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ADS1131: how to connect ads1131 in parallel or simmilar daisy chain

Ernesto Ramirez
Prodigy 70 points
Part Number: ADS1131
Other Parts Discussed in Thread: SN74LVC125A, ADS1234, ADS1120, ADS1220, ADS114S08, ADS124S08

Hello everyone, I have a design of an application with an ads1131 that is used with a load cell on a Weighing scale, I would like to extend that application and connect 4 load cells to 4 Weighing scales, because the ads1131 does not have a daisy chain or pin select , I am looking for a solution. the first would be to put 4 ads1131 in parallel to a spi, the second would be to redesign the card with a similar adc like a ads1131 with daisy chain (which I recommend).
thanks in advanced.

  • 0
    TI__Guru** 113765 points

    Hi Ernesto,

    The ADS1131 was designed for a single SPI connection (no CS pin to select device communication).  However you can connect a number of devices together by using some form of logic that you can use to disable the communication.  You could use a device such as the SN74LVC125A so that only one device at a time can communicate through the SPI bus by enabling/disabling particular channels.  The only drawback is keeping the communication synchronized so that you do not try to retrieve data while the ADS1131 is updating conversion results.

    Another possibility would be to use the ADS1234 which is a 24-bit version very similar in operation to the ADS1131, but with 4 input channels to a single device. 

    If you would like to stay with a 16-bit version you could also look at the ADS114S08 which allows for 4 differential signals to be muxed to on ADC.  The last potential method I would suggest is to use the ADS1120 (or ADS1220) instead of the ADS1131 as the ADS1120 also has the low-side switch to break excitation and also contains CS pin for connecting multiple devices.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points

    Thanks for your answer Bob, it has helped me a lot, just out of curiosity, the ads1131 and the ads1234 have a maximum of 80 samples per second both are 18 bits, the number of samples is related to the resolution?
    and if so, that adc similar to both but with less resolution and more sampling per second.

    Thanks in advanced.

  • 0 in reply to Ernesto Ramirez
    TI__Guru** 113765 points

    EDITED to correct timing information regarding the ADS1234 when cycling through the mux.....

    Hi Ernesto,

    The ADS1131 is 18-bit, but the ADS1234 is 24-bit.  The maximum data rate for both is 80sps.  As the ADS1234 uses a mux to measure the four channels separately the cycle time will be reduced greatly from 12.5ms (80sps).  The ADS1234 uses a sinc4 digital filter, so the digital filter will not be fully settled until after the 4th conversion completes.  The timing is demonstrated in the ADS1234 datasheet on page 17 and shows what happens when the mux is changed in Figure 32.  First there is a small setup time before conversion starts following a mux change which can range from 40-50us.  DRDY/DOUT can be used as the signal for when valid conversion data are ready to be read out from the ADS1234.  The nominal time is 51ms at 80sps data output rate. So when the mux is changed, DRDY/DOUT will go high and 51.05 ms later will go low signaling a valid data are available.  If the mux is not changed, the next transition of DRDY/DOUT from high to low takes place at 12.5 ms after the previous.

    Added to this time will be the actual time it takes to read out the conversion result prior to the next mux change.  Depending on SCLK speed this could range from 10 to 50us or more.  Let's say 50us which would be added to the 51.05ms so that the time of each channel averages to 51.1ms.  So for all four input channels to convert and be read will take about 204.4ms or 4.89 measurement cycles per second to measure all four input channels.

    As I mentioned previously, you could use a device such as the ADS1220 which is 24-bit and allows for faster operation.  You could use the ADS1220 in the same way as you use the ADS1131 and use 4 devices.  The ADS1220 has a CS pin so you could easily connect 4 devices together on the same SPI bus.

    Or you could use a device such as the ADS124S08 which has a single ADC with a mux but is faster than the ADS1234.  The ADS124S08 has a low-latency digital filter option for cycling through the mux, and also has faster data rates.

    In general, the faster the data rate the higher the noise.  So if you operate at higher data rates, then noise will affect the resolution.  Depending on the desired scale resolution you are targeting, the load cell sensitivity and the rate you need to capture data both the ADS124S08 and the ADS1220, although they are 24-bit devices, may not have the desired performance as the full-scale range of these devices are different from the ADS1131.

    If you tell me your target resolution and more about your use case I may be able to give you a better idea on overall performance.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points 
    Thanks for your reply
    

    

    

    

    

    In our application we have an encoder which handles 66ms in high state and 200ms in low state. The sampling time is 66ms. Currently we have an ADS1131 connected to a 5kg load cell, The ADS1131 offers us around 10 samples in 66ms, to which we apply an average that will be the final weight evaluated in 66ms (in this application the ADS1131 is working well). Our question is about the ADS1234 which is 4 channels, We want to know if we can get 10 samples for each channel in 66ms.
    

    

     
    

    

    

    

    

    Thanks in advanced.
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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						in reply to Ernesto Ramirez
					
									
    
	
    
	                                    
                      
    
                        
                          TI__Guru**
                        113765 points
                      
    
                          
                   		
		
    
					
    Hi Ernesto,
    

    I'm not sure I totally follow what you are trying to tell me.  The ADS1131 running at 80sps provides a new conversion result every 12.5ms.  So it is not possible to capture more than 5 conversion results within a 66ms window.
    

    The ADS1234 requires you to cycle through the input mux.  As I stated in my previous post, it will take a minimum of 204.4ms to cycle through all four mux input channels.  It would be impossible to use the ADS1234 and take a single reading of all four load cells let alone 10 within a 66ms window.  So based on this information you have provided and time window allowed, the ADS1234 would not be a good fit for your application.
    

    You could take a look at using the ADS124S08 as this device has the potential for higher data output rates.  Using the 800sps data output rate and the low-latency digital filter you should be able to cycle through all four input channels and measure 10 times on each channel.
    

    Best regards,
    

    Bob B
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    

	            
                 	
	
    
				
		                
                
                
                
                
                
                
        		
		
				
    
			
		
    
		
		 
		
		
     
    
	
    

    
			
    
			
    
		
    

		
    
	
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