ADS114S08: Low input offset voltage opamp

Prodigy 220 points

Replies: 8

Views: 177

Part Number: ADS114S08

8 Replies

  • Hi Arumugam,

    Can you send me the configuration register settings you are using for the ADS114S08?  In your first point you state you are not getting the accuracy that you expected.  Basically you are finding out that offset error is causing you an issue, but you have other potential issues related to offset/gain error and drift.  I'm not an op amp expert, but it would appear that the OPA2317 would be a good substitute.  You should pose the same question on the amplifiers forum.

    As far as error sources, you are using the SMF5.0A TVS diode that has high leakage.  This may actually be a much bigger issue than the op amp.  Also, you have removed the antialiasing filter you had on your previous schematic (cap to ground on the analog inputs) where we were discussing the oscillation you were seeing. You should be able to have that cap as long is you do not directly connect the cap to the op amp output.  That said, the cap may not be that helpful as you have some filtering at the op amp input.  The noise of the amplifier would be the noise that might aliasing back into the pass band.

    R265 will also have some error associated with it and can potentially drift.

    Best regards,

    Bob B

  • In reply to Bob Benjamin:

    Hi Bob,

    Thanks for your comments.. Please find detailed snap which i have given below. 

    We are using RC low pass filter ADC input ( missed to add in the existing snap). The R265 we are using 0.01% so i hope that is OK.

    Resister settings comments available in snap. Also Removed SMF5.0A TVS diode and measured voltage differents between input and op-amp output ~ 0.3 to 0.7mV. 

    I believe the the 0.6mV due to op-amp offset as per op-amp datasheet (page 3). 

    Need your advise while we go OPA2317  opamp ; is it will affect existing RC low pass filter design (R = 1Kohm, C = 0.1 uF) due to op-amp capacitor load ?

    Thanks and Regards,

    Arumugam.P

  • In reply to Arumugam Palaniyandi:

    Hi Arumugam,

    You should not need to change the RC filter values when using the OPA2317.

    Best regards,

    Bob B

  • In reply to Bob Benjamin:

    Hi Bob,

    I have replaced with OPA2317IDR opamp. Find the attached op-amp output. The ripple output voltage measured at op-amp output.

    This confirms the RC filter value need to be re-designed. could you share your comments 

    - 200 Deg = 350 Hz

    - 100 Deg = 566 Hz

    0 Deg = 700Hz

    100 Deg = 780Hz

    200 Deg = 860Hz 

    300 Deg = 920 Hz

    800 Deg = 1.15KHzOPA2317DIR.xlsx

    Thanks and Regards,

    Arumugam.P

  • In reply to Arumugam Palaniyandi:

    Hi Arumugam,

    I see that you have also been discussing the op amp circuit with Luis in the amplifier E2E.  Similar to what Luis has done I created a TINA simulation of the circuit you are using along with the 1k/100nF input filter and I don't see any oscillation using the OPA317 macro model. What does the inputs to the op amp look like?  What are you using as the sensor input?  Is it a simulator, PT100 or true resistance?

    If we look strictly at the 0 deg C case shown in the spreadsheet, the op amp is peaking at around 800mV.  A PT100 would be around 100mV output at that temperature as the resistance would be 100 Ohms.  So things just don't come together for the circuit you have given and the response of the output.  The output should be stable and at the correct voltage for the input resistance applied.  My concern is that the pulse does not appear to be like a normal oscillation of the op amp output as there is a peak and a rapid decay back to 0V with no sustained oscillation.  You will need to do a lot of probing to determine the cause.

    Best regards,

    Bob B

  • As far as error sources, you are using the SMF5.0A TVS diode that has high leakage.  This may actually be a much bigger issue than the op amp.  Also, you have removed the antialiasing filter you had on your previous schematic (cap to ground on the analog inputs) where we were discussing the oscillation you were seeing. You should be able to have that cap as long is you do not directly connect the cap to the op amp output.  That said, the cap may not be that helpful as you have some filtering at the op amp input.  The noise of the amplifier would be the noise that might aliasing back into the pass band.

  • In reply to snow jhon:

    Thanks for your comments. I was able to see the oscillation in inverting input of the opamp. So i removed inverting input cap (2 no's 0.1uF) and the oscillation completely removed both op-amp input and output.

    Now ADC accuracy is meeting our requirement.

    Thanks...