    | |   +-|   | |   -----     | AIN7         |----'T'-Mux-----|  | |   |     +++     | 2.5pF | -----#------#-------#--+ +----#-------+ +--# |   |      |1M0   |       | +--+ +----#-------+ +----# =====  =====  =====     | | |  ===    ===    ===      | | |   =      =      =       | | |                         | | |   P: Single-ended       | | |   P&N: Differential     | PIN_VOLTAGE_0                     | |                         | |     |   AIN_VDD       |----'T'-Mux-----|  | |                         | |     +-----#--------#--+ +----#-------+ +--+ |                         | +--+ +----#-------+ +----#                         | |                         | |    GND                |----'T'-Mux-----|    |                         | -----#-----------------+ +----#-------+ +----+                         | |     |                                                                 | |   =====                                                               | |    ===                                                                | |     =                                                                 | +-----------------------------------------------------------------------+ = (3pF+2.5pF)*2^14=80nF voltage V1 and V2 in SAADC counts voltage V2 and V3 in SAADC counts

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Fri, 28 Mar 2025 15:58:09 GMT

UPDATE:

I have tested on a fixed Rx (real value 99ohm) and the results are much better (64X average):

Vout(diff)	Vin	Temp	RTD_meas	error
00> = -0.16mV	vin = 2.71V	temp = -0.78	rtd = 99.70	error: 0.71%
00> = -0.08mV	vin = 2.71V	temp = -0.39	rtd = 99.85	error: 0.86%
00> = -0.59mV	vin = 2.72V	temp = -2.95	rtd = 98.86	error: -0.14%
00> = -0.37mV	vin = 2.71V	temp = -1.87	rtd = 99.28	error: 0.28%
00> = -0.28mV	vin = 2.72V	temp = -1.40	rtd = 99.46	error: 0.47%
00> = -0.14mV	vin = 2.71V	temp = -0.70	rtd = 99.73	error: 0.74%

When I test on a real 3 wire RTD, the results are so unstables.

RE: RTD measurement with Wheatstone Bridge and ADC

jelsesser — Thu, 27 Mar 2025 15:02:34 GMT

Hi! Thanks, I will check it out

RE: RTD measurement with Wheatstone Bridge and ADC

hmolesworth — Thu, 27 Mar 2025 06:48:03 GMT

I wrote some extensive code to get very accurate samples from an RTD with SAADC 12-bits effective ENOB using alternate differential measurements; the code is low-level bare-metal, but you can follow the guidance I provide and use either nrfx or zephyr (if you must). The code is valid for nRF52832, nRF52840 and nRF52833:

seeking-more-information-on-nrf52833-adc-reference-and-buffers