sigma delta adc

maduba · Feb 21, 2022

Please see the attached circuit. Can this be circuit connected to sigma delta adc. Does the reference voltge of the sigma delta adc needs to be the same as the comparator input 1.65V. I highly appreciated if you could give me idea about this. Thank you.

danadak · Feb 22, 2022

The output amp, U12B, is clearly offset by 1.65V, 1/2 its supply V. You can connect
your DelSig either single ended , ADC is essentially unsigned output. Or if it has
differential input use the 1.65 as other input to ADC creating a signed output
for the ADC.

Are you trying to do CDS ? Correlated Double Sampling ?

Not knowing the functionality and design requirements of the signal path cannot
comment further. I see you are sampling the input and then LPF the output, but
thats about all.....

Regards, Dana.

maduba · Feb 22, 2022

danadak said:
The output amp, U12B, is clearly offset by 1.65V, 1/2 its supply V. You can connect
your DelSig either single ended , ADC is essentially unsigned output. Or if it has
differential input use the 1.65 as other input to ADC creating a signed output
for the ADC.

Are you trying to do CDS ? Correlated Double Sampling ?

Not knowing the functionality and design requirements of the signal path cannot
comment further. I see you are sampling the input and then LPF the output, but
thats about all.....

Regards, Dana.

Thank you for the reply. I am trying to understand this circuit. This circuit to be use as low current amplifier.I dont get why 1.65V is applied as a reference for the U15. What I understand is 1.65V added as offset of the output voltage. Please correct me if I am wrong. Thank you

danadak · Feb 22, 2022

Would be used to maximize the dynamic range for the sensor, not knowing what
the sensor is rest is a guess.

Regards, Dana.

kubeek · Feb 22, 2022

there are no comparators in the first schematic. What exaxtly are you trying to ask? Also, which adc are we talking about?

maduba · Feb 22, 2022

danadak said:
Would be used to maximize the dynamic range for the sensor, not knowing what
the sensor is rest is a guess.

View attachment 135964

Regards, Dana.

Thank you for the reply. This is very helpful. this is the article about this circuit. https://www.analog.com/en/analog-di...librated-conductivity-measurement-system.html

I dont think I need U12B. Can I just use the output of the U10 and feed it to sigma delta ADC input

maduba · Feb 22, 2022

kubeek said:
there are no comparators in the first schematic. What exaxtly are you trying to ask? Also, which adc are we talking about?

Yes there are no comparators in the first schematic. I wanted to use this circuit as a low current sensing. And feed the output to sigma delta adc (MCP3553). Looking at the circuit I dont think I need U12B. Can I just use the output of the U10 and feed it to sigma delta ADC input(MCP3553)

kubeek · Feb 22, 2022

maduba said:
Yes there are no comparators in the first schematic. I wanted to use this circuit as a low current sensing. And feed the output to sigma delta adc (MCP3553). Looking at the circuit I dont think I need U12B. Can I just use the output of the U10 and feed it to sigma delta ADC input(MCP3553)

So... you don´t actually need to measure conductivitiy using a fully automated and self calibrated system?
What do you then actually want to measure? And please describe what you actually want to measure as best as possible otherwise this is going nowhere.

maduba · Feb 22, 2022

kubeek said:
So... you don´t actually need to measure conductivitiy using a fully automated and self calibrated system?
What do you then actually want to measure? And please describe what you actually want to measure as best as possible otherwise this is going nowhere.

I wanted to measure the current of a sense resistor which will be in mili amps.

kubeek · Feb 22, 2022

what is the value of the resistor and what is the accuracy you want to achieve?

maduba · Feb 22, 2022

I don't know the value of it but probably in the order of magnitude of 1.8 MΩ

Pommie · Feb 22, 2022

maduba said:
I wanted to measure the current of a sense resistor which will be in mili amps.

maduba said:
but probably in the order of magnitude of 1.8 MΩ

A current of 1mA will give a voltage of 1,800V. Something is wrong with your figures.

Mike.

maduba · Feb 22, 2022

Pommie said:
A current of 1mA will give a voltage of 1,800V. Something is wrong with your figures.

Mike.

Thank you for the reply. Since this is going to be low current measurement. Does U15 Reference voltage needed because the offset creates errors on the output of ADC. Am I correct about this.

