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.

ADS1282 input circuit for Seismometer

Other Parts Discussed in Thread: ADS1282, PGA281, TINA-TI, OPA1632

Dear Colleagues,

I have to develop a circuit using ADS1282 as the analog digital converter. We have to connect a seismometer like this http://www.eentec.com/SP-400_data.htm to provide the 3 necessary differential signals to my device. If you check in the sensor specifications you can see that the output signal swing can reach +-20Vpp in the differential mode and of course this is too much for the ADS1282 inputs. In the ADS1282 manual I can see on page 42 an Geophone interface Application, Do you have any circuit interface sample for Seismometer? Should I use a simple voltage divider or another solution? (maximum symmetrical supply +-5V)

Best Regards

Flávio Cavalieri

  • Hi Flavio,

    Welcome to the E2E forums and thank you for the seismometer link!

    While I don't know the output impedance of the SP-400, my guess is that you'll want to connect it to a high impedance input. (I found an example circuit of an FBV seismometer here that shows 1k series resistors on the output.) You may want to contact Eentec support to be sure.

    My recommendation would be to use the PGA281 in a gain of 1/8, and use a 2.5V common-mode voltage. Then you could connect the PGA281 to the ADS1282 (using a 5V reference and gain of 1).

    Attached is a TINA-TI simulation of the PGA281 circuit: 1351.PGA281 Attenuator.TSC

    Best Regards,
    Chris

  • Hi Christopher,

    First of all, Thank you so much by the fast and detailed answer.
    I'm sorry about the impedance information, but I think with the following information you can help me to decide the
    best circuit option to the requirements I have. Unfortunatelly if we try to find information about seismometer
    specifications at google we normally find only a few like Operating principle, Dynamic Range, Output swing
    and so on. So I decided to check for some comercial seismic records that supports this sensor and I found at
    least two options with the following specifications:

    http://www.eentec.com/DR-4000_data.htm
    http://gachon.eri.u-tokyo.ac.jp/~hitosi/NECESSArray/data/1301_startup.pdf (PAGE 14)

    If you check for the parameter "Input Impedance" throught the specification table you find the values:
    REFTEK 130-01 (PAGE 14), (2Mohms - Gain x32 / 25Kohms - Gain x1)
    EENTEC DR4000 (1Mohm - +-2.5V / 26Kohms - +-20V)

    Digging deeper I found the following link: http://gachon.eri.u-tokyo.ac.jp/~hitosi/NECESSArray/data/Manual/130_01/RT505Dall.pdf
    As you can see, this is the manual for the Ref Tek A/D Board and on the page 53 (6-45) you can find the electrical diagram for one input,
    The desired board is related to the Bill of materials (B05) and I think the 25Kohms is a result of
    R31, R30 in series with (parallel of R41 and R40). With the relays they can select or not the resistor network to provide
    the desired voltage division and match the manual specification (gain selection: x1 and x32).

    In our case we can use the advantage of the internal PGA gains of ADS1282 (x1 to x64), but of course I need to decrease the sensor
    voltage before reach the inputs AINP1 and AINN1. My question is, should I use in this case the PGA281 to select the gain before
    the AINP1 and AINN1 ADS1282 inputs? Using a resistor network like the diagram I send you seems to be a quite simple solution, but do you
    think I can get more benefit using an fully-differential amplifier like OPA1632 as a buffer between the ADS1282 inputs and the voltage divider?
    In case of using the PGA281 I'll probably have to use at least a DC-DC converter of +-15V to provide the necessary supply to the amplifier.
    Sorry about my english and also by the long questions before, I just want to make sure I'm going in a correct way before draw the first diagram.

    Best Regards and thanks again

    Flávio Cavalieri

  • Hi Flavio,

    Thanks for all the details and references! (BTW, your English is great!)

    On the last link (for the RT505 A/D Board), the schematic shows a differential resistor divider. R30 and R31 are the series elements and R40 or R41 are the shunt elements that can be switched in or out of the circuit. You could very easily do the same in your system to divide down the input signal.

    My concern was that the output impedance of the SP-400 would add to the resistive divider and change the signal attenuation. This is okay, as long as you measure the attenuation and correct for it by way of gain calibration.

