Post History
Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance ...
#3: Post edited
by
Andy aka
·
2026-03-18T15:11:51Z (6 months ago)
- Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance of the ADC is about 10 pF so, if that was directly fed from C44 in a slightly different arrangement it is likely to work with low errors.
- **Proposal:**
- 
- So, if the 15 nF capacitor (C44) on the feed to the ADC were suddenly loaded with 10 pF (the ADC input), how much would the voltage dip. Well, assume the charge is the same before as after and, 2 volts was on the 15 nF capacitor (C44), adding 10 pF would reduce the voltage to 1.99867 volts.
- Is that too much of an error (1.33 mV in 2 volts or 0.067%).
It might be a little too much but, on the other hand it may be very reasonable and, allows you to get rid of an op-amp. That op-amp (OP350) would erode accuracy by up to 500 μV (at 25° C) anyway due to its input offset error. Then when you take into account the likely errors from your flux sensor, not using an op-amp is really the optimum route to choose.- Is an op-amp somehow needed? Well, in the data sheet this diagram is shown: -
- 
- Clearly there is no op-amp and, using CTRL-F there are zero mentions of `op-amp`, `op amp` or `opamp`.
- ----------
- You should also examine the effectiveness of the RC filter as an anti-alias filter. Do you know what ADC sampling rate you use?
- > _<sub>In my circuit, I am using an OPA333 op amp to buffer a sensor signal before it goes through a two stage RC low pass filter (R13–C44 and R14–C45).</sub>_
- <sub>No, what you have is the flux sensor output feeding though a low-pass filter then being buffered by the op-amp prior to feeding into the ADC. In other words, your words are jumbled and imply that the filter comes between op-amp and ADC (which it doesn't according to your schematic).</sub>
- Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance of the ADC is about 10 pF so, if that was directly fed from C44 in a slightly different arrangement it is likely to work with low errors.
- **Proposal:**
- 
- So, if the 15 nF capacitor (C44) on the feed to the ADC were suddenly loaded with 10 pF (the ADC input), how much would the voltage dip. Well, assume the charge is the same before as after and, 2 volts was on the 15 nF capacitor (C44), adding 10 pF would reduce the voltage to 1.99867 volts.
- Is that too much of an error (1.33 mV in 2 volts or 0.067%).
- It might be a little too much but, on the other hand it may be very reasonable and, allows you to get rid of an op-amp. That op-amp (OP350) would erode accuracy by up to 500 μV (at 25° C) anyway due to its input offset error.
- **When you take into account the likely errors from your flux sensor, not using an op-amp is really the optimum route to choose.**
- Is an op-amp somehow needed? Well, in the data sheet this diagram is shown: -
- 
- Clearly there is no op-amp and, using CTRL-F there are zero mentions of `op-amp`, `op amp` or `opamp`.
- ----------
- You should also examine the effectiveness of the RC filter as an anti-alias filter. Do you know what ADC sampling rate you use?
- > _<sub>In my circuit, I am using an OPA333 op amp to buffer a sensor signal before it goes through a two stage RC low pass filter (R13–C44 and R14–C45).</sub>_
- <sub>No, what you have is the flux sensor output feeding though a low-pass filter then being buffered by the op-amp prior to feeding into the ADC. In other words, your words are jumbled and imply that the filter comes between op-amp and ADC (which it doesn't according to your schematic).</sub>
#2: Post edited
by
Andy aka
·
2026-03-18T15:10:22Z (6 months ago)
- Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance of the ADC is about 10 pF so, if that was directly fed from C44 in a slightly different arrangement it is likely to work with low errors.
- **Proposal:**
- 
- So, if the 15 nF capacitor (C44) on the feed to the ADC were suddenly loaded with 10 pF (the ADC input), how much would the voltage dip. Well, assume the charge is the same before as after and, 2 volts was on the 15 nF capacitor (C44), adding 10 pF would reduce the voltage to 1.99867 volts.
- Is that too much of an error (1.33 mV in 2 volts or 0.067%).
- It might be a little too much but, on the other hand it may be very reasonable and, allows you to get rid of an op-amp. That op-amp (OP350) would erode accuracy by up to 500 μV (at 25° C) anyway due to its input offset error. Then when you take into account the likely errors from your flux sensor, not using an op-amp is really the optimum route to choose.
