ADS1282: ADS1282 seismic

Hi Chris

thanks for your reply.

1. following data is main  parameter of the sensor：

Output impedance of the sensor： 2 x 150Ω ±1%

output voltage of sensor: 40V peak-to-peak differential Selectable XYZ or UVW mode

the frequence range of the sensor: 120s-100HZ

now i used the resistance divide circuit to test it. BUT, Perhaps , TI have more good idea or reference to resolution for this.

2. I need four channel for my datalogger  ADS1282+STM32F407. So i need used four ADS1282 for my design? and i see the datasheet of the ads1282(Figure 66. Microcontroller Interface with Dual ADS1282s) do i need much more IO port for the design？

Hi Kevin,

Do you know what the common-mode output voltage of the sensor is? Are the output voltages centered around 0V or some other voltage potential? Also, are you using bipolar (+/- 2.5V) or unipolar (AVDD=5V, AVSS=0V) supplies for the ADS1282?

...I ask because you may need to level-shift the sensor output so that the output common-mode voltage is centered around the ADC's mid-supply voltage. We certainly can help to recommend an active circuit to do this; however, I generally prefer to use passive components whenever possible, since active devices may introduce significant noise, offset, gain, and drift errors into the measurement.

For your second question, that is correct...You would need four ADCs. For I/Os, you would either need four SPI peripherals OR four GPIOs that could act as chip select signals. While the ADS1282 does not have a "/CS" pin, you could use the following work around to select between multiple ADS1282s on the same SPI peripheral: e2e.ti.com/.../2748.where-is-the-cs-pin

Best regards,
Chris

Hi chris

thank you very much！

i meet a new problem in my debug process, when the input signal cross zero（just like sine wave ，near zero），the output data often distortion. following is the output data when input standard sine ware：

ch3 -4572
ch3 -8295
ch3 -11749
ch3 -14813
ch3 -17418
ch3 -19478
ch3 -20938
ch3 -21752
ch3 -21883
ch3 -21332
ch3 -20129
ch3 -18304
ch3 -15915
ch3 -13009
ch3 -9691
ch3 -6068
ch3 -2238
ch3 -8386937
ch3 5550
ch3 9263
ch3 12698
ch3 15742
ch3 18311
ch3 20332
ch3 21731
ch3 22454
ch3 22496
ch3 21836
ch3 20520
ch3 18575
ch3 16063
ch3 13059
ch3 9652
ch3 5971
ch3 2119
ch3 8386800
ch3 -5653
ch3 -9314
ch3 -12661

Best regards！

Kevin

Hi Chris

thank you for your  patience！following picture is the input circuit and code of the ads1282.

void SPI3_WriteByte(int TxData)
{
int i;
for(i = 0; i < 8; i++)
{
if (TxData & 0x80)
ADS1282_DIN3_1();
else
ADS1282_DIN3_0();

SCK3_1();
ADS1282_DelaySCLK(7);
TxData <<= 1;
SCK3_0();
ADS1282_DelaySCLK(10);
}
}

unsigned char SPI3_ReadByte(void)
{
unsigned char i;
int read = 0;
SCK3_0();
for (i = 0; i < 8; i++)
{
SCK3_1();
if (ADS1282_DOUT3)
{
read++;
}
ADS1282_DelaySCLK(5);
SCK3_0();

ADS1282_DelaySCLK(10);
read = read<<1;

}
read=read>>1;  
return read;
}

int testSPI3()
{
unsigned char temp1_read31;
unsigned char temp1_read32;
unsigned char temp1_read33;
unsigned char temp1_read34;     // for 32bit
unsigned long int temp1_read3_U;
signed long int temp1_read3_S;

if(!ADS1282_DRDY3)
{
while(!ADS1282_DRDY3);
temp1_read31 = SPI3_ReadByte();
temp1_read32 = SPI3_ReadByte();
temp1_read33 = SPI3_ReadByte();
temp1_read34 = SPI3_ReadByte();

temp1_read3_U = 0x00000000;
temp1_read3_U += temp1_read31;
temp1_read3_U <<= 8;
temp1_read3_U += temp1_read32;
temp1_read3_U <<= 8;
temp1_read3_U += temp1_read33;
temp1_read3_U <<= 8;
temp1_read3_U += temp1_read34;

temp1_read3_S = temp1_read3_U;
if( temp1_read3_S & 0x80000000 )
{
temp1_read3_S = ~(unsigned long)temp1_read3_S;
temp1_read3_S &= 0x7fffffff;
temp1_read3_S += 1;
temp1_read3_S = -temp1_read3_S;
}

printf("ch3 %d\r\n",temp1_read3_S); //double
}
return temp1_read3_S;
}

the 32bit or 24bit is the same problem.

