ADS1113: Continuous Mode

Hi Mark, 

The caveat when using continuous conversion mode is that you can read a previous conversion and may not always be 'fresh'. However, using single-shot mode, you can. In single shot mode, you tell the device when you want to perform a conversion and you can poll bit-15 of the configuration register to identify when a conversion is done. The ADS1113 will always act as a target device. 

Is using single-shot mode something you would be capable of doing?

Regards,

Aaron Estrada

Hi Aaron,

Thanks for your reply. When doing an application like ECG, single-shot mode isn't really an option. We want periodic samples because then we can remove spectral content via filtering. If the samples are done via single-shot, then the time delta between them isn't known, so you can't discern the spectral content of the signal. Imagine the difference between the two cases: 1) sample 100Hz sine wave at 200Hz rate and do FFT on the results [you'd get 100Hz peak and then all the alias frequencies; and 2) sample 100Hz sine wave randomly and do FFT (it would be a mess).

Regards,

Mark

Hi Mark,

Understood! With continuous conversion mode, using the Alert pin would be the best way to know when a conversion is finished. I suppose you can read the conversion data at a slower rate than the data rate to avoid reading the same data twice. However, there is a +/- 10% variation on the data rate so that would need to be taken into account. 

I know we do offer some other devices that would be more suitable for an ECG application so if you are interested, I can give you some part numbers. Also, if you don't mind, what drew you to the ADS1113 for this type of application?

Regards,
Aaron Estrada

Hi Aaron,

Thanks for the feedback. The ADS111x was attractive because of the following: 1) in stock!!;)   2) small size; 3) I can try three parts ('13, '14, '15) all in the same footprint; 4) has continuous mode w/o needing to have SCL continuous (ADS7142 uses this method). But if you think there is a more applicable PN for ECG, please pass on those part numbers. Thanks!

Regards,

Mark

Hi Mark, 

Understood! Well some of the devices I found may be a bit overkill if you are looking at the ADS111x family but I would still like to give you options We have the ADS119x and ADS129x families of ADC's for biopotential measurements. If you only need 1 channel, then I would look into the ADS1191 which is a 16-bit, 1CH device. 

If these are overkill then I think your best bet would probably be going with the ADS1114 due to the integrated PGA that allows for a wider full scale range. Regarding the continuous conversion mode, it will ultimately be best to utilize the Alert pin. Are you sure that your uP cannot spare a GPIO pin for the Alert signal? You can also read the conversion results at a slower data rate but after some thinking, that may cause you to read repeated data. 

Regards,
Aaron

Hi Aaron,

Whoops, I should have also added "I2C interface" to my list-of-needs. We're looking to cable this a fair distance and we're pin-limited, so we chose I2C. Are all the "biopotential" targeted parts SPI interface?

I did look at the I2C driver, and I guess basically the driver inserts delays to gate the reads for the samples (if I understand it correctly -- and maybe I don't). So that would make the samples "periodic", presumably with some +/-% in the period (we're not talkin' realtime SW here). 

Regarding using the Alert, I may be able to cobble it into my existing hardware with a rework. It's just that my baseboard is already designed so, preferably, I'd like to live without needing it. But it's possible I could rework it.

Regards,
Mark

Hi Mark, 

To my knowledge, all of the biopotential parts are SPI. I can still look around but ultimately, I think a workaround on the ADS1113 or ADS1114 will need to be done. Would be much easier with the Alert pin but I understand your constraints. 

Regards,
Aaron Estrada