Improving CT conditioning circuit

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
102
The circuit attached is an inverting amplifier with variable gains. The CT measures 60 Hz currents from 0 to 30A. It has high sensitivity since it's used to measure low ground fault currents. I added a DC bias to the AC signal as shown for the ADC processing. I have the following questions:

1. As I'm using an IC (LM4041) for the voltage reference, would I need to buffer it with the op amp?
2. I added a coupling cap C3, but not sure how it would affect my signal. It's a high pass with a cut off frequency of ~20Hz. C19 forms a low pass filter. Should I leave C3 for this application?
3. If op amps are cheap or low quality, would the offset voltage affect significantly?
4. If for some reason the primary current exceeds 30A, would diodes across the CT help clamp any over voltage? I usually see diodes used when the signal swings around zero, but in this case it swings around 1.65V (3.3/2).

Any suggestion is appreciated.
 

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MisterBill2

Joined Jan 23, 2018
5,804
The circuit attached is an inverting amplifier with variable gains. The CT measures 60 Hz currents from 0 to 30A. It has high sensitivity since it's used to measure low ground fault currents. I added a DC bias to the AC signal as shown for the ADC processing. I have the following questions:

1. As I'm using an IC (LM4041) for the voltage reference, would I need to buffer it with the op amp?
2. I added a coupling cap C3, but not sure how it would affect my signal. It's a high pass with a cut off frequency of ~20Hz. C19 forms a low pass filter. Should I leave C3 for this application?
3. If op amps are cheap or low quality, would the offset voltage affect significantly?
4. If for some reason the primary current exceeds 30A, would diodes across the CT help clamp any over voltage? I usually see diodes used when the signal swings around zero, but in this case it swings around 1.65V (3.3/2).

Any suggestion is appreciated.
1. Buffering a reference is usually OK, except that you are adding contributions from the amp used as a buffer. That might be a problem in your (unknown) application of the final signal.
2. with both gain switches open you may have some kind of oscillation. It does not seem like there is any benefit to adding C3, since there is no DC part of your input except for the reference.
3. You will indeed have the amplifier offset voltage in the output, and with some amplifiers offset does vary with temperature and other influences.
4. if there is a bit of over-current then the output will probably not be accurate. R35 will provide some current limiting.
But I am not sure just where your output voltage will be with no current, unless you have chosen a "rail to rail" op amp.
 

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
102
1. Buffering a reference is usually OK, except that you are adding contributions from the amp used as a buffer. That might be a problem in your (unknown) application of the final signal.
2. with both gain switches open you may have some kind of oscillation. It does not seem like there is any benefit to adding C3, since there is no DC part of your input except for the reference.
3. You will indeed have the amplifier offset voltage in the output, and with some amplifiers offset does vary with temperature and other influences.
4. if there is a bit of over-current then the output will probably not be accurate. R35 will provide some current limiting.
But I am not sure just where your output voltage will be with no current, unless you have chosen a "rail to rail" op amp.
OK. Thanks.
1. What kind of contributions do you refer to?
2. I think it's better to remove C3 then. The switches are analog, and they are controlled by a micro. It's hard that both of them will be open at the same time.
3. Maybe I will have to choose a zero drift amplifier to mitigate any potential issue with the offset.
4. Maybe a couple of diodes help clamp any overvoltage? I have to make sure I don't turn on the diodes unintentionally.
 

MisterBill2

Joined Jan 23, 2018
5,804
OK. Thanks.
1. What kind of contributions do you refer to?
2. I think it's better to remove C3 then. The switches are analog, and they are controlled by a micro. It's hard that both of them will be open at the same time.
3. Maybe I will have to choose a zero drift amplifier to mitigate any potential issue with the offset.
4. Maybe a couple of diodes help clamp any overvoltage? I have to make sure I don't turn on the diodes unintentionally.
The contributions may be noise, drift, or varying gain with temperature. And some devices have a problem rejecting power supply noise. So none of the possible contributions would be what you want.
 

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
102
How would I choose a right op amp for this application? The minimum voltage across the burden resistor is 100uV, and I want to mitigate any effect from the op amp offset, if significant. Zero drift op amps I think solve this issue, some of them are expensive though. It's a trade-off.
 

