why ads1148's two RTD channel get different temperature when testing the same pt100 at the same time?

Hi Henry,

It appears the device is being used to measure the RTD in the 3-Wire configuration with Hardware compensation (with R100 used as a compensation resistor).  A potential issue with the RC filter configuration shown in the schematic,  there is a mismatch in the corner filter frequencies seen the positive channel (AIN+) and the negative channel (AIN-) due to the compensation resistor.  In other words, the time constant seen by AIN6 (RTD1-) is provided by two resistors: R100+R99 with C117; and the time constant seen by AIN7 (RTD1+) is provided  by only one resistor:  R101 with C117 .  This will cause a mismatch at the noise seen by the positive and negative inputs of the device, and this differential noise signal will be amplified by the internal programmable gain amplifier of the device.  A suggestion to better match the filter would be to place the differential filter capacitor (C117) on the other side of RCOMP, and keep the RCOMP resistor close to the device. Make this change on both the RTD1 and RTD2 channels.    Please see figure below.

 When using the 3-wire RTD configuration, since the IDAC currents flow through the positive and negative RTD sense connections, the resistances through the RTD+ and RTD- wire lines and through the PCB traces must be closely matched.  The resistance of the components in the path must be also closely matched.  What is the tolerance of the RCOMP (R100) resistor and the filter resistors R99 and R101? 

Can you please provide detail of the PGA gain settings, data rate and IDAC settings being used? Also, please provide a code snippet or the code algorithm; including the commands and delays used between the commands.

Thank you and Best Regards,

Luis

Hi Luis,

Thank you for your suggestion.
I moved the differential capacitor to the other side of Rcomp,the situation does not get better.
the following are  the tolerance and values:
49.9R 1%, 47PF 20%, 110R 0.1%, IDAC1=IDAC2=1.5mA, PGA=128
The pcb traces of RTD1+/RTD1- pair and RTD2+/RTD2- pair do  mismatch, the maximum tolerance of the two pair is about 300mil.
the following are the steps to test the two channels:
1、initializing ads1148
2、read rtd1 channel value
3、change the value to temprature and print
4、the board is power off
5、initializing ads1148
6、read rtd2 channel value
7、change the value to temprature and print

the attachment is the code snippet.

Thank you and Best Regards

#define	REG_MUX0	0x00
#define REG_VBIAS	0x01
#define REG_MUX1	0x02
#define REG_SYS0	0x03
#define REG_IDAC0	0x0A
#define REG_IDAC1	0x0B

#define CMD_RESET	0x06
#define CMD_RDATA	0x12
#define CMD_RREG1(r)	(0x20|((r)&0x0F))
#define CMD_RREG2(n)	((n)&0x0F)
#define CMD_WREG1(r)	(0x40|((r)&0x0F))
#define CMD_WREG2(n)	((n)&0x0F)
#define CMD_SELFOCAL	0x62

#define START_H()	MCF_GPIO_PPDSDR_D = 0x02		//pin START high
#define START_L()	MCF_GPIO_PCLRR_D = ~0x02		//pin START low

//pin START output a pulse
#define START()	do{	\
					START_H();\
					udelay(1);				\
					START_L();\
}while(0)

#define DRDY()		(!(MCF_GPIO_PPDSDR_D & 0x8))	//check pin DRDY 

void ads114x_init(void)
{
	spi_init();

	MCF_GPIO_PAR_DSPI0WL &= ~(3 << 4);	//PD3 as GPIO, connect DRDY
	MCF_GPIO_PAR_TIMER  &= ~(3 << 4);	//PD1 as GPIO, connect START

	MCF_GPIO_PDDR_D |= (0x01<<1);		//PD3 as input
	MCF_GPIO_PDDR_D &= ~(0x01<<3);		//PD1 as output
	
	START_H();							
	//reset the ads device
	tx_buf[0] = CMD_RESET;
	spi_write(tx_buf, 1);
	udelay(1*1000);
	START_L();

}

int ads114x_read_rtd(int channal, float &result)
{	
	unsigned char tx_buf[1];
	short rx_data;
	unsigned char *rx_buf = rx_data;

	
	if(result == NULL)
		return -2;
	if(channal == 1)
	{
		START_H();
		ads_write_reg(REG_MUX0, 0x07<<3|6);
		ads_write_reg(REG_MUX1, 0x20);		//use REF0
		ads_write_reg(REG_SYS0, 0x7F);	
		//Off the current out before slect the current dir.
		ads_write_reg(REG_IDAC0, 0);	
		ads_write_reg(REG_IDAC1, 0x67);	
		ads_write_reg(REG_IDAC0, 0x07);	
		START_L();
	}
	else if(channal == 2)
	{	
		START_H();
		ads_write_reg(REG_MUX0, 0x03<<3|2);
		ads_write_reg(REG_MUX1, 0x20);	//use REF0
		ads_write_reg(REG_SYS0, 0x7F);	
		//Off the current out before slect the current dir.
		ads_write_reg(REG_IDAC0, 0);	
		ads_write_reg(REG_IDAC1, 0x23);	
		ads_write_reg(REG_IDAC0, 0x07);	
		START_L();
	}
	else
		return -1;

	udelay(2);

	START();
	while(!DRDY())
		udelay(10);
	
	tx_buf[0] = CMD_RDATA;
	status = spi_write_then_read(tx_buf, 1, rx_buf, 2);

	if(status < 0)
	{
		printf("ADS114x""spi_write_then_read failed with status %d\n",
				status);
		return status;
	}

	rx_data = NTOHS(rx_data);
	
	*result = RTD_V2T(rx_data);	//change the adc outdata to temprature
	return 0;
}