Part Number: ADS1299EEGFE-PDK
I recently purchased this PDK. I replaced the bias filter in order to include 150 Hz BW instead of 41 Hz BW(which comes as default) Because I wanted to get rid of the 50,100 and 150 Hz noise from power lines.
I configured one channel to be used with bias electrode(selected appropriate sense bits for derivation). Used short electrodes with GND shield. Confirmed the bias electrode function by measuring the bias voltage from another channel. Using bipolar power supply and batteries are used to power the PDK.
But still I am not getting a decent noise floor as expected from this amplifier. Noise floor(pk - pk) seems to be around 10-20 uV. Please let me know the some suggestions to improve or find the cause of this.
Thank you for your post.
Can you please share the complete register map settings? The noise floor will be most dependent on the data rate and the PGA gain.
It makes sense that you would increase the R-C filter cutoff frequency for the BIAS amplifier to cancel the second and third harmonics of 50 Hz. What is the output of the BIAS amplifier connected to? I imagine you are shorting INxP, INxN for one channel and BIASOUT together?
Applications Engineer | Precision ADCs
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In reply to Ryan Andrews:
Thank you for your response. Please find the register map settings attached. I connected it to a subject in development environment, with many different electrode configurations, but couldn't get a signal less than 10 uV (pk - pk) . Is there any modification needs to be done for the RC filter of the INxN , INxP?
I am a new to this EMG playground and would appreciate your support to get started with this low noise amplifier.
In reply to Saba Eas:
There are several factors which will contribute noise to the signal chain. At 500 SPS, Gain = 24 V/V, the ADS1299 itself contributes 1.39 uVpp of noise (typical), but this is not the limiting factor. Noise will also couple onto the body, the electrode wires, and the PCB. The input RC filters are not intended to filter out this noise - they are used to prevent higher frequency noise from aliasing.The active BIAS electrode will help suppress some of the common-mode noise on the body, but some of this noise may still convert to a differential noise due to mismatches between the INxP and INxN signal paths. This includes contact impedance from the electrodes, which can vary quite a bit. The best you can do is to match the components as closely as possible using very low tolerance components. Most EEG applications still require additional averaging and other post-processing algorithms to reduce the noise further. I think with 10 uVpp, you're already very close!
Thank you Ryan,
Apologies for the late response and thank you for your detailed response. Please help me with the following questions.
1) If the bandwidth of interest is 300 Hz, does decreasing the bandwidth of those anti aliasing filters to somewhere around 800Hz help (without affecting the fMod)? I am thinking of minimising the components in the analog path to increase the CMRR.
2) Could you please recommend the components needed for the bias shield circuitry or suggest how to choose them?
The -3 dB bandwidth of the digital filter is about 0.262 * f_DATA. At 500 SPS, this gives you a -3 dB bandwidth of about 131 Hz. I do not see a benefit is reducing the cutoff frequency of the antialiasing filters.
The shield drive circuitry can be as simple as a unity-gain voltage follower with limited filtering. A general purpose amplifier such as the TLV6741 would work fine.
Thank you for your response.
Does it means that the amplifier is converting signals reliably if they are under 131Hz?
How does the 0.262 calculated? is that amplifier specific?
Thank you for suggesting the amplifier for shield drive.
The ADS1299 is a delta-sigma analog-to-digital converter (ADC). There are some peripheral amplifiers included, but it is primarily an 8-channel ADC.
The ADCs are followed by a digital filter that decimates the output and contains a low-pass filter response. This response scales with the ADC output data rate as detailed in Figures 27 - 31.
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