MisterBill2

Joined Jan 23, 2018
5,804
How would I choose a right op amp for this application? The minimum voltage across the burden resistor is 100uV, and I want to mitigate any effect from the op amp offset, if significant. Zero drift op amps I think solve this issue, some of them are expensive though. It's a trade-off.
To decide which op-amp you use, first you need to decide what your requirements are. How accurate does the output need to be, and over what range. Then what temperature range will you need to have it working in. If your power supply is a battery, as shown, how important is battery life?
Unfortunately the arrangement for setting the gain with the two switches is not the best choice. A permanently connected resistor to set the higher gain and then an additional resistor switched across it to provide the lower gain, is a better choice. And many op-amps do not like large capacitors across the output because it leads to instability.
Powering a device from a higher supply voltage allows a wider range of voltages and that may be an advantage worth considering, unless this is a battery powered application.
 

DickCappels

Joined Aug 21, 2008
6,317
The circuit attached is an inverting amplifier with variable gains. The CT measures 60 Hz currents from 0 to 30A. It has high sensitivity since it's used to measure low ground fault currents. I added a DC bias to the AC signal as shown for the ADC processing. I have the following questions:

1. As I'm using an IC (LM4041) for the voltage reference, would I need to buffer it with the op amp?
2. I added a coupling cap C3, but not sure how it would affect my signal. It's a high pass with a cut off frequency of ~20Hz. C19 forms a low pass filter. Should I leave C3 for this application?
3. If op amps are cheap or low quality, would the offset voltage affect significantly?
4. If for some reason the primary current exceeds 30A, would diodes across the CT help clamp any over voltage? I usually see diodes used when the signal swings around zero, but in this case it swings around 1.65V (3.3/2).

Any suggestion is appreciated.
1. The LM4041 is way more than you need for a bias source, but it is very stable and quiet. You don't need to buffer it since it has a very low output impedance already (1Ω±0.5Ω_)

2. C3 is interrupting the DC feedback in the highest gain position. Replace it with a short.

3. What is your application?

4. You probably don't need the diode protection with that 8.2k resistor in series with that 10 mv/ma scale factor unless you get WAY over 30 amps, but I would still include the diodes just in case. but I would put it on the op amp side of the 8.2k resistor. At a penny each, why not?

5. C4 is a good idea to deal with parasitic capacitance coupling voltage glitches from the primary. It might cause the opamp to oscillate, if so, see the datasheet for a suggestion as to how to drive a high capacitance load.
 

MisterBill2

Joined Jan 23, 2018
5,804
At some current level you may get into saturation of the CT core and at that point the power transferred will be limited. So that can limit the amplifier input voltage.
 

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
102
1. The LM4041 is way more than you need for a bias source, but it is very stable and quiet. You don't need to buffer it since it has a very low output impedance already (1Ω±0.5Ω_)

2. C3 is interrupting the DC feedback in the highest gain position. Replace it with a short.

3. What is your application?

4. You probably don't need the diode protection with that 8.2k resistor in series with that 10 mv/ma scale factor unless you get WAY over 30 amps, but I would still include the diodes just in case. but I would put it on the op amp side of the 8.2k resistor. At a penny each, why not?

5. C4 is a good idea to deal with parasitic capacitance coupling voltage glitches from the primary. It might cause the opamp to oscillate, if so, see the datasheet for a suggestion as to how to drive a high capacitance load.
1. I may get away with using the voltage reference without buffering it.
2. Ok, makes sense.
3. Current measurement, being the minimum current 10 mA for a 1000:1 high sensitivity CT, so 100uV min in the burden resistor. Max current I will measure is 25A.
4. I put anti parallel diodes across the secondary
5. If I'm not using the buffer, then I don't need C4 I think. Or maybe I can put it at the reference IC output.
 

DickCappels

Joined Aug 21, 2008
6,317
5. Sorry, when writing the comment I failed to look at the circuit as a whole. I was looking for a place to do something that would keep the entire secondary from "jumping" if there were a fast change in the voltage on the primary, such as the circuit being activated or noise on the line like a switching transient or a nearby lightning strike. This is not necessarily to protect only the buffer but everything connected to the secondary and especially the ADC input so I would still leave C4 in for that purpose.
 
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