ChrisP58 · Feb 22, 2022

maduba said:
I don't know the value of it but probably in the order of magnitude of 1.8 MΩ

While high resistor values are typical for voltage measurements where you don't want the measurement to affect the circuit operation, very low resistance values are typically used when measuring current. This is so that the voltage to the load doesn't change much with changing current. A circuit that I'm working on right now uses 5 milliOhms.

Let me ask a few questions.
What range of milliAmps do you need to measure? Min to Max.
What is the voltage with respect to ground at the sense resistor?
Is the load current fairly constant DC, or does it vary?
Is it always in one direction?
What is the load? How stable does it's input voltage need to be?

rjenkinsgb · Feb 22, 2022

maduba said:
I wanted to use this circuit as a low current sensing. And feed the output to sigma delta adc (MCP3553). Looking at the circuit I dont think I need U12B. Can I just use the output of the U10 and feed it to sigma delta ADC input(MCP3553)

I don't believe you need anything after U15, or possibly U10 as a low pass filter with the analog switch bypassed?

The sigma-delta ADC uses a switch capacitor input circuit that acts as a sample & hold.

Connect input high to the opamp out and input low to the mid-supply re (U22B). That circuit is configured to work with either polarity of current in the sense resistor, so "zero" is half supply, to give the widest range either way.

The ADC reference should be set to suit the maximum input range needed.

U22B input is presumably shown as 1.65V on the basis the circuit feeds an ADC operating on 3.3V
Use 2.5V (again, half supply; two equal resistors and a cap to analog ground) instead, if the ADC will run from 5V.

maduba said:
probably in the order of magnitude of 1.8 MΩ

I'm guessing [hoping] you meant
"probably in the order of magnitude of 1.8 mΩ" - Milliohms, not Megaohms?

As shown, that circuit can only work as long as both ends of the sense resistor stay within the common mode range of the opamps; less than +/- 15V.

If you need a wider range, you need a different input circuit.

maduba · Feb 22, 2022

rjenkinsgb said:
I don't believe you need anything after U15, or possibly U10 as a low pass filter with the analog switch bypassed?

The sigma-delta ADC uses a switch capacitor input circuit that acts as a sample & hold.

Connect input high to the opamp out and input low to the mid-supply re (U22B). That circuit is configured to work with either polarity of current in the sense resistor, so "zero" is half supply, to give the widest range either way.

The ADC reference should be set to suit the maximum input range needed.

U22B input is presumably shown as 1.65V on the basis the circuit feeds an ADC operating on 3.3V
Use 2.5V (again, half supply; two equal resistors and a cap to analog ground) instead, if the ADC will run from 5V.

I'm guessing [hoping] you meant
"probably in the order of magnitude of 1.8 mΩ" - Milliohms, not Megaohms?

As shown, that circuit can only work as long as both ends of the sense resistor stay within the common mode range of the opamps; less than +/- 15V.

If you need a wider range, you need a different input circuit.

Thank you for the reply.I meant Megaohms to measure

Nigel Goodwin · Feb 23, 2022

It's rather vague and confusing about exactly what you're trying to do?, but you might try googling for 'current ranger', which is a device for measuring tiny currents, without losing too much voltage across the sense resistor. It's also open source, so all the details you need are there.

danadak · Feb 23, 2022

Measuring Femtoamps -

https://www.edn.com/femtoamp-fa-measurements/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-2-component-selection/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-3-low-current-design-techniques/

Regards, Dana.

maduba · Feb 23, 2022

ChrisP58 said:
While high resistor values are typical for voltage measurements where you don't want the measurement to affect the circuit operation, very low resistance values are typically used when measuring current. This is so that the voltage to the load doesn't change much with changing current. A circuit that I'm working on right now uses 5 milliOhms.

Let me ask a few questions.
What range of milliAmps do you need to measure? Min to Max.
What is the voltage with respect to ground at the sense resistor?
Is the load current fairly constant DC, or does it vary?
Is it always in one direction?
What is the load? How stable does it's input voltage need to be?

Thank you for the reply. I need to measure the resistance in megaohms. Its constant DC. It is always in one direction.

maduba · Feb 23, 2022

danadak said:
Measuring Femtoamps -

https://www.edn.com/femtoamp-fa-measurements/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-2-component-selection/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-3-low-current-design-techniques/

Regards, Dana.

Thank you for the links. Would that circuit viable to use for measuring.

sigma delta adc

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