- Is an op-amp somehow needed? Well, in the data sheet this diagram is shown: -
- 
- Clearly there is no op-amp and, using CTRL-F there are zero mentions of `op-amp`, `op amp` or `opamp`.
- ----------
- You should also examine the effectiveness of the RC filter as an anti-alias filter. Do you know what ADC sampling rate you use?
> *<sub>In my circuit, I am using an OPA333 op amp to buffer a sensor signal before it goes through a two stage RC low pass filter (R13–C44 and R14–C45).</sub>*- <sub>No, what you have is the flux sensor output feeding though a low-pass filter then being buffered by the op-amp prior to feeding into the ADC. In other words, your words are jumbled and imply that the filter comes between op-amp and ADC (which it doesn't according to your schematic).</sub>
- Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance of the ADC is about 10 pF so, if that was directly fed from C44 in a slightly different arrangement it is likely to work with low errors.
- **Proposal:**
- 
- So, if the 15 nF capacitor (C44) on the feed to the ADC were suddenly loaded with 10 pF (the ADC input), how much would the voltage dip. Well, assume the charge is the same before as after and, 2 volts was on the 15 nF capacitor (C44), adding 10 pF would reduce the voltage to 1.99867 volts.
- Is that too much of an error (1.33 mV in 2 volts or 0.067%).
- It might be a little too much but, on the other hand it may be very reasonable and, allows you to get rid of an op-amp. That op-amp (OP350) would erode accuracy by up to 500 μV (at 25° C) anyway due to its input offset error. Then when you take into account the likely errors from your flux sensor, not using an op-amp is really the optimum route to choose.
- Is an op-amp somehow needed? Well, in the data sheet this diagram is shown: -
- 
- Clearly there is no op-amp and, using CTRL-F there are zero mentions of `op-amp`, `op amp` or `opamp`.
- ----------
- You should also examine the effectiveness of the RC filter as an anti-alias filter. Do you know what ADC sampling rate you use?
- > _<sub>In my circuit, I am using an OPA333 op amp to buffer a sensor signal before it goes through a two stage RC low pass filter (R13–C44 and R14–C45).</sub>_
- <sub>No, what you have is the flux sensor output feeding though a low-pass filter then being buffered by the op-amp prior to feeding into the ADC. In other words, your words are jumbled and imply that the filter comes between op-amp and ADC (which it doesn't according to your schematic).</sub>
#1: Initial revision
by
Andy aka
·
2026-03-18T15:03:36Z (6 months ago)
Your opening words appear to be at odds with your schematic but, assuming the schematic is correct I'm thinking why bother with an op-amp at all. According to sources on-line the input capacitance of the ADC is about 10 pF so, if that was directly fed from C44 in a slightly different arrangement it is likely to work with low errors. **Proposal:**  So, if the 15 nF capacitor (C44) on the feed to the ADC were suddenly loaded with 10 pF (the ADC input), how much would the voltage dip. Well, assume the charge is the same before as after and, 2 volts was on the 15 nF capacitor (C44), adding 10 pF would reduce the voltage to 1.99867 volts. Is that too much of an error (1.33 mV in 2 volts or 0.067%). It might be a little too much but, on the other hand it may be very reasonable and, allows you to get rid of an op-amp. That op-amp (OP350) would erode accuracy by up to 500 μV (at 25° C) anyway due to its input offset error. Then when you take into account the likely errors from your flux sensor, not using an op-amp is really the optimum route to choose. Is an op-amp somehow needed? Well, in the data sheet this diagram is shown: -  Clearly there is no op-amp and, using CTRL-F there are zero mentions of `op-amp`, `op amp` or `opamp`. ---------- You should also examine the effectiveness of the RC filter as an anti-alias filter. Do you know what ADC sampling rate you use? > *<sub>In my circuit, I am using an OPA333 op amp to buffer a sensor signal before it goes through a two stage RC low pass filter (R13–C44 and R14–C45).</sub>* <sub>No, what you have is the flux sensor output feeding though a low-pass filter then being buffered by the op-amp prior to feeding into the ADC. In other words, your words are jumbled and imply that the filter comes between op-amp and ADC (which it doesn't according to your schematic).</sub>