    Here are my thoughts on the various solutions:

    1. Simply using the resistor dividers prior to the ADS1282 would be the simplest solution. You may need to test the attenuation and adjust the resistor values as needed. Also, consider the resistors tolerances and temperature effects. Offset and Gain calibration could correct for initial resistor tolerances; however, resistor drift may cause additional offset and gain error that would require re-calibration as temperature changes.

    2. Using the PGA281 would require higher supply voltages, as you mentioned. However, it would likely provide better matching and less drift over temperature, but it  also adds additional noise and distortion to the signal.

    3. The OPA1632 is a low distortion fully-differential amplifier, I think you're right to suggest this device over the PGA281. You would also be able to use it with your existing +/-5V supplies. As you would be configuring the OPA1632 in a difference amplifier configuration, you would still need to consider the resistor matching, resistor drift, as well as additional amplifier and resistor noise. I would probably still want to use solution #1 above, as it uses fewer components and will likely provide similar or perhaps slightly better performance. (The OPA1632 really isn't need to drive the ADS1282, all you need to do is divide down the signal to be in range of the integrated PGA).

    Look for low temperature coefficient resistors such as metal-, thick-, or thin-film. Also, precision wire-wound resistors will have low temperature coefficients but will be higher cost.

    Let me know if that helps and if you have any additional questions!

    Best Regards,
    Chris

  • Hi Christopher,

    Thanks again for sharing your thoughts about the possible solutions.
    As you said the solution 1 use fewer components and I'll probably go on this direction following your
    advices related to resistors tolerances and temperature effects. I have one additional question:
    To choose the proper circuit I made two different diagrams as you can see in the pictures (Input1 and Input2)

    In the first solution (Input1) resistors R5, R6, R7 define my differencial gain and also my impedance (I have
    to consider in this calculation R3 and R4).

    INPUT1:

    R3, R4 and C3 is a differential filter for the seismometer input.

    I'll also use an external diode clamps for ADC input protection.

    In this firs circuit (Input1) is also possible to change the differential filter (R3, R4, C3) moving forward
    and positioning between the diode clamps and the attenuation circuit, in this case I think would be interesting
    to use and differential amplifier as a buffer so the RC differential filter will be not affected by the attenuation
    circuit (R5, R6, R7)

    The second solution (Input 2) it looks more simple.

    INPUT2:

    The attenuation circuit can be calculated by the following formulas: R5 / (R1 + R3 + R5) or R5 / (R1 + R5)
    The input impedance is a sum of 2*(R1 + R3 + R5) and it's important, as you said, to make the proper gain and
    offset calibration for the PGA of ADS1282.

    R1, R2 and C1 is a differential filter for the seismometer input.

    Should I use an TSV diode for circuit protection between the inputs SIG+ and SIG-?

    In my final circuit I want to have the possibility to connect differential or even single ended signals, may
    I use the second solution instead of first?

    Best Regards and thanks again

    Flávio Cavalieri

  • Hi Flavio,

    Both of your circuits have some advantages and disadvantages...

    I like that circuit #2 doesn't require the 1M resistors; however, you then have one additional precision resistor to purchase.

    With either circuit, I recommend putting the differential filter capacitor LAST! This way, you get the best noise performance. I would probably order my circuit order my input circuit like the following (going from the sensor to the ADC):

    1. Inductors (if you use them, to block high frequency content such as ESD events).
    2. Diodes (to clamp over voltage events to the rails - I could also see these being placed before the inductors ).
    3. Resistor attenuator (here this circuit is only loaded by the seismometer output impedance, because the ADC input is high impedance. The series resistors will also protect the ADC inputs and will help ensure the clamp diodes remain on during an over voltage event)
    4. Common-mode filter capacitors (the filtering benefits from all the the series impedance up to this point)
    5. Differential filter capacitor (this needs to be as close to the ADC as possible and should be a C0G type for best noise performance and linearity!)

    Also as a rule of thumb, I like to make sure the differential filter capacitor is 10x times larger than the common-mode filter capacitors. Any mismatching in in the common-mode filter capacitors will convert common-mode signals to differential signals (very bad!!!). A larger differential filter capacitor will help attenuate any such differential signals that result from common-mode capacitor mismatches! Alternatively, use an X2Y (TM) capacitor for the common-mode capacitors. An X2Y cap has can be used as a common-mode filter and the capacitors will be very well matched!

