AD8400 Digipot with Arduino Uno R3: Non Linear Output Resistance

[avis_17122002]

I am controlling the AD8400(1 kohm) digipot with Arduino Uno R3 using SPI interface. My connections are as follows:

Digipot <-> Arduino

CS(3) <->10

SDI(4) <-> 11

CLK(5) <->13

VDD(6) <-> 5v

GND <-> GND

I am checking the resistance across terminal W and B using multimeter after giving input between 0 to 255.But the resistance obtained is not varying linearly and it also exceeds the Resistor Nonlinearity((-4) -(+4) LSB) provided in datasheet.

Below is the graph obtained for values between 0-255

Blue line indicates the observed values

Red line is for the equation:

Resistance = 43.2 + (Data/ 255) * (1000)

Below is the code used:

#include <SPI.h> // include the SPI library

#define slaveSelectPin 10 // set pin 10 as the slave select for the digital pot

int address = 0; //addr bits = 0)
int value = 72; // value can be anything between 0 to 255

void setup() {
  pinMode(slaveSelectPin, OUTPUT);// set the slaveSelectPin as an output:
  Serial.begin(115200);  
  SPI.begin(); // initialize SPI:
}

void DigitalPotWrite(int addr, int val){
  digitalWrite(slaveSelectPin, LOW);
  SPI.beginTransaction(SPISettings(10000000, MSBFIRST, SPI_MODE1)); //initialises SPI clock speed, mode and bit order
  digitalWrite(slaveSelectPin, LOW);
  SPI.transfer(addr);
  SPI.transfer(val);
  digitalWrite(slaveSelectPin, HIGH);
  SPI.endTransaction(); 
}

void loop() {
  digitalWrite(slaveSelectPin, HIGH); // ensure default CS stays high for now
  DigitalPotWrite(address, value);
  delay(5000); //arbitrary delay 
}

Can someone please check what could be the error and how to reduce it??

[JohnLincoln]

Does your multimeter 'auto range'?
There could be discontinuities as the meter changes range.

Most autorange multimeters can be set to stay on a particular range.
Choose a range that will read up to the maximum resistance of the AD8400.

[jim-p]

@avis_17122002

SPI.beginTransaction(SPISettings(10000000, MSBFIRST, SPI_MODE1));

That statement should be in setup()

 SPI.endTransaction();

Remove this statement

There should be a delay after setting SS pin LOW and before setting it HIGH

Value and address should be byte

[jim-p]

In addition to what @JohnLincoln said, if you are using this on a breadboard, consistent readings could be problematic.

[avis_17122002]

Yes I had used an autoranging multimeter... will try that

[avis_17122002]

How much delay would you recommend?.... I have kept SPI speed 10 MHz

[jim-p]

@avis_17122002

How much delay would you recommend?

The minimum you can use will depend on you physical set-up but for debugging purposes use 1ms

[oqibidipo]

@avis_17122002

SPI.beginTransaction(SPISettings(10000000, MSBFIRST, SPI_MODE1));

I would try SPI_MODE0.

https://en.wikipedia.org/wiki/Serial_Peripheral_Interface#Mode_numbers

[avis_17122002]

Sorry for the late update.... I tried with SPI Mode0 and it worked! I also added made changes in code according to what @jim-p suggested. Thank you so much everyone. The graph obtained is quite linear now