ADS1299: Can not get full range readings even if my leads are open

Hi Yf,

Thanks for your post!

As per table 9, 7FFFFFh is the max code while 800000h is the minimum code. Is 2500000 in decimal? Are you using the internal test signal to generate this value as your registers indicate, open input as your post title indicates, or applying an external signal?

To get the full scale reading you will have to apply a signal that reaches the full scale input of the ADC. Section 9.3.1.3.2 of the datasheet covers this.

A couple of notes from your register settings:
Internal test signal 1x-(Vrefp-Vrefn)/2400
Bias buffer powered down
Every ch enabled to bias
every ch lead off enabled, lead off comparator disabled, config4

Hi Alex,

Thanks for the following up and sorry for the confusions. 

I am using decimal, 2^23-1 (0x7FFFFF) is 8388607 in decimal, which is what I mean by 8000000 (8M) in my post. Actually I did test again and found that: with 24x gain I get 2.5M readings, with 1x gain I get 1.25M readings.

I was not intended to select the internal test signals, you can see this from the CH*SET registers where MUX[2:0] bits all set to 0 (normal input case). I leave the SRB1 (which connects to all channels' negative input), the BIAS and the CH*P all open to create a definite lead off case. I am expecting to get some readings close to 0x7FFFFF(8M) but can only get 2.5M or 1.25M so far.

I turned the lead off comparator off to see how it affects the range -- my finding is no effect.

I am unsure my BIAS related configuration is right or not, so I turned the BIAS buffer off and find the range is still the same.

Regards,

yf

Hi Yf,

What is the magnitude and frequency of the signal being applied and to which channel?

To get the full scale reading you will have to apply a signal that reaches the full scale input of the ADC. Section 9.3.1.3.2 of the datasheet covers this.

Alex,

I am only using three electrodes now, namely the Ch1, SRB1, and BIAS. Currently there is no input to these electrodes since I am forcing the case of "lead off" and see if the signals can trigger the LOFF comparator.

I will study 9.3.1.3.2 again but would you please teach me how to properly test the lead off case?

Also please give a look my schematic where I connected all unused Ch*N pins to AVDD... is this proper?

Regards,

yf

Hi Yf,

Are you able to get full-scale signals when not using the lead off function?

If not, I think we should work on that first before adding another variable (lead off) to the equation.

If there is nothing applied to the inputs, then how would the lead-off comparator sense a change in impedance?

This app note is helpful for understanding lead-off:www.ti.com/.../sbaa196a.pdf

As per the datsheet unused analog inputs should be connected to AVDD so that's fine.

Alex,

No, I got smaller readings (-50k) when LOFF_SENSP==00h and LOFF_SENSN==00h. I got same 1.25M readings when LOFF_SENSP==FFh and LOFF_SENSN==00h.

The PD_LOFF_COMP bit in CONF4 register seems have no effect on the readings range based on test.

I agree that we should work on simpler (non-LOFF) case before touching LOFF, so please suggest what I should check?

I guess the problem will be:

1. Turn off lead off functions:
   1. LOFF_SENSP=00h,  
   2. LOFF_SENSN=00h,
   3. CONF3.BIAS_LOFF_SENS=0
   4. CONF4.PD_LOFF_COMP=0 
2. Using normal MUX mode: CH*SET=0
3. Connect SRB1 to all channels' Negative electrode: MISC1=20h
4. Keep the electrodes open
5. Check the readings from SPI interface

Regards,

yf

Hi Yf,

Can you please clarify what exactly you are trying to do or what you having trouble understanding?

I would like to get more information on your problem.