ADC - strictly managing sampling timing

Takushi-san,


Many devices have timing that is exact in terms of when data comes out. If you look at Tables 15 and 16 of the ADS1248 datasheet you can see the time in which it takes to get a data out. This of course assumes that you have an oscillator clock of exactly 4.096MHz. In the first table, the timing is for a START pin used to initiate the conversion. In the second table, it is the timing for changes in the configuration register. Many of data converters will have this type of exact timing. This might work for you customer.

However, there are reasons why this type of data converter might not work for your customer. Most precision data converters in our product line are delta sigma type data converters. In this case, the data that comes out is close to an average of the input from the start of the conversion to the end of the conversion time. If your customer needs a conversion with a sample-and-hold for the data at the moment of the start of the conversion, then this type of converter will not work for them. I don't think any of our devices in the 24-bit range have this.

I think we would want to know the data rate, resolution, and the nature of their sampling to get an idea if we have what they need. We'll probably need some description of their system and to help them make a selection.


Joseph Wu

Kawai-san,


Based on the 860 SPS data rate, it sounds like your customer is interested in the ADS1115 or the ADS1118. The first device uses I2C while the second uses SPI.

These devices all have the basic features that you describe, including the ability to shut it down when not in use.

I would however, be concerned about the data rate. In your first post, you mention strictly managing the sample timing. In these devices, there is an internal oscillator which does have some variability in the internal clock rate. Both of these devices do have an 860 SPS data rate, but it can be off by +/-10%.

If there's more information about what the customer would like to do, I'd be interested in hearing about it. However, it's likely that the either the ADS1115 or the ADS1118 will work for your customer.


Joseph Wu

Hello Kawai-san

Looking at your requirements, a SAR ADC may also be a good fit for your application:
1) In a typical SAR ADC, an external CONVST or CSz signal is used to control the sampling instant for the device. The device holds the input signal value at the rising or falling edge (device dependent) of the sampling control signal after which it goes into conversion phase to provide a digital output.
2) Since the sampling control is user controlled, it can be applied after any time delay. In some new ADC architectures, the device consumes zero power after completing a user initiated conversion. The zero power state is maintained until the next sampling control is initiated. This results in significant power savings if a device is used at a low sampling rate.

Some of our new SAR ADC devices support the above described power scaling feature, namely below:
1) 18 bits --> ADS8881/3/5/7 ; ADS9110
2) 16 bits --> ADS8860/1/2/3/4/5/6/7
3) 12 bits --> ADS7042/3/4

Please let us know if you have any follow up questions in this regard.

Thanks