[MAXI Automation] Analog IN and internal 5V supply deviation

[Friedl_AP]

I’ve tried the AnalogOutAnalogIn demo and discovered the following:

• ADC on AI12/AI13 reads 1023 already at 9.749V (should be 10.0V)
• +5V (X1) vs. GND (X1) measures 4.874V (which should be 5.0V)
• 9.749V * (5V / 4.874V) = 10.001V interesting!

Between the AIxx inputs and the analog inputs of the AVR, there must be only a voltage divider with factor 2:1 because the 10V input measurement is off by the same factor the 5V supply is off.

The 0-10V analog in/out’s are missing on the MAXI Automation pinout diagram – please fix this.

External lab power supply connected to GND of MAXI Automation and voltage measurements taken direct from this ground to the input pin.

Nothing’s connected to X1/X2, no load there, its just the MAXI Automation by itself. Multimeter used: Keysight 34461A (calibration date: 23.03.2020)

How can I tune the 5V as it is used as ADC reference voltage?

• This discussion was modified 3 years, 8 months ago by  Friedl_AP. Reason: removed some stray html tags which ended as clear text, which the editor inserted erroneously
• This discussion was modified 3 years, 8 months ago by  Friedl_AP. Reason: removed some more clear text div tags

[Friedl_AP]

May I feed 5.000V via X1 into the MAXI Automation to overwrite the internal power supply?

[Jarda]

Dear Friedl_AP,

When you need more precise measurement of 5V and 10V, please use internal INTERNAL2V56: a built-in 2.56V reference.

If you want to use precise external votlage, you can also use external votlage applied to the AREF pin (0 to 5V only).

BR

Jarda

[Friedl]

Dear Jarda!

1) As shown above, the 0-10V inputs are nothing more than 2:1 resistive divider, correct? They are not documented in the manual nor on the pinout diagram.

2) If we’d switch to the 2.56V reference, we could only measure 0-5.12V (2.56V * 2), right?

3) Am I missing the secret pin to control the switch for the voltage divider on AI12 and AI13 from ratio 2:1 to ratio 3.90625:1?

Greetings from Graz
Friedl

• This reply was modified 3 years, 8 months ago by  Friedl. Reason: wording
• This reply was modified 3 years, 8 months ago by  Friedl. Reason: typo

[Friedl]

Dear Jarda!

Regarding your suggestion of using the AREF pin.

If you open the microcontroller manual https://ww1.microchip.com/downloads/en/DeviceDoc/ATmega640-1280-1281-2560-2561-Datasheet-DS40002211A.pdf, there on page 9, chapter “2.3 Pin Descriptions” you can read:

AVCC is the supply voltage pin for Port F and the A/D Converter. It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter.

Therefore we can assume AVCC = VCC = 4.874V as measured before.

When you scroll down to page 365, chapter ” 31.8 ADC Characteristics” you will discover that:

Vref_MIN = 1.0V and Vref_MAX = AVCC

Because we know from above that AVCC = VCC, so AREF must always be less than or equal to VCC.

If VCC = 4.874V, as demonstrated before, then supplying 5V to AREF without supplying 5V to VCC as well is out of specification.

This suggestion could be dangerous, don’t you think so?

Greetings
Friedl

• This reply was modified 3 years, 8 months ago by  Friedl. Reason: typo

[Friedl_AP]

Dear Jarda!

I’ve now tried the following:

1. Switched ADC to external reference as first command of setup()
2. After loading the code I switched on a lab supply (4.500 V) connected to GND/VREF on X2
3. AI12 is driven by another lab supply with 4.500 V (GND=screw terminal of Controllino, measured versus GND of Controllino)
4. ADC value: 602 (expected (1024/2-1)=511)
5. reduce VREF till ADC value gives 1023
6. VREF measured: 2.653 V (expected 4.500V/2 = 2.25V)

The internal reference is off and connecting an external reference does not really work either.

Code:

#include <Controllino.h>

/*
 * CONTROLLINO MAXI AUTOMATION - using analog VREF input
 * 
 * Reads out converted analog values from a screw terminal input AI12
 * of CONTROLLINO using VREF on X2 as reference.
 *                               
 *                             ------------------------
 * 24V ---------o--------------| 24V                  |  
 * external     |              |                      |  
 * supply       |              |                      |        
 *       --------------        |   CONTROLLINO MAXI   |        
 *       | Lab Supply |--------| AI12                 |        ---------------
 *       |   4.500V   |        |              X2-VREF |--------| Lab Supply  |        
 *       --------------        |                      |        |  4.500V     |               
 *              |              |                      |        ---------------
 *              |              |                      |               |
 * GND ---------o--------------| GND           X2-GND |---------------o
 *                             ------------------------
 * remark: AI12 = internal divider 2:1
 * ATTENTION: Only switch on the lab supply on VREF after you've downloaded the code the first time!
*/

void setup() {
  analogReference(EXTERNAL);
  pinMode(CONTROLLINO_AI12, INPUT);
  Serial.begin(115200);
  delay(300);
}

void loop() {
  int val = analogRead(CONTROLLINO_AI12);
  Serial.print(F("raw value: ")); Serial.println(val);
  delay(100);
}

• This reply was modified 3 years, 8 months ago by  Friedl_AP. Reason: stray DIV tags removed