Problems with ADS1178?

Jeremy said:

I am using the TI sigma-delta ADC ADS1178 with my C6747 board. There are some problems i don't understand about the sigma-delta AD ADS1178? would you pls give me some advices?

1. The group delay and settling time in the ADS1178 datasheet page 3 is 38/fData and 76/fData? What does the fData mean? Does it mean the sampling rate? If so, I am gona to sample the data with 8k, and so the group delay is 38/8k>4ms, the settling time is 76/8k >9ms? How does the chip adapt to the industrial control system with such a long group delay and settling time?

You are correct about the group delay and settling time for the ADS1178. Fdata is the data rate of the part, the max data rate is specified at 52,734 SPS. The group delay is incorporated into the settling time. It does take 38 sample for the group delay. It takes 76 samples (or full conversions) for the group delay and the settling time. Using a 52,734 SPS data rate, this is ~2.45 ms. Because of this delay, Delta Sigma converters are sometimes not suitable for industrial control systems - it depends upon how fast the control loop needs to be. A SAR converter would be a better device for your application. SAR converters have a much lower latency. For information on the two types of converters, please see this application note:

http://focus.ti.com/lit/an/slyt191/slyt191.pdf

 

2. What are the occasions where the sigma-delta ADCs are suitable? In the real-time control system, the long group delay will limit the wide use of the sigma-delta ADC. I am puzzled that how to use this kind of ADCs? In my system, a timer is used to generate the 1/8k period interrupt, and in the ISR program, read back the AD sampling data, handling the AD data with function program and then output the handled data with DA. How can I use the ADS1178 in my system? Bye the way, ADS1178 works in high-speed mode, TDM data transfer with EDMA..

Delta Sigma ADCs are more suitable where the delay does not matter. Applications such as data loggers and data monitors are a few places delta sigma converters may be seen. Delta sigma converters are used in cases where you would need high precision and accuracy. These converters are  designed to average many samples while digitally filtering the signal as best as possible. In your system, we recommend using a SAR converter. The ADS1178 in high speed mode is not fast enough for your system. SAR converters run at much faster speeds.

 

3. What is the usage of pin SYNC signal in ADS1178? From the datasheet, it can  be synchronized by pulsing the SYNC to low and return to high again. In SPI mode, the time for new data to be ready  is 176/fData(if fData is 52k, there will be almost 3ms delay).  If the SYNC pin is connected to a reference time pulse with 1/8k, will the ADS1178 work properly(that is sample at an interval of 1/8k)?

The SYNC signal allows for the conversion to be aligned with an external event. In SPI mode, the signal is pulsed and 129 conversions (max) later the data will be settled and ready to be read. Using the max Fdata rate (52734 SPS) the time needed for the new data to be ready is 2.441 ms. You will not be able to connect the SYNC pin to the reference time pulse with a 1/8k pulse interval. 1/8k is not enough time for the data to be ready to be read. Again, we recommend looking into using a SAR converter to fit your system.