Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 46 subscribers
  • Views 932 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS8924B: Geophone Signal Conditioning with Range Extender

Mahabir Prasad
Intellectual 300 points
Part Number: ADS8924B
Other Parts Discussed in Thread: CC1310, OPA192, ADS1282, ADS8685

Dear TI Team,

I need your help/Review to correct the attached TINA Circuit for Geophone amplifier. I am not analog hardware expert, have basic knowledge. In attached circuit, have used information from this blog as well from web. I need to design a geophone sensor amplifier using HS-6, 10Hz, possible to extend the range to lower side till 100mHz to 1Hz using electronic methods, e.g. NVIC or any suitable method. The OPA192 and ADS8924B are planned for amplification and ADC. I have attached documents - data sheets of Geophone, document to derive the transfer function of Geophone and TINA simulation circuit. The unit will powered by 5V battery using LDO with 7.2V Li-Ion Battery. This signal amplifier will be connected to CC1310 module to send the data. The use of external ADC is needed, max use of freq will be 250Hz.

1. Does placement of RC filters after each stage of amplifier needed?

2. Anti-aliasing Filter (3rd order) after amplifier is good or need to insert between both gain stages?

3. Selection of parts are OK, with 16 Bits ADC measurement?

Datasheet Geophone HG-6.pdfGeophone_freq_resp.pdf

Geophone HG-6 10Hz Amplifier V2.TSC

  • 0
    TI__Mastermind 41090 points

    Hello Mahabir,

    I ran a quick simulation of your circuit, and obtained the below response.Your total circuit gain is around 1600, or 64dB.This suggests that the maximum output of your geophone will be around 1mVrms.

    I noticed in your schematic that you had made reference to a sampling rate of 32ksps, in which case your Nyquist frequency will be 16kHz and your filter will attenuate relative to the passband -107dB, which is quite good for most applications.

    1. Does placement of RC filters after each stage of amplifier needed?

    I assume you are talking about the high pass stages on the first two G=40 stages.  These are needed to set the DC operating point to 2.5V, and to reduce the effect of any offset voltages getting gained up and saturating the output of the amplifiers.

    2. Anti-aliasing Filter (3rd order) after amplifier is good or need to insert between both gain stages?

    Based on my earlier assumption that you are sampling at 32ksps, your anti-alias filters are giving you around -107dB of attenuation at 16kHz.  This is quite good for most applications, but it really depends on your specific system requirements.  You may be able to use a lower order filter, but again, it depends on how large your out of band signals (>16kHz) can be in amplitude vs your 0.1-250Hz signal of interest.

    3. Selection of parts are OK, with 16 Bits ADC measurement?

    The frequency response of your front end looks good.  In order to drive the ADS8924, you will need to convert the single ended signal to a differential, which will require 1 additional amplifier.  Take a look at this cookbook circuit that shows how to implement this:

    http://www.ti.com/lit/an/sbaa265/sbaa265.pdf

    Also, the total noise in your system will be rather high due to the overall gain in the front end amplifier.  The SNR will be limited to around 73dB.  You could use lower noise amplifiers, but these will require more current and possible wider supply voltages, complicating your design.

    We do have an ADC specifically designed for geophone applications, ADS1282. 

    This device integrates the ADC and the front-end amplifier.  Based on your gain, I estimate the output of your geophone to be around 1mVrms.  The ADS1282 can achieve an SNR of around 84dB in this case.

    Regards,

    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Intellectual 300 points

    Dear keith,

    Thanks for your detailed reply of every point.

    Due to certain doubts:

    1. I did change the uni-polar to bipolar power supply in design.

    2. Used new ADC ADS8685 with +/-4.0V range as input.

    3. I had seen your suggested Delta-Sigma ADC, did not select due to reasons, may be I am wrong. 1) More power than SAR (in application, ADC will never goes in sleep mode, always conversion), 2) In case I need more gain, there was not option, 3) Most of circuit in market goes with lots of amplifier, filters etc.

    Once I get confidence on input signal range from Geophone for said application, surely like to go less chip solution, which Delta/Sigma ADC and few external parts.

    Any further inputs from you as team, always welcome. Specially next target will be "how to extent lower side natural frequency of Geophone electronically"