ADS125P08: External sampling synchronization using START pin

Hi Olav Staberg,

The answer to this question depends on what else you are trying to do:

• Are you using the START pin so you can synchronize sampling to an external event?
• Assuming you are toggling the START pin at 1kHz, are you just taking a single sample every 1ms? Are you trying to take multiple samples?
• Are you trying to power cycle so you take data quickly then put the device in standby mode to save power?
• In any of these scenarios, how long do you want the sampling process to take? And then what is the noise / resolution target at this speed?

The ADS125P08 digital filter latency is given in tables 7-9 (sinc3 or sinc4 filter), 7-12 (sinc4+scin1 filter), and 7-14 (FIR filter) so you can understand how long it takes for the ADC to produce the first conversion after toggling START. Second and subsequent conversions on the same channel will be available at approximately 1/data rate

The ADS125P08 has a lot of digital features in the sequencer that might be able to help you achieve your goals, so if you can provide more information I can confirm if the ADS125P08 is the correct device to choose.

-Bryan

Hi Bryan

Yes, we want to synchronize sampling on independent adcs to an external event using a time synchronized 1 or 8kHz signal that is distributed to all the devices. The external sync signal is used to toggle Start pin at 1 or 8kHz (depending of operatuion mode) to sample two channels. After reading both channels start pin is pulled adc is put into idle until next toggle of start pin (1ms or 125us later)
For now we are just interrested in verifying if this is feasable using the ADS125P08
We have tested same operation for other adcs but the settling time has typically been to long for this to work 

-Olav-

Thanks for verification 

Do you know of any alternative simultaneous-sampling ADCs (minimum two channels) that can
achieve similar operation and data rates as in the above case?
The devices we have evaluated so far (e.g., ADS131 series and ADS1174) have not been
suitable due to long settling times as far as we can see.

Regards
Olav 

Hi Olav Staberg,

Again, it would be helpful to understand the specifics of what you are trying to do. I understand the ADS131 and ADS127x devices didn't do what you needed them to do, but I don't understand why. You said that there are long "settling times", but again, I don't know how long is too long

Without additional information you could at least consider our highest sampling rate precision ADC device: the ADS9219, a 2-channel, 18-bit, 20-MSPS SAR ADC

-Bryan

Hi Bryan

We want to synchronize sample and conversion of severeal independent ADCs with a accuracy of better than 10us 
For this purpose we have an external sampling signal with accuracy <5us that is avilable in all devices 
We want to use this signal, which can be 1 - 8kHz depending on mode of operation, to control conversion via 
START/SYNC pin in the ADC. However, for several ADCs the settling time when using START pin is greater than the
minimum required sampling time (min 125us). Hence we are looking for ADC with short settling time and an input 
pin that can be used for controlling conversion.

In addition, this is a battery-powered application, so the current consumption must be as low
as possible, preferably below 5 mA overall.

Regards
Olav

Hi Olav Staberg,

Understood, thanks. I did present a scenario using the ADS125P08 where the settling time to measure 1x sample on 2x channels would only take ~60us, which is well within your 125us requirement - why would this not work?

Another option would be to use 2x ADS127L11 devices with the sinc filter (the ADS1174 only has a wideband filter, which has significant group delay and is therefore not ideal for "stop / start" sampling). This device also has different speed modes that also scale the power consumption so you can tradeoff between power and latency. There is also an ADS127L14, where you could powerdown two of the channels if you need to reduce the current

You could also consider something like 2x ADS7056 (not sure what resolution you need), which is a 14-bit, 2.5MSPS low power SAR ADC. Then you would control conversions using the CS pin

Let me know if you have any questions about these options

-Bryan

Hi

The presented solution for ADS125P08 will work - but it's always nice to have a few other option for consideration
as well (might be price current consumption complexity in implementation and so on)
And yes, I have looked into using ADS127Lxx series for our application  
We presumably need 24-bit resolution for our application 

Thank's for all input we will evaluate the suggested solutions

Regards
-Olav

Hi Olav Staberg,

Understood, let me know if we can help answer any more questions

-Bryan