there is another matter about the program. you can see i have a read=read>>1;  in read SPI ,i don't know why need this, because read SPI and write SPI are with sck rising edge， if i don't use this statement，the output data will move to right one bit. 

thank your again！

best regards

Kevin

Hi Kevin,

Do you have access to a dedicated SPI peripheral on your microcontroller?
If you can, I'd suggest using it instead of bit banging here.

For the erroneous data, it looks like you are trying to convert the ADC result to a two's compliment number, here:

temp1_read3_S = temp1_read3_U;
if( temp1_read3_S & 0x80000000 )
{
	temp1_read3_S = ~(unsigned long)temp1_read3_S;
	temp1_read3_S &= 0x7fffffff;
	temp1_read3_S += 1;
	temp1_read3_S = -temp1_read3_S;
}

This is unnecessary since the conversion result is already in the two's compliment format. All you need to do is concatenate the bytes into the signed data type and remove the duplicate sign bit, like so (note I'm using the signed long instead of the unsigned type here):

temp1_read3_S = 0x00000000;
temp1_read3_S += temp1_read31;
temp1_read3_S <<= 8;
temp1_read3_S += temp1_read32;
temp1_read3_S <<= 8;
temp1_read3_S += temp1_read33;
temp1_read3_S <<= 8;
temp1_read3_S += temp1_read34;
temp1_read3_S >>= 1;            // Make sure right-shift maintains the sign bit

I think that change might help resolve the issue.

Regarding the "read = read>>1;" line, that looks like it's necessary because you're shifting the data left at the end of each FOR loop iteration, and you don't need to do the left shift on the very last iteration (because 0x01 << 8 = 0x100).

Best regards,
Chris

Hi Kevin,

I don't have another other documentation with synchronization instructions. However, if what you're wanting to do is to ensure that all four ADCs are simultaneously sampling, then I would recommend using a single clock source for the ADC's fCLK (master clock) and sending a SYNC command or SYNC HW pulse to all four devices at the same time.

You might refer to these related E2E threads:

Let me know if you still have questions.

Best regards,
Chris

Hi Chris

To disturb you again. I want to get three channel data with ads1282 in sync mode. I design my circuit board like figure 1. Is it correct?

And I want to program it following the ads1282 datasheet and refer the figure 47 and figure 48, but I don’t realize it. do you have the sequence figure of DOUT1 DOUT2 DOUT3 ,DRDY1 DRDY2 DRDY3 with sync？or can you give me more detail instruction about this?

PS:  The data output rate of ADS1282 is 500SPS and the frequency of the SYNC signal is 500 Hz

Best regards!

Kevin

Hi Kevin,

I don't see any problems with your circuit diagram. You should be okay using 3x different SPI peripherals to communicate with 3x ADS1282s.

...One thing to note is that you probably wont need to monitor all 3 DRDY/IRQ signals if the devices are in sync.

...The only other suggestion I have would be to source the ADC clocks from the MCU if you intend to use continuous-sync mode of the SYNC GPIO (hardware) signal instead of the SYNC SPI (software) command.

Do you intend to use pulse-sync mode or continuous-sync mode?

Since you show an independent clock (independent from the MCU), I might recommend only using the pulse-sync mode and sending the SYNC SPI command (instead of the SYNC GPIO signal). The reason for this is that your SYNC GPIO and CLK signals are not derived from one another and it is very easy for these clocks to get out of sync. When this happens in continuous-sync mode the ADC(s) may re-synchronize unnecessarily. See this related E2E thread: 

If you use pulse-sync mode and send the SYNC (software) command, you can issue this command at any time to synchronize the ADCs. If you use the SYNC GPIO signal, then please pay attention to the "t_CSHD" timing requirement, which requires that the rising edge of the SYNC signal not occur within 10 ns of the rising edge of the CLK signal.

Best regards,
Chris