    About the TVS diode, you could add it if you need additional protection. Again, I would place it before the series resistors for best protection. As will all your diodes, try to find low leakage didoes to minimize the resulting gain error.

    Best Regards,
    Chris

  • Hi Christopher,

    First of all thank you very much for the precise answer. The reason I put in the circuit 2 the Differencial and common-mode filter before the resistor attenuator is because in this case I think the cutoff frequency fc can be calculated using the fixed value resistors R1, R2 because resistors R5/R6 (R5, R6 >> R1,R2) are much bigger than R1/R2 and I think they'll not influence considerably in the R1, R2, C1 network (probably the seisimometer output impedance will also influence this). As you can see in the circuit 2 it's possible to select the circuit gain by the switch S1/S2 and maybe if I use the filter capacitors after this point, the cutoff frequency will be directly dependent of S1/S2?. Can I use the Differential and common-mode filter capacitors directly after the resistor attenuator without a circuit buffer like an differential amplifier?

    Thank you very much for all the support, I think based on the information you share with me I'll use the differential filter capacitor (10x larger them the common-mode) and calculate the circuit impedance before this component to find a desirable cutoff frequency for my project.

    Best Regards,

    Flávio

  • Hi Flavio,

    What you're describing makes senses and I don't see a problem with doing that way!

    If you decide to put your filters before the attenuator, you could always place one additional differential capacitor after the attenuator (at the ADC inputs), to attenuate the resistor noise. Either way, both circuits will be affected by the seismometer output impedance.

    The nice thing about positioning the attenuator first (followed by the RC filters), is that the RC filters do not put a load on your attenuation circuit. You will have additional series elements that affect the cutoff frequencies, but they are easy to account for. If you switch the shunt resistive element (instead of the series resistors) then you will not affect the cutoff frequencies when you change the attenuation - so no buffer is needed.

    Best Regards,
    Chris

  • Hi Christopher,

    Great! Based on all your advices I decided to draw the following circuit diagram:

    Thanks for the tip to positioning the attenuator first (RC filters do not put a load on it).
    Just to explain why I used such values for the components in my circuit.
    (R3-R5; R4-R6) is the attenuator with a factor of 1/16, the reason is because with a sensor input (+-20)
    it can decrease the signal amplitude to +-1.25V to match the ADS1282 input specification (AVSS + 0.7 ; AVDD – 1.25)

    R5 and R6 also provide the bias current path for the ADS1282 inputs, even when the switchs S1/S2 are positioning in the shunt resistive element.

    If I turn off the attenuator (switch the shunt resistive element) the maximun input allowed in Sig+ and Sig- is (+-1.25).
    The signal is filtered differentially by components R1, R2 and C1, and independently (common-mode) by R1, R2, C2 and C3.
    As the ADS1282 inputs are high impedance, may I change the R1,R2 resistor to bigger values (at same time decrease the capacitor values)?
    I just want to know this because maybe if I increase these resistor values (R1,R2), the additional series elements (R5,R6) will not affect the cutoff frequencies? So I can calculate the cutoff frequencies for my differential filter based only on R1, R2 and C1 values.

    Thank you so much Christopher,

    Best Regards,

    Flávio

  • Hi Flavio,

    Thanks for sharing, you seem to be on the right track!

    I recommend not increasing R1 and R2 too much, maybe 100k maximum. The ADS1282 input are high impedance, but at 100 MOhm common-mode impedance or 1-100 GOhm (depending on CHOP) they will have some effects as the resistor values get larger.

    By making R1 and R2 larger you can make the cutoff frequencies mainly dependent on the filter stage elements.There will still be some effect on the cutoff frequencies as you switch the attenuation - unless, you were to use some kind of double pole double throw switch that had an equal and opposite effect on the circuit, ie the attenuation changes but the Thevenin equivalent impedance remains the same. However, I don't think you want to deal with too many switches and components. It may be better to use through components that are easily swappable if you want to adjust the attenuation and filter values.

    Best Regards,
    Chris

  • Hi Christopher,

    First of all thank you very much for spending your time helping me with this issue.
    I'm really glad about the way TI provide web solutions for sharing knowledge, like the E2E community, FANTASTIC!!!

    Wish you and all your team a nice week!

    Best Regards

    Flávio

  • Hi Flavio,

    Thank you for the kind compliment.

    Let us know if you have any additional questions we can help with!

    Best Regards,
    Chris